diff --git a/.github/workflows/php-workshop.yml b/.github/workflows/php-workshop.yml
index 2c17e0af..f55b1402 100644
--- a/.github/workflows/php-workshop.yml
+++ b/.github/workflows/php-workshop.yml
@@ -35,8 +35,8 @@ jobs:
       - name: Run phpcs
         run: composer cs
 
-#      - name: Run phpstan
-#        run: composer static
+      - name: Run phpstan
+        run: composer static
 
       - name: Coverage upload
         if: matrix.php == '7.4'
diff --git a/.scrutinizer.yml b/.scrutinizer.yml
deleted file mode 100644
index 231b1b13..00000000
--- a/.scrutinizer.yml
+++ /dev/null
@@ -1,5 +0,0 @@
-before_commands:
-  - "composer install --prefer-source"
-
-checks:
-    php: true
\ No newline at end of file
diff --git a/composer.json b/composer.json
index d4c963ef..3b578b6c 100644
--- a/composer.json
+++ b/composer.json
@@ -34,7 +34,10 @@
     "require-dev": {
         "composer/composer": "^1.2",
         "squizlabs/php_codesniffer": "^3.4",
-        "symfony/phpunit-bridge": "^5.1"
+        "symfony/phpunit-bridge": "^5.1",
+        "phpstan/phpstan": "^0.12.50",
+        "phpstan/extension-installer": "^1.0",
+        "phpstan/phpstan-symfony": "^0.12.8"
     },
     "autoload" : {
         "psr-4" : {
@@ -54,6 +57,7 @@
         "cs" : [
             "phpcs src --standard=PSR12 --encoding=UTF-8",
             "phpcs test --standard=PSR12 --encoding=UTF-8"
-        ]
+        ],
+        "static": "phpstan --ansi analyse --level max src"
     }
 }
diff --git a/composer.lock b/composer.lock
index b65444ed..e5d7568e 100644
--- a/composer.lock
+++ b/composer.lock
@@ -4,7 +4,7 @@
         "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#installing-dependencies",
         "This file is @generated automatically"
     ],
-    "content-hash": "0ea09ae1f35bb06e5b04906b19e16b78",
+    "content-hash": "bf9d4cab83dfee8219e63d4e0d93c71c",
     "packages": [
         {
             "name": "aydin-hassan/cli-md-renderer",
@@ -1656,6 +1656,177 @@
             ],
             "time": "2020-05-27T16:41:55+00:00"
         },
+        {
+            "name": "phpstan/extension-installer",
+            "version": "1.0.5",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/phpstan/extension-installer.git",
+                "reference": "5c2da3846819f951385cb6a25d3277051481c48a"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/phpstan/extension-installer/zipball/5c2da3846819f951385cb6a25d3277051481c48a",
+                "reference": "5c2da3846819f951385cb6a25d3277051481c48a",
+                "shasum": ""
+            },
+            "require": {
+                "composer-plugin-api": "^1.1 || ^2.0",
+                "php": "^7.1 || ^8.0",
+                "phpstan/phpstan": ">=0.11.6"
+            },
+            "require-dev": {
+                "composer/composer": "^1.8",
+                "consistence/coding-standard": "^3.8",
+                "dealerdirect/phpcodesniffer-composer-installer": "^0.7.0",
+                "ergebnis/composer-normalize": "^2.0.2",
+                "phing/phing": "^2.16",
+                "php-parallel-lint/php-parallel-lint": "^1.2.0",
+                "phpstan/phpstan-strict-rules": "^0.11",
+                "slevomat/coding-standard": "^5.0.4"
+            },
+            "type": "composer-plugin",
+            "extra": {
+                "class": "PHPStan\\ExtensionInstaller\\Plugin"
+            },
+            "autoload": {
+                "psr-4": {
+                    "PHPStan\\ExtensionInstaller\\": "src/"
+                }
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "MIT"
+            ],
+            "description": "Composer plugin for automatic installation of PHPStan extensions",
+            "time": "2020-08-30T12:06:42+00:00"
+        },
+        {
+            "name": "phpstan/phpstan",
+            "version": "0.12.50",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/phpstan/phpstan.git",
+                "reference": "b8248f9c81265af75d6d969ca3252aaf3e998f3a"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/phpstan/phpstan/zipball/b8248f9c81265af75d6d969ca3252aaf3e998f3a",
+                "reference": "b8248f9c81265af75d6d969ca3252aaf3e998f3a",
+                "shasum": ""
+            },
+            "require": {
+                "php": "^7.1|^8.0"
+            },
+            "conflict": {
+                "phpstan/phpstan-shim": "*"
+            },
+            "bin": [
+                "phpstan",
+                "phpstan.phar"
+            ],
+            "type": "library",
+            "extra": {
+                "branch-alias": {
+                    "dev-master": "0.12-dev"
+                }
+            },
+            "autoload": {
+                "files": [
+                    "bootstrap.php"
+                ]
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "MIT"
+            ],
+            "description": "PHPStan - PHP Static Analysis Tool",
+            "funding": [
+                {
+                    "url": "https://github.com/ondrejmirtes",
+                    "type": "github"
+                },
+                {
+                    "url": "https://www.patreon.com/phpstan",
+                    "type": "patreon"
+                },
+                {
+                    "url": "https://tidelift.com/funding/github/packagist/phpstan/phpstan",
+                    "type": "tidelift"
+                }
+            ],
+            "time": "2020-10-16T12:22:23+00:00"
+        },
+        {
+            "name": "phpstan/phpstan-symfony",
+            "version": "0.12.9",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/phpstan/phpstan-symfony.git",
+                "reference": "9da032e0874ea6bb21aa04ddf009e36ff595478f"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/phpstan/phpstan-symfony/zipball/9da032e0874ea6bb21aa04ddf009e36ff595478f",
+                "reference": "9da032e0874ea6bb21aa04ddf009e36ff595478f",
+                "shasum": ""
+            },
+            "require": {
+                "ext-simplexml": "*",
+                "php": "^7.1 || ^8.0",
+                "phpstan/phpstan": "^0.12.26"
+            },
+            "conflict": {
+                "symfony/framework-bundle": "<3.0"
+            },
+            "require-dev": {
+                "consistence/coding-standard": "^3.10",
+                "dealerdirect/phpcodesniffer-composer-installer": "^0.7.0",
+                "ergebnis/composer-normalize": "^2.0.2",
+                "phing/phing": "^2.16.2",
+                "php-parallel-lint/php-parallel-lint": "^1.2",
+                "phpstan/phpstan-phpunit": "^0.12.8",
+                "phpstan/phpstan-strict-rules": "^0.12.2",
+                "phpunit/phpunit": "^7.5.20",
+                "slevomat/coding-standard": "^6.4",
+                "squizlabs/php_codesniffer": "^3.5.6",
+                "symfony/console": "^4.0",
+                "symfony/framework-bundle": "^4.0",
+                "symfony/http-foundation": "^4.0",
+                "symfony/messenger": "^4.2",
+                "symfony/serializer": "^4.0"
+            },
+            "type": "phpstan-extension",
+            "extra": {
+                "branch-alias": {
+                    "dev-master": "0.12-dev"
+                },
+                "phpstan": {
+                    "includes": [
+                        "extension.neon",
+                        "rules.neon"
+                    ]
+                }
+            },
+            "autoload": {
+                "psr-4": {
+                    "PHPStan\\": "src/"
+                }
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "MIT"
+            ],
+            "authors": [
+                {
+                    "name": "Lukáš Unger",
+                    "email": "looky.msc@gmail.com",
+                    "homepage": "https://lookyman.net"
+                }
+            ],
+            "description": "Symfony Framework extensions and rules for PHPStan",
+            "time": "2020-10-21T15:45:33+00:00"
+        },
         {
             "name": "psr/log",
             "version": "1.1.3",
diff --git a/phpstan.neon b/phpstan.neon
new file mode 100644
index 00000000..ae54b621
--- /dev/null
+++ b/phpstan.neon
@@ -0,0 +1,9 @@
+parameters:
+    treatPhpDocTypesAsCertain: false
+    ignoreErrors:
+        -
+            message: '#PhpSchool\\PhpWorkshop\\ExerciseRunner\\CliRunner\:\:preserveOldArgFormat\(\) should return#'
+            path: src/ExerciseRunner/CliRunner.php
+        -
+            message: '#Call to an undefined method PhpParser\\Node\\Expr\|PhpParser\\Node\\Name\:\:__toString\(\)#'
+            path: src/Check/FunctionRequirementsCheck.php
diff --git a/src/Application.php b/src/Application.php
index 18bff628..352f4d4c 100644
--- a/src/Application.php
+++ b/src/Application.php
@@ -22,17 +22,17 @@ final class Application
     private $workshopTitle;
 
     /**
-     * @var array
+     * @var array<class-string>
      */
     private $checks = [];
 
     /**
-     * @var ExerciseInterface[]
+     * @var array<class-string>
      */
     private $exercises = [];
 
     /**
-     * @var array
+     * @var array<array{resultClass: class-string, resultRendererClass: class-string}>
      */
     private $results = [];
 
@@ -68,7 +68,7 @@ final class Application
      * @param string $workshopTitle The workshop title - this is used throughout the application
      * @param string $diConfigFile The absolute path to the DI configuration file
      */
-    public function __construct($workshopTitle, $diConfigFile)
+    public function __construct(string $workshopTitle, string $diConfigFile)
     {
         Assertion::string($workshopTitle);
         Assertion::file($diConfigFile);
@@ -81,9 +81,9 @@ public function __construct($workshopTitle, $diConfigFile)
      * Register a custom check with the application. Exercises will only be able to use the check
      * if it has been registered here.
      *
-     * @param string $check The FQCN of the check
+     * @param class-string $check The FQCN of the check
      */
-    public function addCheck($check)
+    public function addCheck(string $check): void
     {
         $this->checks[] = $check;
     }
@@ -92,18 +92,18 @@ public function addCheck($check)
      * Register an exercise with the application. Only exercises registered here will
      * be displayed in the exercise menu.
      *
-     * @param string $exercise The FQCN of the check
+     * @param class-string $exercise The FQCN of the check
      */
-    public function addExercise($exercise)
+    public function addExercise(string $exercise): void
     {
         $this->exercises[] = $exercise;
     }
 
     /**
-     * @param string $resultClass
-     * @param string $resultRendererClass
+     * @param class-string $resultClass
+     * @param class-string $resultRendererClass
      */
-    public function addResult($resultClass, $resultRendererClass)
+    public function addResult(string $resultClass, string $resultRendererClass): void
     {
         Assertion::classExists($resultClass);
         Assertion::classExists($resultRendererClass);
@@ -120,7 +120,7 @@ public function addResult($resultClass, $resultRendererClass)
      *
      * @param string $logo The logo
      */
-    public function setLogo($logo)
+    public function setLogo(string $logo): void
     {
         Assertion::string($logo);
         $this->logo = $logo;
@@ -132,7 +132,7 @@ public function setLogo($logo)
      *
      * @param string $colour The colour
      */
-    public function setFgColour($colour)
+    public function setFgColour(string $colour): void
     {
         Assertion::string($colour);
         $this->fgColour = $colour;
@@ -144,7 +144,7 @@ public function setFgColour($colour)
      *
      * @param string $colour The colour
      */
-    public function setBgColour($colour)
+    public function setBgColour(string $colour): void
     {
         Assertion::string($colour);
         $this->bgColour = $colour;
@@ -156,7 +156,7 @@ public function setBgColour($colour)
      *
      * @return int The exit code
      */
-    public function run()
+    public function run(): int
     {
         $container = $this->getContainer();
         
@@ -218,7 +218,7 @@ public function run()
     /**
      * @return \DI\Container
      */
-    private function getContainer()
+    private function getContainer(): \DI\Container
     {
         $containerBuilder = new ContainerBuilder();
         $containerBuilder->addDefinitions(
diff --git a/src/Check/CheckRepository.php b/src/Check/CheckRepository.php
index 7565061c..19f245d3 100644
--- a/src/Check/CheckRepository.php
+++ b/src/Check/CheckRepository.php
@@ -11,12 +11,12 @@
 class CheckRepository
 {
     /**
-     * @var CheckInterface[]
+     * @var array<CheckInterface>
      */
     private $checks = [];
 
     /**
-     * @param CheckInterface[] $checks An array of checks available to the workshop framework.
+     * @param array<CheckInterface> $checks An array of checks available to the workshop framework.
      */
     public function __construct(array $checks = [])
     {
@@ -27,6 +27,7 @@ public function __construct(array $checks = [])
 
     /**
      * Add a new check to the repository.
+     * @param CheckInterface $check
      */
     public function registerCheck(CheckInterface $check): void
     {
@@ -36,7 +37,7 @@ public function registerCheck(CheckInterface $check): void
     /**
      * Get all of the checks in the repository.
      *
-     * @return array
+     * @return array<CheckInterface>
      */
     public function getAll(): array
     {
@@ -61,6 +62,9 @@ public function getByClass(string $class): CheckInterface
 
     /**
      * Query whether a check instance exists in this repository via its class name.
+     *
+     * @param string $class
+     * @return bool
      */
     public function has(string $class): bool
     {
diff --git a/src/Check/CodeParseCheck.php b/src/Check/CodeParseCheck.php
index 1330ecf1..02b95bfe 100644
--- a/src/Check/CodeParseCheck.php
+++ b/src/Check/CodeParseCheck.php
@@ -51,13 +51,12 @@ public function getName(): string
      */
     public function check(ExerciseInterface $exercise, Input $input): ResultInterface
     {
-        
-        $code = file_get_contents($input->getArgument('program'));
+        $code = (string) file_get_contents($input->getRequiredArgument('program'));
 
         try {
             $this->parser->parse($code);
         } catch (Error $e) {
-            return Failure::fromCheckAndCodeParseFailure($this, $e, $input->getArgument('program'));
+            return Failure::fromCheckAndCodeParseFailure($this, $e, $input->getRequiredArgument('program'));
         }
         
         return Success::fromCheck($this);
diff --git a/src/Check/ComposerCheck.php b/src/Check/ComposerCheck.php
index 1bef0aab..a2ba915f 100644
--- a/src/Check/ComposerCheck.php
+++ b/src/Check/ComposerCheck.php
@@ -41,19 +41,19 @@ public function check(ExerciseInterface $exercise, Input $input): ResultInterfac
             throw new \InvalidArgumentException();
         }
         
-        if (!file_exists(sprintf('%s/composer.json', dirname($input->getArgument('program'))))) {
+        if (!file_exists(sprintf('%s/composer.json', dirname($input->getRequiredArgument('program'))))) {
             return new Failure($this->getName(), 'No composer.json file found');
         }
 
-        if (!file_exists(sprintf('%s/composer.lock', dirname($input->getArgument('program'))))) {
+        if (!file_exists(sprintf('%s/composer.lock', dirname($input->getRequiredArgument('program'))))) {
             return new Failure($this->getName(), 'No composer.lock file found');
         }
 
-        if (!file_exists(sprintf('%s/vendor', dirname($input->getArgument('program'))))) {
+        if (!file_exists(sprintf('%s/vendor', dirname($input->getRequiredArgument('program'))))) {
             return new Failure($this->getName(), 'No vendor folder found');
         }
         
-        $lockFile = new LockFileParser(sprintf('%s/composer.lock', dirname($input->getArgument('program'))));
+        $lockFile = new LockFileParser(sprintf('%s/composer.lock', dirname($input->getRequiredArgument('program'))));
         $missingPackages = array_filter($exercise->getRequiredPackages(), function ($package) use ($lockFile) {
             return !$lockFile->hasInstalledPackage($package);
         });
diff --git a/src/Check/DatabaseCheck.php b/src/Check/DatabaseCheck.php
index 2f1f65d1..d0c79e59 100644
--- a/src/Check/DatabaseCheck.php
+++ b/src/Check/DatabaseCheck.php
@@ -37,7 +37,7 @@ class DatabaseCheck implements ListenableCheckInterface
     private $solutionDatabasePath;
 
     /**
-     * @var
+     * @var string
      */
     private $userDsn;
 
diff --git a/src/Check/FileExistsCheck.php b/src/Check/FileExistsCheck.php
index d569eaad..8327ff94 100644
--- a/src/Check/FileExistsCheck.php
+++ b/src/Check/FileExistsCheck.php
@@ -31,11 +31,14 @@ public function getName(): string
      */
     public function check(ExerciseInterface $exercise, Input $input): ResultInterface
     {
-        if (file_exists($input->getArgument('program'))) {
+        if (file_exists($input->getRequiredArgument('program'))) {
             return Success::fromCheck($this);
         }
 
-        return Failure::fromCheckAndReason($this, sprintf('File: "%s" does not exist', $input->getArgument('program')));
+        return Failure::fromCheckAndReason(
+            $this,
+            sprintf('File: "%s" does not exist', $input->getRequiredArgument('program'))
+        );
     }
 
     /**
diff --git a/src/Check/FunctionRequirementsCheck.php b/src/Check/FunctionRequirementsCheck.php
index ba42f3ea..e275daff 100644
--- a/src/Check/FunctionRequirementsCheck.php
+++ b/src/Check/FunctionRequirementsCheck.php
@@ -4,6 +4,7 @@
 
 use PhpParser\Error;
 use PhpParser\Node\Expr\FuncCall;
+use PhpParser\Node\Name;
 use PhpParser\NodeTraverser;
 use PhpParser\Parser;
 use PhpSchool\PhpWorkshop\Exercise\ExerciseInterface;
@@ -15,6 +16,7 @@
 use PhpSchool\PhpWorkshop\Result\FunctionRequirementsFailure;
 use PhpSchool\PhpWorkshop\Result\ResultInterface;
 use PhpSchool\PhpWorkshop\Result\Success;
+use PhpSchool\PhpWorkshop\Utils\Collection;
 
 /**
  * This check verifies that the student's solution contains usages of some required functions
@@ -22,7 +24,7 @@
  */
 class FunctionRequirementsCheck implements SimpleCheckInterface
 {
-    
+
     /**
      * @var Parser
      */
@@ -62,12 +64,12 @@ public function check(ExerciseInterface $exercise, Input $input): ResultInterfac
         $requiredFunctions  = $exercise->getRequiredFunctions();
         $bannedFunctions    = $exercise->getBannedFunctions();
 
-        $code = file_get_contents($input->getArgument('program'));
+        $code = (string) file_get_contents($input->getRequiredArgument('program'));
 
         try {
-            $ast = $this->parser->parse($code);
+            $ast = $this->parser->parse($code) ?? [];
         } catch (Error $e) {
-            return Failure::fromCheckAndCodeParseFailure($this, $e, $input->getArgument('program'));
+            return Failure::fromCheckAndCodeParseFailure($this, $e, $input->getRequiredArgument('program'));
         }
 
         $visitor    = new FunctionVisitor($requiredFunctions, $bannedFunctions);
diff --git a/src/CodeInsertion.php b/src/CodeInsertion.php
index 0b4c3f48..b1866a7e 100644
--- a/src/CodeInsertion.php
+++ b/src/CodeInsertion.php
@@ -23,7 +23,7 @@ class CodeInsertion
     public const TYPE_AFTER    = 'after';
 
     /**
-     * @var array
+     * @var array<string>
      */
     private $types = [
         self::TYPE_BEFORE,
@@ -47,11 +47,10 @@ class CodeInsertion
      * @param string $type
      * @param string $code
      */
-    public function __construct($type, $code)
+    public function __construct(string $type, string $code)
     {
         Assertion::inArray($type, $this->types);
-        Assertion::string($code);
-        
+
         $this->type = $type;
         $this->code = $code;
     }
@@ -61,7 +60,7 @@ public function __construct($type, $code)
      *
      * @return string
      */
-    public function getType()
+    public function getType(): string
     {
         return $this->type;
     }
@@ -71,7 +70,7 @@ public function getType()
      *
      * @return string
      */
-    public function getCode()
+    public function getCode(): string
     {
         return $this->code;
     }
diff --git a/src/CodePatcher.php b/src/CodePatcher.php
index aabac59f..6a38671f 100644
--- a/src/CodePatcher.php
+++ b/src/CodePatcher.php
@@ -3,6 +3,7 @@
 namespace PhpSchool\PhpWorkshop;
 
 use PhpParser\Error;
+use PhpParser\Node\Stmt;
 use PhpParser\Parser;
 use PhpParser\PrettyPrinter\Standard;
 use PhpSchool\PhpWorkshop\Exercise\ExerciseInterface;
@@ -26,7 +27,7 @@ class CodePatcher
     private $printer;
     
     /**
-     * @var Patch
+     * @var Patch|null
      */
     private $defaultPatch;
 
@@ -40,12 +41,12 @@ class CodePatcher
      *
      * @param Parser $parser
      * @param Standard $printer
-     * @param Patch $defaultPatch
+     * @param Patch|null $defaultPatch
      */
     public function __construct(Parser $parser, Standard $printer, Patch $defaultPatch = null)
     {
-        $this->parser       = $parser;
-        $this->printer      = $printer;
+        $this->parser = $parser;
+        $this->printer = $printer;
         $this->defaultPatch = $defaultPatch;
     }
     
@@ -61,7 +62,7 @@ public function __construct(Parser $parser, Standard $printer, Patch $defaultPat
      * @param string $code
      * @return string
      */
-    public function patch(ExerciseInterface $exercise, $code)
+    public function patch(ExerciseInterface $exercise, string $code): string
     {
         if (null !== $this->defaultPatch) {
             $code = $this->applyPatch($this->defaultPatch, $code);
@@ -79,7 +80,7 @@ public function patch(ExerciseInterface $exercise, $code)
      * @param string $code
      * @return string
      */
-    private function applyPatch(Patch $patch, $code)
+    private function applyPatch(Patch $patch, string $code): string
     {
         $statements = $this->parser->parse($code);
         foreach ($patch->getModifiers() as $modifier) {
@@ -99,15 +100,15 @@ private function applyPatch(Patch $patch, $code)
 
     /**
      * @param CodeInsertion $codeInsertion
-     * @param array $statements
-     * @return array
+     * @param array<Stmt> $statements
+     * @return array<Stmt>
      */
-    private function applyCodeInsertion(CodeInsertion $codeInsertion, array $statements)
+    private function applyCodeInsertion(CodeInsertion $codeInsertion, array $statements): array
     {
         try {
             $codeToInsert = $codeInsertion->getCode();
             $codeToInsert = sprintf('<?php %s', preg_replace('/^\s*<\?php/', '', $codeToInsert));
-            $additionalStatements = $this->parser->parse($codeToInsert);
+            $additionalStatements = $this->parser->parse($codeToInsert) ?? [];
         } catch (Error $e) {
             //we should probably log this and have a dev mode or something
             return $statements;
diff --git a/src/Command/CreditsCommand.php b/src/Command/CreditsCommand.php
index c5951225..182a1805 100644
--- a/src/Command/CreditsCommand.php
+++ b/src/Command/CreditsCommand.php
@@ -21,52 +21,52 @@ class CreditsCommand
     private $color;
 
     /**
-     * @var array
+     * @var array<string>
      */
     private $coreContributors;
 
     /**
-     * @var array
+     * @var array<string>
      */
     private $appContributors;
 
     /**
-     * @param array $coreContributors
-     * @param array $appContributors
+     * @param array<string> $coreContributors
+     * @param array<string> $appContributors
      * @param OutputInterface $output
      * @param Color $color
      */
     public function __construct(array $coreContributors, array $appContributors, OutputInterface $output, Color $color)
     {
         $this->coreContributors = $coreContributors;
-        $this->appContributors  = $appContributors;
-        $this->output           = $output;
-        $this->color            = $color;
+        $this->appContributors = $appContributors;
+        $this->output = $output;
+        $this->color = $color;
     }
 
     /**
      * Output contributors in columns
      *
-     * @param array $contributors
+     * @param array<string> $contributors
      */
-    private function writeContributors(array $contributors)
+    private function writeContributors(array $contributors): void
     {
         $nameColumnSize = max(array_map('strlen', array_values($contributors)));
-        $columns        = sprintf('%s  GitHub Username', str_pad('Name', $nameColumnSize));
+        $columns        = sprintf('%s  GitHub Username', str_pad('Name', (int) $nameColumnSize));
 
         $this->output->writeLine($columns);
         $this->output->writeLine(str_repeat('-', strlen($columns)));
 
         foreach ($contributors as $gitHubUser => $name) {
-            $this->output->writeLine(sprintf("%s  %s", str_pad($name, $nameColumnSize), $gitHubUser));
+            $this->output->writeLine(sprintf("%s  %s", str_pad($name, (int) $nameColumnSize), $gitHubUser));
         }
     }
     
     /**
      *
-     * @return int|void
+     * @return void
      */
-    public function __invoke()
+    public function __invoke(): void
     {
         if (empty($this->coreContributors)) {
             return;
diff --git a/src/Command/MenuCommandInvoker.php b/src/Command/MenuCommandInvoker.php
index 9341c751..2faaea33 100644
--- a/src/Command/MenuCommandInvoker.php
+++ b/src/Command/MenuCommandInvoker.php
@@ -25,7 +25,7 @@ public function __construct(callable $command)
     /**
      * @param CliMenu $menu
      */
-    public function __invoke(CliMenu $menu)
+    public function __invoke(CliMenu $menu): void
     {
         $menu->close();
         $command = $this->command;
diff --git a/src/Command/PrintCommand.php b/src/Command/PrintCommand.php
index d8b88857..c6b62447 100644
--- a/src/Command/PrintCommand.php
+++ b/src/Command/PrintCommand.php
@@ -2,6 +2,7 @@
 
 namespace PhpSchool\PhpWorkshop\Command;
 
+use PhpSchool\PhpWorkshop\Exercise\ExerciseInterface;
 use PhpSchool\PhpWorkshop\ExerciseRepository;
 use PhpSchool\PhpWorkshop\MarkdownRenderer;
 use PhpSchool\PhpWorkshop\Output\OutputInterface;
@@ -66,7 +67,7 @@ public function __invoke()
         $currentExercise = $this->userState->getCurrentExercise();
         $exercise = $this->exerciseRepository->findByName($currentExercise);
 
-        $markDown = file_get_contents($exercise->getProblem());
+        $markDown = (string) file_get_contents($exercise->getProblem());
         $doc = $this->markdownRenderer->render($markDown);
         $doc = str_replace('{appname}', $this->appName, $doc);
         $this->output->write($doc);
diff --git a/src/CommandArgument.php b/src/CommandArgument.php
index e282df8c..2c69f85d 100644
--- a/src/CommandArgument.php
+++ b/src/CommandArgument.php
@@ -21,7 +21,7 @@ class CommandArgument
      * @param string $name The name of the argument
      * @param bool $optional Whether it is required or not
      */
-    public function __construct($name, $optional = false)
+    public function __construct(string $name, bool $optional = false)
     {
         $this->name = $name;
         $this->optional = $optional;
@@ -29,20 +29,20 @@ public function __construct($name, $optional = false)
 
     /**
      * @param string $name
-     * @return static
+     * @return self
      */
     public static function optional($name)
     {
-        return new static($name, true);
+        return new self($name, true);
     }
 
     /**
      * @param string $name
-     * @return static
+     * @return self
      */
     public static function required($name)
     {
-        return new static($name);
+        return new self($name);
     }
 
     /**
diff --git a/src/CommandRouter.php b/src/CommandRouter.php
index e5cc1e7e..15d65357 100644
--- a/src/CommandRouter.php
+++ b/src/CommandRouter.php
@@ -15,9 +15,8 @@
  */
 class CommandRouter
 {
-
     /**
-     * @var CommandDefinition[]
+     * @var array<string, CommandDefinition>
      */
     private $commands;
 
@@ -43,14 +42,14 @@ class CommandRouter
      * Also accepts an instance of the container so it can look for services in there which may by defined
      * as the callable for one of the command definitions.
      *
-     * @param CommandDefinition[] $commands An array of command definitions
+     * @param array<CommandDefinition> $commands An array of command definitions
      * @param string $default The default command to use (if the workshop was invoked with no arguments)
      * @param EventDispatcher $eventDispatcher
      * @param ContainerInterface $container An instance of the container
      */
     public function __construct(
         array $commands,
-        $default,
+        string $default,
         EventDispatcher $eventDispatcher,
         ContainerInterface $container
     ) {
@@ -61,15 +60,15 @@ public function __construct(
         if (!isset($this->commands[$default])) {
             throw new \InvalidArgumentException(sprintf('Default command: "%s" is not available', $default));
         }
-        $this->defaultCommand   = $default;
-        $this->eventDispatcher  = $eventDispatcher;
-        $this->container        = $container;
+        $this->defaultCommand = $default;
+        $this->eventDispatcher = $eventDispatcher;
+        $this->container = $container;
     }
 
     /**
      * @param CommandDefinition $c
      */
-    private function addCommand(CommandDefinition $c)
+    private function addCommand(CommandDefinition $c): void
     {
         if (isset($this->commands[$c->getName()])) {
             throw new \InvalidArgumentException(sprintf('Command with name: "%s" already exists', $c->getName()));
@@ -91,11 +90,11 @@ private function addCommand(CommandDefinition $c)
      * Finally, the callable is invoked with the arguments passed from the cli. The return value of
      * callable is returned (if it is an integer, if not zero (success) is returned).
      *
-     * @param array $args
+     * @param array<string>|null $args
      * @return int
      * @throws CliRouteNotExistsException
      */
-    public function route(array $args = null)
+    public function route(array $args = null): int
     {
 
         if (null === $args) {
@@ -112,7 +111,7 @@ public function route(array $args = null)
         if (!isset($this->commands[$commandName])) {
             $command = $this->findNearestCommand($commandName, $this->commands);
 
-            if (false === $command) {
+            if (null === $command) {
                 throw new CliRouteNotExistsException($commandName);
             }
 
@@ -131,17 +130,17 @@ public function route(array $args = null)
      * @param string $commandName
      * @param CommandArgument[] $definitionArgs
      * @param string $appName
-     * @param array $givenArgs
+     * @param array<string> $givenArgs
      * @return Input
      */
-    private function parseArgs($commandName, array $definitionArgs, $appName, array $givenArgs)
+    private function parseArgs(string $commandName, array $definitionArgs, string $appName, array $givenArgs): Input
     {
         $parsedArgs = [];
 
         while (null !== ($definitionArg = array_shift($definitionArgs))) {
             $arg = array_shift($givenArgs);
 
-            if (null == $arg && !$definitionArg->isOptional()) {
+            if (null === $arg && !$definitionArg->isOptional()) {
                 throw new MissingArgumentException($commandName, array_map(function (CommandArgument $argument) {
                     return $argument->getName();
                 }, array_merge([$definitionArg], $definitionArgs)));
@@ -158,10 +157,10 @@ private function parseArgs($commandName, array $definitionArgs, $appName, array
      * characters different
      *
      * @param string $commandName
-     * @param array $commands
-     * @return string|false
+     * @param array<string, CommandDefinition> $commands
+     * @return string|null
      */
-    private function findNearestCommand($commandName, array $commands)
+    private function findNearestCommand(string $commandName, array $commands): ?string
     {
         $distances = [];
         foreach (array_keys($commands) as $command) {
@@ -173,10 +172,11 @@ private function findNearestCommand($commandName, array $commands)
         });
 
         if (empty($distances)) {
-            return false;
+            return null;
         }
 
-        return array_search(min($distances), $distances);
+        $name = array_search(min($distances), $distances, true);
+        return is_string($name) ? $name : null;
     }
 
     /**
@@ -184,12 +184,13 @@ private function findNearestCommand($commandName, array $commands)
      * @param Input $input
      * @return int
      */
-    private function resolveCallable(CommandDefinition $command, Input $input)
+    private function resolveCallable(CommandDefinition $command, Input $input): int
     {
         $commandCallable = $command->getCommandCallable();
 
         if (is_callable($commandCallable)) {
-            return $this->callCommand($command, $commandCallable, $input);
+            $return = $this->callCommand($command, $commandCallable, $input);
+            return is_int($return) ? $return : 0;
         }
 
         if (!is_string($commandCallable)) {
@@ -208,18 +209,14 @@ private function resolveCallable(CommandDefinition $command, Input $input)
 
         $return = $this->callCommand($command, $callable, $input);
 
-        if (is_int($return)) {
-            return $return;
-        }
-
-        return 0;
+        return is_int($return) ? $return : 0;
     }
 
     /**
      * @param CommandDefinition $command
      * @param callable $callable
      * @param Input $input
-     * @return int
+     * @return ?int
      */
     private function callCommand(CommandDefinition $command, callable $callable, Input $input)
     {
diff --git a/src/ComposerUtil/LockFileParser.php b/src/ComposerUtil/LockFileParser.php
index 6535b60f..6620c0ce 100644
--- a/src/ComposerUtil/LockFileParser.php
+++ b/src/ComposerUtil/LockFileParser.php
@@ -10,7 +10,7 @@
 class LockFileParser
 {
     /**
-     * @var string
+     * @var array<string, mixed>
      */
     private $contents;
 
@@ -18,13 +18,17 @@ class LockFileParser
      * @param string $lockFilePath The absolute path to the `composer.lock` file.
      * @throws InvalidArgumentException If the file does not exist.
      */
-    public function __construct($lockFilePath)
+    public function __construct(string $lockFilePath)
     {
         if (!file_exists($lockFilePath)) {
             throw new InvalidArgumentException(sprintf('Lock File: "%s" does not exist', $lockFilePath));
         }
         
-        $this->contents = json_decode(file_get_contents($lockFilePath), true);
+        $this->contents = json_decode((string) file_get_contents($lockFilePath), true);
+
+        if (!isset($this->contents['packages'])) {
+            throw new InvalidArgumentException(sprintf('Lock File: "%s" does not contain packages key', $lockFilePath));
+        }
     }
 
     /**
@@ -37,9 +41,9 @@ public function __construct($lockFilePath)
      * ];
      * ```
      *
-     * @return array
+     * @return array<array{name: string, version: string}>
      */
-    public function getInstalledPackages()
+    public function getInstalledPackages(): array
     {
         return array_map(function (array $packageDetails) {
             return [
@@ -55,7 +59,7 @@ public function getInstalledPackages()
      * @param string $packageName
      * @return bool
      */
-    public function hasInstalledPackage($packageName)
+    public function hasInstalledPackage($packageName): bool
     {
         foreach ($this->contents['packages'] as $packageDetails) {
             if ($packageName === $packageDetails['name']) {
diff --git a/src/Event/CgiExecuteEvent.php b/src/Event/CgiExecuteEvent.php
index d5559383..e174aa14 100644
--- a/src/Event/CgiExecuteEvent.php
+++ b/src/Event/CgiExecuteEvent.php
@@ -10,7 +10,6 @@
  */
 class CgiExecuteEvent extends Event
 {
-
     /**
      * @var RequestInterface
      */
@@ -19,9 +18,9 @@ class CgiExecuteEvent extends Event
     /**
      * @param string $name The event name.
      * @param RequestInterface $request The request that will be performed.
-     * @param array $parameters The event parameters.
+     * @param array<mixed> $parameters The event parameters.
      */
-    public function __construct($name, RequestInterface $request, array $parameters = [])
+    public function __construct(string $name, RequestInterface $request, array $parameters = [])
     {
         $parameters['request'] = $request;
         parent::__construct($name, $parameters);
@@ -32,9 +31,9 @@ public function __construct($name, RequestInterface $request, array $parameters
      * Add a header to the request.
      *
      * @param string $header
-     * @param string|string[] $value
+     * @param string|array<string> $value
      */
-    public function addHeaderToRequest($header, $value)
+    public function addHeaderToRequest(string $header, $value): void
     {
         $this->request = $this->request->withHeader($header, $value);
     }
@@ -44,7 +43,7 @@ public function addHeaderToRequest($header, $value)
      *
      * @param callable $callback
      */
-    public function modifyRequest(callable $callback)
+    public function modifyRequest(callable $callback): void
     {
         $this->request = $callback($this->request);
     }
@@ -54,7 +53,7 @@ public function modifyRequest(callable $callback)
      *
      * @return RequestInterface
      */
-    public function getRequest()
+    public function getRequest(): RequestInterface
     {
         return $this->request;
     }
diff --git a/src/Event/CliExecuteEvent.php b/src/Event/CliExecuteEvent.php
index 8db99dd9..43d520b7 100644
--- a/src/Event/CliExecuteEvent.php
+++ b/src/Event/CliExecuteEvent.php
@@ -12,16 +12,16 @@
 class CliExecuteEvent extends Event
 {
     /**
-     * @var ArrayObject
+     * @var ArrayObject<string>
      */
     private $args;
 
     /**
      * @param string $name The event name.
-     * @param ArrayObject $args The arguments that should be/have been passed to the program.
-     * @param array $parameters The event parameters.
+     * @param ArrayObject<string> $args The arguments that should be/have been passed to the program.
+     * @param array<mixed> $parameters The event parameters.
      */
-    public function __construct($name, ArrayObject $args, array $parameters = [])
+    public function __construct(string $name, ArrayObject $args, array $parameters = [])
     {
         $parameters['args'] = $args;
         parent::__construct($name, $parameters);
@@ -33,9 +33,8 @@ public function __construct($name, ArrayObject $args, array $parameters = [])
      *
      * @param string $arg
      */
-    public function prependArg($arg)
+    public function prependArg(string $arg): void
     {
-        Assertion::string($arg);
         $this->args = $this->args->prepend($arg);
     }
 
@@ -44,18 +43,17 @@ public function prependArg($arg)
      *
      * @param string $arg
      */
-    public function appendArg($arg)
+    public function appendArg(string $arg): void
     {
-        Assertion::string($arg);
         $this->args = $this->args->append($arg);
     }
 
     /**
      * Get the arguments to be passed to the program.
      *
-     * @return ArrayObject
+     * @return ArrayObject<string>
      */
-    public function getArgs()
+    public function getArgs(): ArrayObject
     {
         return $this->args;
     }
diff --git a/src/Event/ContainerListenerHelper.php b/src/Event/ContainerListenerHelper.php
index c96149ba..7ee2614d 100644
--- a/src/Event/ContainerListenerHelper.php
+++ b/src/Event/ContainerListenerHelper.php
@@ -18,7 +18,7 @@ class ContainerListenerHelper
     private $method;
 
     /**
-     * @param $service
+     * @param string $service
      * @param string $method
      */
     public function __construct($service, $method = '__invoke')
diff --git a/src/Event/Event.php b/src/Event/Event.php
index 4b47fd90..9abe8d26 100644
--- a/src/Event/Event.php
+++ b/src/Event/Event.php
@@ -15,15 +15,15 @@ class Event implements EventInterface
     private $name;
 
     /**
-     * @var array
+     * @var array<mixed>
      */
     protected $parameters;
 
     /**
      * @param string $name The event name.
-     * @param array $parameters The event parameters.
+     * @param array<mixed> $parameters The event parameters.
      */
-    public function __construct($name, array $parameters = [])
+    public function __construct(string $name, array $parameters = [])
     {
         $this->name = $name;
         $this->parameters = $parameters;
@@ -34,7 +34,7 @@ public function __construct($name, array $parameters = [])
      *
      * @return string
      */
-    public function getName()
+    public function getName(): string
     {
         return $this->name;
     }
@@ -42,9 +42,9 @@ public function getName()
     /**
      * Get an array of parameters that were triggered with this event.
      *
-     * @return mixed[]
+     * @return array<mixed>
      */
-    public function getParameters()
+    public function getParameters(): array
     {
         return $this->parameters;
     }
@@ -56,7 +56,7 @@ public function getParameters()
      * @return mixed The value.
      * @throws InvalidArgumentException If the parameter by name does not exist.
      */
-    public function getParameter($name)
+    public function getParameter(string $name)
     {
         if (!array_key_exists($name, $this->parameters)) {
             throw new InvalidArgumentException(sprintf('Parameter: "%s" does not exist', $name));
diff --git a/src/Event/EventDispatcher.php b/src/Event/EventDispatcher.php
index 88fd80f5..b8c7ee5e 100644
--- a/src/Event/EventDispatcher.php
+++ b/src/Event/EventDispatcher.php
@@ -11,7 +11,7 @@
 class EventDispatcher
 {
     /**
-     * @var array
+     * @var array<string, array<callable>>
      */
     private $listeners = [];
 
@@ -49,7 +49,7 @@ public function dispatch(EventInterface $event): EventInterface
      * Attach a callback to an event name. `$eventNames` can be an array of event names in order to attach the same
      * callback to multiple events or it can just be one event name as a string.
      *
-     * @param string|array $eventNames
+     * @param string|array<string> $eventNames
      * @param callable $callback
      */
     public function listen($eventNames, callable $callback): void
@@ -64,10 +64,10 @@ public function listen($eventNames, callable $callback): void
     }
 
     /**
-     * @param string|array $eventName
+     * @param string $eventName
      * @param callable $callback
      */
-    private function attachListener($eventName, callable $callback): void
+    private function attachListener(string $eventName, callable $callback): void
     {
         if (!array_key_exists($eventName, $this->listeners)) {
             $this->listeners[$eventName] = [$callback];
@@ -81,10 +81,10 @@ private function attachListener($eventName, callable $callback): void
      * A verifier should return an object which implements `PhpSchool\PhpWorkshop\Result\FailureInterface`
      * or `PhpSchool\PhpWorkshop\Result\SuccessInterface`. This result object will be added to the result aggregator.
      *
-     * @param string|array $eventName
+     * @param string $eventName
      * @param callable $verifier
      */
-    public function insertVerifier($eventName, callable $verifier): void
+    public function insertVerifier(string $eventName, callable $verifier): void
     {
         $this->attachListener($eventName, function (EventInterface $event) use ($verifier) {
             $result = $verifier($event);
@@ -99,7 +99,7 @@ public function insertVerifier($eventName, callable $verifier): void
     }
 
     /**
-     * @return array
+     * @return array<string, array<callable>>
      */
     public function getListeners(): array
     {
diff --git a/src/Event/EventInterface.php b/src/Event/EventInterface.php
index 2cb20651..2698989f 100644
--- a/src/Event/EventInterface.php
+++ b/src/Event/EventInterface.php
@@ -2,6 +2,8 @@
 
 namespace PhpSchool\PhpWorkshop\Event;
 
+use PhpSchool\PhpWorkshop\Exception\InvalidArgumentException;
+
 /**
  * An event representation.
  */
@@ -12,14 +14,14 @@ interface EventInterface
      *
      * @return string
      */
-    public function getName();
+    public function getName(): string;
 
     /**
      * Get an array of parameters that were triggered with this event.
      *
-     * @return mixed[]
+     * @return array<mixed>
      */
-    public function getParameters();
+    public function getParameters(): array;
 
     /**
      * Get a parameter by it's name.
@@ -28,5 +30,5 @@ public function getParameters();
      * @return mixed The value.
      * @throws InvalidArgumentException If the parameter by name does not exist.
      */
-    public function getParameter($name);
+    public function getParameter(string $name);
 }
diff --git a/src/Event/ExerciseRunnerEvent.php b/src/Event/ExerciseRunnerEvent.php
index 153b7411..06eedcd8 100644
--- a/src/Event/ExerciseRunnerEvent.php
+++ b/src/Event/ExerciseRunnerEvent.php
@@ -10,7 +10,6 @@
  */
 class ExerciseRunnerEvent extends Event
 {
-
     /**
      * @var ExerciseInterface
      */
@@ -25,9 +24,9 @@ class ExerciseRunnerEvent extends Event
      * @param string $name
      * @param ExerciseInterface $exercise
      * @param Input $input
-     * @param array $parameters
+     * @param array<mixed> $parameters
      */
-    public function __construct($name, ExerciseInterface $exercise, Input $input, array $parameters = [])
+    public function __construct(string $name, ExerciseInterface $exercise, Input $input, array $parameters = [])
     {
         $parameters['input'] = $input;
         $parameters['exercise'] = $exercise;
@@ -40,7 +39,7 @@ public function __construct($name, ExerciseInterface $exercise, Input $input, ar
     /**
      * @return Input
      */
-    public function getInput()
+    public function getInput(): Input
     {
         return $this->input;
     }
@@ -48,7 +47,7 @@ public function getInput()
     /**
      * @return ExerciseInterface
      */
-    public function getExercise()
+    public function getExercise(): ExerciseInterface
     {
         return $this->exercise;
     }
diff --git a/src/Exception/CheckNotApplicableException.php b/src/Exception/CheckNotApplicableException.php
index 5f90e0e0..7606cf9d 100644
--- a/src/Exception/CheckNotApplicableException.php
+++ b/src/Exception/CheckNotApplicableException.php
@@ -17,11 +17,11 @@ class CheckNotApplicableException extends RuntimeException
      *
      * @param CheckInterface $check The check Instance.
      * @param ExerciseInterface $exercise The exercise Instance.
-     * @return static
+     * @return self
      */
     public static function fromCheckAndExercise(CheckInterface $check, ExerciseInterface $exercise)
     {
-        return new static(
+        return new self(
             sprintf(
                 'Check: "%s" cannot process exercise: "%s" with type: "%s"',
                 $check->getName(),
diff --git a/src/Exception/CodeExecutionException.php b/src/Exception/CodeExecutionException.php
index 4a23ac5a..758d8000 100644
--- a/src/Exception/CodeExecutionException.php
+++ b/src/Exception/CodeExecutionException.php
@@ -14,11 +14,11 @@ class CodeExecutionException extends RuntimeException
      * Static constructor to create an instance from a failed `Symfony\Component\Process\Process` instance.
      *
      * @param Process $process The `Symfony\Component\Process\Process` instance which failed.
-     * @return static
+     * @return self
      */
     public static function fromProcess(Process $process)
     {
-        return new static(
+        return new self(
             sprintf(
                 'PHP Code failed to execute. Error: "%s"',
                 trim($process->getErrorOutput() ?: $process->getOutput())
diff --git a/src/Exception/ExerciseNotAssignedException.php b/src/Exception/ExerciseNotAssignedException.php
new file mode 100644
index 00000000..490276f9
--- /dev/null
+++ b/src/Exception/ExerciseNotAssignedException.php
@@ -0,0 +1,19 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PhpSchool\PhpWorkshop\Exception;
+
+/**
+ * When a student has no exercise assigned
+ */
+class ExerciseNotAssignedException extends \RuntimeException
+{
+    /**
+     * @return self
+     */
+    public static function notAssigned(): self
+    {
+        return new self('Student has no exercise assigned');
+    }
+}
diff --git a/src/Exception/ExerciseNotConfiguredException.php b/src/Exception/ExerciseNotConfiguredException.php
index 00cee384..533a7eb3 100644
--- a/src/Exception/ExerciseNotConfiguredException.php
+++ b/src/Exception/ExerciseNotConfiguredException.php
@@ -16,10 +16,10 @@ class ExerciseNotConfiguredException extends RuntimeException
      *
      * @param ExerciseInterface $exercise The exercise instance.
      * @param string $interface The FQCN of the interface.
-     * @return static
+     * @return self
      */
     public static function missingImplements(ExerciseInterface $exercise, $interface)
     {
-        return new static(sprintf('Exercise: "%s" should implement interface: "%s"', $exercise->getName(), $interface));
+        return new self(sprintf('Exercise: "%s" should implement interface: "%s"', $exercise->getName(), $interface));
     }
 }
diff --git a/src/Exception/InvalidArgumentException.php b/src/Exception/InvalidArgumentException.php
index 8fa6cf31..3db70f61 100644
--- a/src/Exception/InvalidArgumentException.php
+++ b/src/Exception/InvalidArgumentException.php
@@ -12,11 +12,11 @@ class InvalidArgumentException extends \InvalidArgumentException
      *
      * @param string $expected
      * @param mixed $actual
-     * @return static
+     * @return self
      */
-    public static function typeMisMatch($expected, $actual)
+    public static function typeMisMatch(string $expected, $actual): self
     {
-        return new static(
+        return new self(
             sprintf(
                 'Expected: "%s" Received: "%s"',
                 $expected,
@@ -31,11 +31,11 @@ public static function typeMisMatch($expected, $actual)
      * @param string $parameterName
      * @param mixed[] $allowedValues
      * @param mixed $actualValue
-     * @return static
+     * @return self
      */
-    public static function notValidParameter($parameterName, array $allowedValues, $actualValue)
+    public static function notValidParameter(string $parameterName, array $allowedValues, $actualValue): self
     {
-        return new static(
+        return new self(
             sprintf(
                 'Parameter: "%s" can only be one of: "%s" Received: "%s"',
                 $parameterName,
@@ -47,12 +47,12 @@ public static function notValidParameter($parameterName, array $allowedValues, $
 
     /**
      * @param object $object
-     * @param string $requiredInterface
-     * @return static
+     * @param class-string $requiredInterface
+     * @return self
      */
-    public static function missingImplements($object, $requiredInterface)
+    public static function missingImplements($object, string $requiredInterface): self
     {
-        return new static(
+        return new self(
             sprintf(
                 '"%s" is required to implement "%s", but it does not',
                 get_class($object),
@@ -62,17 +62,17 @@ public static function missingImplements($object, $requiredInterface)
     }
 
     /**
-     * @param $value
+     * @param mixed $value
      * @return string
      */
-    private static function stringify($value)
+    private static function stringify($value): string
     {
         if (is_object($value)) {
             return get_class($value);
         }
 
         if (is_array($value)) {
-            return implode('", "', array_map([static::class, 'stringify'], $value));
+            return implode('", "', array_map([self::class, 'stringify'], $value));
         }
 
         if (is_bool($value)) {
@@ -83,7 +83,6 @@ private static function stringify($value)
             return (string) $value;
         }
 
-
         return 'unknown';
     }
 }
diff --git a/src/Exception/MissingArgumentException.php b/src/Exception/MissingArgumentException.php
index aeebbc6e..3b4937c5 100644
--- a/src/Exception/MissingArgumentException.php
+++ b/src/Exception/MissingArgumentException.php
@@ -10,7 +10,7 @@
 class MissingArgumentException extends RuntimeException
 {
     /**
-     * @var array
+     * @var array<string>
      */
     private $missingArguments = [];
 
@@ -18,9 +18,9 @@ class MissingArgumentException extends RuntimeException
      * Create the exception, requires the command name and missing arguments.
      *
      * @param string $commandName The command name.
-     * @param array $missingArguments An array of missing arguments (strings).
+     * @param array<string> $missingArguments An array of missing arguments.
      */
-    public function __construct($commandName, array $missingArguments)
+    public function __construct(string $commandName, array $missingArguments)
     {
         $this->missingArguments = $missingArguments;
         parent::__construct(
@@ -35,9 +35,9 @@ public function __construct($commandName, array $missingArguments)
     /**
      * Retrieve the list of missing arguments.
      *
-     * @return array
+     * @return array<string>
      */
-    public function getMissingArguments()
+    public function getMissingArguments(): array
     {
         return $this->missingArguments;
     }
diff --git a/src/Exercise/AbstractExercise.php b/src/Exercise/AbstractExercise.php
index f2ad9360..5488aa87 100644
--- a/src/Exercise/AbstractExercise.php
+++ b/src/Exercise/AbstractExercise.php
@@ -19,7 +19,7 @@ abstract class AbstractExercise
      *
      * @return string
      */
-    abstract public function getName();
+    abstract public function getName(): string;
 
     /**
      * This returns a single file solution named `solution.php` which
@@ -31,13 +31,13 @@ abstract public function getName();
      *
      * @return SolutionInterface
      */
-    public function getSolution()
+    public function getSolution(): SolutionInterface
     {
         return SingleFileSolution::fromFile(
-            realpath(
+            (string) realpath(
                 sprintf(
                     '%s/../../exercises/%s/solution/solution.php',
-                    dirname((new ReflectionClass(static::class))->getFileName()),
+                    dirname((string) (new ReflectionClass(static::class))->getFileName()),
                     self::normaliseName($this->getName())
                 )
             )
@@ -50,10 +50,10 @@ public function getSolution()
      *
      * @return string
      */
-    public function getProblem()
+    public function getProblem(): string
     {
         $name = self::normaliseName($this->getName());
-        $dir  = dirname((new ReflectionClass(static::class))->getFileName());
+        $dir  = dirname((string) (new ReflectionClass(static::class))->getFileName());
         return sprintf('%s/../../exercises/%s/problem/problem.md', $dir, $name);
     }
 
@@ -63,7 +63,7 @@ public function getProblem()
      *
      * @return void
      */
-    public function tearDown()
+    public function tearDown(): void
     {
     }
 
@@ -71,9 +71,9 @@ public function tearDown()
      * @param string $name
      * @return string
      */
-    public static function normaliseName($name)
+    public static function normaliseName(string $name): string
     {
-        return preg_replace('/[^A-Za-z\-]+/', '', str_replace(' ', '-', strtolower($name)));
+        return (string) preg_replace('/[^A-Za-z\-]+/', '', str_replace(' ', '-', strtolower($name)));
     }
 
     /**
@@ -82,7 +82,7 @@ public static function normaliseName($name)
      *
      * @param ExerciseDispatcher $dispatcher
      */
-    public function configure(ExerciseDispatcher $dispatcher)
+    public function configure(ExerciseDispatcher $dispatcher): void
     {
     }
 }
diff --git a/src/Exercise/ExerciseInterface.php b/src/Exercise/ExerciseInterface.php
index 9f049e00..ff16ba64 100644
--- a/src/Exercise/ExerciseInterface.php
+++ b/src/Exercise/ExerciseInterface.php
@@ -17,21 +17,21 @@ interface ExerciseInterface
      *
      * @return string
      */
-    public function getName();
+    public function getName(): string;
 
     /**
      * Return the type of exercise. This is an ENUM. See `PhpSchool\PhpWorkshop\Exercise\ExerciseType`.
      *
      * @return ExerciseType
      */
-    public function getType();
+    public function getType(): ExerciseType;
 
     /**
      * Get the absolute path to the markdown file which contains the exercise problem.
      *
      * @return string
      */
-    public function getProblem();
+    public function getProblem(): string;
 
     /**
      * This is where the exercise specifies the extra checks it may require. It is also
@@ -41,14 +41,14 @@ public function getProblem();
      *
      * @param ExerciseDispatcher $dispatcher
      */
-    public function configure(ExerciseDispatcher $dispatcher);
+    public function configure(ExerciseDispatcher $dispatcher): void;
 
     /**
      * A short description of the exercise.
      *
      * @return string
      */
-    public function getDescription();
+    public function getDescription(): string;
 
     /**
      * Allows to perform some cleanup after the exercise solution's have been executed, for example
@@ -56,5 +56,5 @@ public function getDescription();
      *
      * @return void
      */
-    public function tearDown();
+    public function tearDown(): void;
 }
diff --git a/src/Exercise/ExerciseType.php b/src/Exercise/ExerciseType.php
index 88640292..4259947b 100644
--- a/src/Exercise/ExerciseType.php
+++ b/src/Exercise/ExerciseType.php
@@ -12,6 +12,7 @@
  * $typeCgi = ExerciseType::CGI();
  * $typeCustom = ExerciseType::CUSTOM();
  * ```
+ * @extends Enum<string>
  */
 class ExerciseType extends Enum
 {
@@ -19,16 +20,15 @@ class ExerciseType extends Enum
     public const CGI    = 'CGI';
     public const CUSTOM = 'CUSTOM';
 
-
     /**
      * Map of exercise types to the required interfaces exercises of that particular
      * type should implement.
      *
-     * @var array
+     * @var array<string, class-string>
      */
     private static $exerciseTypeToExerciseInterfaceMap = [
-        self::CLI    => CliExercise::class,
-        self::CGI    => CgiExercise::class,
+        self::CLI => CliExercise::class,
+        self::CGI => CgiExercise::class,
         self::CUSTOM => CustomVerifyingExercise::class,
     ];
 
@@ -36,9 +36,9 @@ class ExerciseType extends Enum
      * Get the FQCN of the interface this exercise should implement for this
      * exercise type.
      *
-     * @return string
+     * @return class-string
      */
-    public function getExerciseInterface()
+    public function getExerciseInterface(): string
     {
         return static::$exerciseTypeToExerciseInterfaceMap[$this->getKey()];
     }
diff --git a/src/Exercise/TemporaryDirectoryTrait.php b/src/Exercise/TemporaryDirectoryTrait.php
index 61ee5790..d8e11e8c 100644
--- a/src/Exercise/TemporaryDirectoryTrait.php
+++ b/src/Exercise/TemporaryDirectoryTrait.php
@@ -18,7 +18,7 @@ public function getTemporaryPath(): string
     {
         return sprintf(
             '%s/%s',
-            str_replace('\\', '/', realpath(sys_get_temp_dir())),
+            str_replace('\\', '/', (string) realpath(sys_get_temp_dir())),
             str_replace('\\', '_', __CLASS__)
         );
     }
diff --git a/src/ExerciseCheck/ComposerExerciseCheck.php b/src/ExerciseCheck/ComposerExerciseCheck.php
index cba2efaf..568863fe 100644
--- a/src/ExerciseCheck/ComposerExerciseCheck.php
+++ b/src/ExerciseCheck/ComposerExerciseCheck.php
@@ -12,7 +12,7 @@ interface ComposerExerciseCheck
      * Returns an array of composer package names that student's solution should
      * have required via composer.
      *
-     * @return array An array of composer package names.
+     * @return array<string> An array of composer package names.
      */
-    public function getRequiredPackages();
+    public function getRequiredPackages(): array;
 }
diff --git a/src/ExerciseDispatcher.php b/src/ExerciseDispatcher.php
index 3606ba12..77292b37 100644
--- a/src/ExerciseDispatcher.php
+++ b/src/ExerciseDispatcher.php
@@ -65,10 +65,10 @@ public function __construct(
         EventDispatcher $eventDispatcher,
         CheckRepository $checkRepository
     ) {
-        $this->runnerManager    = $runnerManager;
-        $this->results          = $resultAggregator;
-        $this->eventDispatcher  = $eventDispatcher;
-        $this->checkRepository  = $checkRepository;
+        $this->runnerManager = $runnerManager;
+        $this->results = $resultAggregator;
+        $this->eventDispatcher = $eventDispatcher;
+        $this->checkRepository = $checkRepository;
     }
 
     /**
@@ -76,10 +76,10 @@ public function __construct(
      * the check specified as the first argument will also be executed. Throws an `InvalidArgumentException`
      * if the check does not exist in the `CheckRepository`.
      *
-     * @param string $requiredCheck The name of the required check.
+     * @param class-string $requiredCheck The name of the required check.
      * @throws InvalidArgumentException If the check does not exist.
      */
-    public function requireCheck($requiredCheck)
+    public function requireCheck(string $requiredCheck): void
     {
         if (!$this->checkRepository->has($requiredCheck)) {
             throw new InvalidArgumentException(sprintf('Check: "%s" does not exist', $requiredCheck));
@@ -124,7 +124,7 @@ public function requireCheck($requiredCheck)
      * @throws ExerciseNotConfiguredException If the exercise does not implement the correct interface based on
      * the checks required.
      */
-    public function verify(ExerciseInterface $exercise, Input $input)
+    public function verify(ExerciseInterface $exercise, Input $input): ResultAggregator
     {
         $exercise->configure($this);
 
@@ -176,7 +176,7 @@ public function verify(ExerciseInterface $exercise, Input $input)
      * @param OutputInterface $output An output instance capable of writing to stdout.
      * @return bool Whether the solution ran successfully or not.
      */
-    public function run(ExerciseInterface $exercise, Input $input, OutputInterface $output)
+    public function run(ExerciseInterface $exercise, Input $input, OutputInterface $output): bool
     {
         $exercise->configure($this);
         $this->eventDispatcher->dispatch(new ExerciseRunnerEvent('run.start', $exercise, $input));
@@ -193,12 +193,12 @@ public function run(ExerciseInterface $exercise, Input $input, OutputInterface $
     }
 
     /**
-     * @param CheckInterface[] $checks
+     * @param SimpleCheckInterface[] $checks
      * @param ExerciseInterface $exercise
      * @throws CheckNotApplicableException
      * @throws ExerciseNotConfiguredException
      */
-    private function validateChecks(array $checks, ExerciseInterface $exercise)
+    private function validateChecks(array $checks, ExerciseInterface $exercise): void
     {
         foreach ($checks as $check) {
             if (!$check->canRun($exercise->getType())) {
@@ -217,7 +217,7 @@ private function validateChecks(array $checks, ExerciseInterface $exercise)
      *
      * @return EventDispatcher
      */
-    public function getEventDispatcher()
+    public function getEventDispatcher(): EventDispatcher
     {
         return $this->eventDispatcher;
     }
diff --git a/src/ExerciseRenderer.php b/src/ExerciseRenderer.php
index 622e6555..ba1c75f5 100644
--- a/src/ExerciseRenderer.php
+++ b/src/ExerciseRenderer.php
@@ -77,7 +77,7 @@ public function __construct(
     /**
      * @param CliMenu $menu
      */
-    public function __invoke(CliMenu $menu)
+    public function __invoke(CliMenu $menu): void
     {
         $menu->close();
 
@@ -89,7 +89,7 @@ public function __invoke(CliMenu $menu)
         $this->userStateSerializer->serialize($this->userState);
 
         $numExercises   = count($exercises);
-        $exerciseIndex  = array_search($exercise, $exercises) + 1;
+        $exerciseIndex  = ((int) array_search($exercise, $exercises)) + 1;
 
         $output  = "\n";
         $output .= $this->color->__invoke(' LEARN YOU THE PHP FOR MUCH WIN! ')->magenta()->bold() . "\n";
@@ -98,7 +98,7 @@ public function __invoke(CliMenu $menu)
         $output .= $this->color->__invoke(" " . $exercise->getName())->yellow()->bold() . "\n";
         $output .= $this->color->__invoke(sprintf(" Exercise %d of %d\n\n", $exerciseIndex, $numExercises))->yellow();
 
-        $content = file_get_contents($exercise->getProblem());
+        $content = (string) file_get_contents($exercise->getProblem());
         $doc     = $this->markdownRenderer->render($content);
         $doc     = str_replace('{appname}', $this->appName, $doc);
         $output .= $doc;
@@ -118,7 +118,7 @@ public function __invoke(CliMenu $menu)
      * @param string $cmd
      * @return string
      */
-    private function helpLine($text, $cmd)
+    private function helpLine(string $text, string $cmd): string
     {
         $cmd = $this->color->__invoke(sprintf('%s %s', $this->appName, $cmd))->yellow()->__toString();
         return sprintf(
diff --git a/src/ExerciseRepository.php b/src/ExerciseRepository.php
index b9210f84..4438d82e 100644
--- a/src/ExerciseRepository.php
+++ b/src/ExerciseRepository.php
@@ -10,6 +10,8 @@
 
 /**
  * Exercise repository, use to locate individual/all exercises by certain criteria.
+ *
+ * @implements IteratorAggregate<int, ExerciseInterface>
  */
 class ExerciseRepository implements IteratorAggregate, Countable
 {
@@ -21,7 +23,7 @@ class ExerciseRepository implements IteratorAggregate, Countable
     /**
      * Requires an array of `ExerciseInterface` instances.
      *
-     * @param ExerciseInterface[] $exercises
+     * @param array<ExerciseInterface> $exercises
      */
     public function __construct(array $exercises)
     {
@@ -34,7 +36,7 @@ public function __construct(array $exercises)
      * @param ExerciseInterface $exercise
      * @return ExerciseInterface
      */
-    private function validateExercise(ExerciseInterface $exercise)
+    private function validateExercise(ExerciseInterface $exercise): ExerciseInterface
     {
         $type = $exercise->getType();
 
@@ -50,9 +52,9 @@ private function validateExercise(ExerciseInterface $exercise)
     /**
      * Retrieve all of the exercises as an array.
      *
-     * @return ExerciseInterface[]
+     * @return array<ExerciseInterface>
      */
-    public function findAll()
+    public function findAll(): array
     {
         return $this->exercises;
     }
@@ -65,7 +67,7 @@ public function findAll()
      * @return ExerciseInterface
      * @throws InvalidArgumentException
      */
-    public function findByName($name)
+    public function findByName(string $name): ExerciseInterface
     {
         foreach ($this->exercises as $exercise) {
             if ($name === $exercise->getName()) {
@@ -79,9 +81,9 @@ public function findByName($name)
     /**
      * Get the names of each exercise as an array.
      *
-     * @return array
+     * @return array<string>
      */
-    public function getAllNames()
+    public function getAllNames(): array
     {
         return array_map(function (ExerciseInterface $exercise) {
             return $exercise->getName();
@@ -93,7 +95,7 @@ public function getAllNames()
      *
      * @return int
      */
-    public function count()
+    public function count(): int
     {
         return count($this->exercises);
     }
@@ -101,9 +103,9 @@ public function count()
     /**
      * Allow to iterate over the repository with `foreach`.
      *
-     * @return ArrayIterator
+     * @return ArrayIterator<int, ExerciseInterface>
      */
-    public function getIterator()
+    public function getIterator(): ArrayIterator
     {
         return new ArrayIterator($this->exercises);
     }
diff --git a/src/ExerciseRunner/CgiRunner.php b/src/ExerciseRunner/CgiRunner.php
index 898dabe3..0d637f6b 100644
--- a/src/ExerciseRunner/CgiRunner.php
+++ b/src/ExerciseRunner/CgiRunner.php
@@ -12,6 +12,7 @@
 use PhpSchool\PhpWorkshop\Exception\CodeExecutionException;
 use PhpSchool\PhpWorkshop\Exception\SolutionExecutionException;
 use PhpSchool\PhpWorkshop\Exercise\CgiExercise;
+use PhpSchool\PhpWorkshop\Exercise\ExerciseInterface;
 use PhpSchool\PhpWorkshop\Input\Input;
 use PhpSchool\PhpWorkshop\Output\OutputInterface;
 use PhpSchool\PhpWorkshop\Result\Cgi\CgiResult;
@@ -32,9 +33,8 @@
  */
 class CgiRunner implements ExerciseRunnerInterface
 {
-
     /**
-     * @var CgiExercise
+     * @var CgiExercise&ExerciseInterface
      */
     private $exercise;
 
@@ -49,7 +49,7 @@ class CgiRunner implements ExerciseRunnerInterface
     private $requestRenderer;
 
     /**
-     * @var array
+     * @var array<class-string>
      */
     private static $requiredChecks = [
         FileExistsCheck::class,
@@ -93,6 +93,7 @@ public function __construct(
                 );
             }
         }
+        /** @var CgiExercise&ExerciseInterface $exercise */
         $this->eventDispatcher = $eventDispatcher;
         $this->exercise = $exercise;
         $this->requestRenderer = $requestRenderer;
@@ -101,7 +102,7 @@ public function __construct(
     /**
      * @return string
      */
-    public function getName()
+    public function getName(): string
     {
         return 'CGI Program Runner';
     }
@@ -109,9 +110,9 @@ public function getName()
     /**
      * Get an array of the class names of the required checks this runner needs.
      *
-     * @return array
+     * @return array<class-string>
      */
-    public function getRequiredChecks()
+    public function getRequiredChecks(): array
     {
         return static::$requiredChecks;
     }
@@ -121,9 +122,10 @@ public function getRequiredChecks()
      * @param string $fileName
      * @return ResultInterface
      */
-    private function checkRequest(RequestInterface $request, $fileName)
+    private function checkRequest(RequestInterface $request, string $fileName): ResultInterface
     {
         try {
+            /** @var CgiExecuteEvent $event */
             $event = $this->eventDispatcher->dispatch(
                 new CgiExecuteEvent('cgi.verify.reference-execute.pre', $request)
             );
@@ -138,6 +140,7 @@ private function checkRequest(RequestInterface $request, $fileName)
         }
 
         try {
+            /** @var CgiExecuteEvent $event */
             $event = $this->eventDispatcher->dispatch(new CgiExecuteEvent('cgi.verify.student-execute.pre', $request));
             $userResponse = $this->executePhpFile($fileName, $event->getRequest(), 'student');
         } catch (CodeExecutionException $e) {
@@ -159,9 +162,9 @@ private function checkRequest(RequestInterface $request, $fileName)
 
     /**
      * @param ResponseInterface $response
-     * @return array
+     * @return array<string,string>
      */
-    private function getHeaders(ResponseInterface $response)
+    private function getHeaders(ResponseInterface $response): array
     {
         $headers = [];
         foreach ($response->getHeaders() as $name => $values) {
@@ -176,7 +179,7 @@ private function getHeaders(ResponseInterface $response)
      * @param string $type
      * @return ResponseInterface
      */
-    private function executePhpFile($fileName, RequestInterface $request, $type)
+    private function executePhpFile(string $fileName, RequestInterface $request, string $type): ResponseInterface
     {
         $process = $this->getProcess($fileName, $request);
 
@@ -202,7 +205,7 @@ private function executePhpFile($fileName, RequestInterface $request, $type)
      * @param RequestInterface $request
      * @return Process
      */
-    private function getProcess($fileName, RequestInterface $request)
+    private function getProcess(string $fileName, RequestInterface $request): Process
     {
         $env = [
             'REQUEST_METHOD'  => $request->getMethod(),
@@ -247,13 +250,13 @@ private function getProcess($fileName, RequestInterface $request)
      * @param Input $input The command line arguments passed to the command.
      * @return CgiResult The result of the check.
      */
-    public function verify(Input $input)
+    public function verify(Input $input): ResultInterface
     {
         $this->eventDispatcher->dispatch(new ExerciseRunnerEvent('cgi.verify.start', $this->exercise, $input));
         $result = new CgiResult(
             array_map(
                 function (RequestInterface $request) use ($input) {
-                    return $this->checkRequest($request, $input->getArgument('program'));
+                    return $this->checkRequest($request, $input->getRequiredArgument('program'));
                 },
                 $this->exercise->getRequests()
             )
@@ -279,15 +282,16 @@ function (RequestInterface $request) use ($input) {
      * @param OutputInterface $output A wrapper around STDOUT.
      * @return bool If the solution was successfully executed, eg. exit code was 0.
      */
-    public function run(Input $input, OutputInterface $output)
+    public function run(Input $input, OutputInterface $output): bool
     {
         $this->eventDispatcher->dispatch(new ExerciseRunnerEvent('cgi.run.start', $this->exercise, $input));
         $success = true;
         foreach ($this->exercise->getRequests() as $i => $request) {
+            /** @var CgiExecuteEvent $event */
             $event = $this->eventDispatcher->dispatch(
                 new CgiExecuteEvent('cgi.run.student-execute.pre', $request)
             );
-            $process = $this->getProcess($input->getArgument('program'), $event->getRequest());
+            $process = $this->getProcess($input->getRequiredArgument('program'), $event->getRequest());
 
             $output->writeTitle("Request");
             $output->emptyLine();
diff --git a/src/ExerciseRunner/CliRunner.php b/src/ExerciseRunner/CliRunner.php
index 2da9654a..bf5728e9 100644
--- a/src/ExerciseRunner/CliRunner.php
+++ b/src/ExerciseRunner/CliRunner.php
@@ -12,6 +12,7 @@
 use PhpSchool\PhpWorkshop\Exception\CodeExecutionException;
 use PhpSchool\PhpWorkshop\Exception\SolutionExecutionException;
 use PhpSchool\PhpWorkshop\Exercise\CliExercise;
+use PhpSchool\PhpWorkshop\Exercise\ExerciseInterface;
 use PhpSchool\PhpWorkshop\Input\Input;
 use PhpSchool\PhpWorkshop\Output\OutputInterface;
 use PhpSchool\PhpWorkshop\Result\Cli\RequestFailure;
@@ -33,7 +34,7 @@
 class CliRunner implements ExerciseRunnerInterface
 {
     /**
-     * @var CliExercise
+     * @var CliExercise&ExerciseInterface
      */
     private $exercise;
 
@@ -43,7 +44,7 @@ class CliRunner implements ExerciseRunnerInterface
     private $eventDispatcher;
 
     /**
-     * @var array
+     * @var array<class-string>
      */
     private static $requiredChecks = [
         FileExistsCheck::class,
@@ -59,6 +60,7 @@ class CliRunner implements ExerciseRunnerInterface
      */
     public function __construct(CliExercise $exercise, EventDispatcher $eventDispatcher)
     {
+        /** @var CliExercise&ExerciseInterface $exercise */
         $this->eventDispatcher = $eventDispatcher;
         $this->exercise = $exercise;
     }
@@ -66,7 +68,7 @@ public function __construct(CliExercise $exercise, EventDispatcher $eventDispatc
     /**
      * @return string
      */
-    public function getName()
+    public function getName(): string
     {
         return 'CLI Program Runner';
     }
@@ -74,20 +76,20 @@ public function getName()
     /**
      * Get an array of the class names of the required checks this runner needs.
      *
-     * @return array
+     * @return array<class-string>
      */
-    public function getRequiredChecks()
+    public function getRequiredChecks(): array
     {
         return static::$requiredChecks;
     }
 
     /**
      * @param string $fileName
-     * @param ArrayObject $args
+     * @param ArrayObject<string> $args
      * @param string $type
      * @return string
      */
-    private function executePhpFile($fileName, ArrayObject $args, $type)
+    private function executePhpFile(string $fileName, ArrayObject $args, string $type): string
     {
         $process = $this->getPhpProcess($fileName, $args);
 
@@ -104,11 +106,11 @@ private function executePhpFile($fileName, ArrayObject $args, $type)
 
     /**
      * @param string $fileName
-     * @param ArrayObject $args
+     * @param ArrayObject<string> $args
      *
      * @return Process
      */
-    private function getPhpProcess($fileName, ArrayObject $args)
+    private function getPhpProcess(string $fileName, ArrayObject $args): Process
     {
         return new Process(
             $args->prepend($fileName)->prepend(PHP_BINARY)->getArrayCopy(),
@@ -135,7 +137,7 @@ private function getPhpProcess($fileName, ArrayObject $args)
      * @param Input $input The command line arguments passed to the command.
      * @return CliResult The result of the check.
      */
-    public function verify(Input $input)
+    public function verify(Input $input): ResultInterface
     {
         $this->eventDispatcher->dispatch(new ExerciseRunnerEvent('cli.verify.start', $this->exercise, $input));
         $result = new CliResult(
@@ -153,14 +155,14 @@ function (array $args) use ($input) {
     /**
      * BC - getArgs only returned 1 set of args in v1 instead of multiple sets of args in v2
      *
-     * @param array $args
-     * @return array
+     * @param array<int, array<string>>|array<int, string> $args
+     * @return array<int, array<string>>
      */
-    private function preserveOldArgFormat(array $args)
+    private function preserveOldArgFormat(array $args): array
     {
         if (isset($args[0]) && !is_array($args[0])) {
             $args = [$args];
-        } elseif (empty($args)) {
+        } elseif (count($args) === 0) {
             $args = [[]];
         }
 
@@ -168,16 +170,17 @@ private function preserveOldArgFormat(array $args)
     }
 
     /**
-     * @param array $args
+     * @param array<string> $args
      * @param Input $input
      * @return ResultInterface
      */
-    private function doVerify(array $args, Input $input)
+    private function doVerify(array $args, Input $input): ResultInterface
     {
         //arrays are not pass-by-ref
         $args = new ArrayObject($args);
 
         try {
+            /** @var CliExecuteEvent $event */
             $event = $this->eventDispatcher->dispatch(new CliExecuteEvent('cli.verify.reference-execute.pre', $args));
             $solutionOutput = $this->executePhpFile(
                 $this->exercise->getSolution()->getEntryPoint(),
@@ -190,8 +193,9 @@ private function doVerify(array $args, Input $input)
         }
 
         try {
+            /** @var CliExecuteEvent $event */
             $event = $this->eventDispatcher->dispatch(new CliExecuteEvent('cli.verify.student-execute.pre', $args));
-            $userOutput = $this->executePhpFile($input->getArgument('program'), $event->getArgs(), 'student');
+            $userOutput = $this->executePhpFile($input->getRequiredArgument('program'), $event->getArgs(), 'student');
         } catch (CodeExecutionException $e) {
             $this->eventDispatcher->dispatch(new Event('cli.verify.student-execute.fail', ['exception' => $e]));
             return GenericFailure::fromArgsAndCodeExecutionFailure($args, $e);
@@ -219,7 +223,7 @@ private function doVerify(array $args, Input $input)
      * @param OutputInterface $output A wrapper around STDOUT.
      * @return bool If the solution was successfully executed, eg. exit code was 0.
      */
-    public function run(Input $input, OutputInterface $output)
+    public function run(Input $input, OutputInterface $output): bool
     {
         $this->eventDispatcher->dispatch(new ExerciseRunnerEvent('cli.run.start', $this->exercise, $input));
         $success = true;
@@ -240,7 +244,7 @@ public function run(Input $input, OutputInterface $output)
             }
 
             $output->writeTitle("Output");
-            $process = $this->getPhpProcess($input->getArgument('program'), $args);
+            $process = $this->getPhpProcess($input->getRequiredArgument('program'), $args);
             $process->start();
             $this->eventDispatcher->dispatch(
                 new CliExecuteEvent('cli.run.student.executing', $args, ['output' => $output])
diff --git a/src/ExerciseRunner/CustomVerifyingRunner.php b/src/ExerciseRunner/CustomVerifyingRunner.php
index 9680ba72..65297896 100644
--- a/src/ExerciseRunner/CustomVerifyingRunner.php
+++ b/src/ExerciseRunner/CustomVerifyingRunner.php
@@ -30,7 +30,7 @@ public function __construct(CustomVerifyingExercise $exercise)
      *
      * @return string
      */
-    public function getName()
+    public function getName(): string
     {
         return 'Custom Verifying Runner';
     }
@@ -38,9 +38,9 @@ public function getName()
     /**
      * Get an array of the class names of the required checks this runner needs.
      *
-     * @return array
+     * @return array<class-string>
      */
-    public function getRequiredChecks()
+    public function getRequiredChecks(): array
     {
         return [];
     }
@@ -52,7 +52,7 @@ public function getRequiredChecks()
      * @param Input $input The command line arguments passed to the command.
      * @return ResultInterface The result of the check.
      */
-    public function verify(Input $input)
+    public function verify(Input $input): ResultInterface
     {
         return $this->exercise->verify();
     }
@@ -65,7 +65,7 @@ public function verify(Input $input)
      * @param OutputInterface $output A wrapper around STDOUT.
      * @return bool If the solution was successfully executed, eg. exit code was 0.
      */
-    public function run(Input $input, OutputInterface $output)
+    public function run(Input $input, OutputInterface $output): bool
     {
         $message  = 'Nothing to run here. This exercise does not require a code solution, ';
         $message .= 'so there is nothing to execute.';
diff --git a/src/ExerciseRunner/ExerciseRunnerInterface.php b/src/ExerciseRunner/ExerciseRunnerInterface.php
index 27b599ff..8a94af4f 100644
--- a/src/ExerciseRunner/ExerciseRunnerInterface.php
+++ b/src/ExerciseRunner/ExerciseRunnerInterface.php
@@ -19,14 +19,14 @@ interface ExerciseRunnerInterface
      *
      * @return string
      */
-    public function getName();
+    public function getName(): string;
 
     /**
      * Get an array of the class names of the required checks this runner needs.
      *
-     * @return array
+     * @return array<class-string>
      */
-    public function getRequiredChecks();
+    public function getRequiredChecks(): array;
 
     /**
      * Verify a solution to an exercise. Verification involves executing the reference solution
@@ -41,7 +41,7 @@ public function getRequiredChecks();
      * @param Input $input The command line arguments passed to the command.
      * @return ResultInterface The result of the check.
      */
-    public function verify(Input $input);
+    public function verify(Input $input): ResultInterface;
 
     /**
      * Run a solution to an exercise. This simply run's the student's solution with the correct input from the exercise
@@ -52,5 +52,5 @@ public function verify(Input $input);
      * @param OutputInterface $output A wrapper around STDOUT.
      * @return bool If the solution was successfully executed, eg. exit code was 0.
      */
-    public function run(Input $input, OutputInterface $output);
+    public function run(Input $input, OutputInterface $output): bool;
 }
diff --git a/src/ExerciseRunner/Factory/CgiRunnerFactory.php b/src/ExerciseRunner/Factory/CgiRunnerFactory.php
index eeb262a2..56a8a50d 100644
--- a/src/ExerciseRunner/Factory/CgiRunnerFactory.php
+++ b/src/ExerciseRunner/Factory/CgiRunnerFactory.php
@@ -5,6 +5,7 @@
 use PhpSchool\PhpWorkshop\CommandArgument;
 use PhpSchool\PhpWorkshop\CommandDefinition;
 use PhpSchool\PhpWorkshop\Event\EventDispatcher;
+use PhpSchool\PhpWorkshop\Exercise\CgiExercise;
 use PhpSchool\PhpWorkshop\Exercise\ExerciseInterface;
 use PhpSchool\PhpWorkshop\Exercise\ExerciseType;
 use PhpSchool\PhpWorkshop\ExerciseRunner\CgiRunner;
@@ -46,7 +47,7 @@ public function __construct(EventDispatcher $eventDispatcher, RequestRenderer $r
      * @param ExerciseInterface $exercise
      * @return bool
      */
-    public function supports(ExerciseInterface $exercise)
+    public function supports(ExerciseInterface $exercise): bool
     {
         return $exercise->getType()->getValue() === self::$type;
     }
@@ -56,7 +57,7 @@ public function supports(ExerciseInterface $exercise)
      *
      * @param CommandDefinition $commandDefinition
      */
-    public function configureInput(CommandDefinition $commandDefinition)
+    public function configureInput(CommandDefinition $commandDefinition): void
     {
         $commandDefinition->addArgument(CommandArgument::required('program'));
     }
@@ -64,10 +65,10 @@ public function configureInput(CommandDefinition $commandDefinition)
     /**
      * Create and return an instance of the runner.
      *
-     * @param ExerciseInterface $exercise
+     * @param ExerciseInterface&CgiExercise $exercise
      * @return ExerciseRunnerInterface
      */
-    public function create(ExerciseInterface $exercise)
+    public function create(ExerciseInterface $exercise): ExerciseRunnerInterface
     {
         return new CgiRunner($exercise, $this->eventDispatcher, $this->requestRenderer);
     }
diff --git a/src/ExerciseRunner/Factory/CliRunnerFactory.php b/src/ExerciseRunner/Factory/CliRunnerFactory.php
index 2dc19636..0461afdf 100644
--- a/src/ExerciseRunner/Factory/CliRunnerFactory.php
+++ b/src/ExerciseRunner/Factory/CliRunnerFactory.php
@@ -5,6 +5,7 @@
 use PhpSchool\PhpWorkshop\CommandArgument;
 use PhpSchool\PhpWorkshop\CommandDefinition;
 use PhpSchool\PhpWorkshop\Event\EventDispatcher;
+use PhpSchool\PhpWorkshop\Exercise\CliExercise;
 use PhpSchool\PhpWorkshop\Exercise\ExerciseInterface;
 use PhpSchool\PhpWorkshop\Exercise\ExerciseType;
 use PhpSchool\PhpWorkshop\ExerciseRunner\CliRunner;
@@ -39,7 +40,7 @@ public function __construct(EventDispatcher $eventDispatcher)
      * @param ExerciseInterface $exercise
      * @return bool
      */
-    public function supports(ExerciseInterface $exercise)
+    public function supports(ExerciseInterface $exercise): bool
     {
         return $exercise->getType()->getValue() === self::$type;
     }
@@ -49,7 +50,7 @@ public function supports(ExerciseInterface $exercise)
      *
      * @param CommandDefinition $commandDefinition
      */
-    public function configureInput(CommandDefinition $commandDefinition)
+    public function configureInput(CommandDefinition $commandDefinition): void
     {
         $commandDefinition->addArgument(CommandArgument::required('program'));
     }
@@ -57,10 +58,10 @@ public function configureInput(CommandDefinition $commandDefinition)
     /**
      * Create and return an instance of the runner.
      *
-     * @param ExerciseInterface $exercise
+     * @param ExerciseInterface&CliExercise $exercise
      * @return ExerciseRunnerInterface
      */
-    public function create(ExerciseInterface $exercise)
+    public function create(ExerciseInterface $exercise): ExerciseRunnerInterface
     {
         return new CliRunner($exercise, $this->eventDispatcher);
     }
diff --git a/src/ExerciseRunner/Factory/CustomVerifyingRunnerFactory.php b/src/ExerciseRunner/Factory/CustomVerifyingRunnerFactory.php
index fe58676d..b3d5de18 100644
--- a/src/ExerciseRunner/Factory/CustomVerifyingRunnerFactory.php
+++ b/src/ExerciseRunner/Factory/CustomVerifyingRunnerFactory.php
@@ -3,6 +3,7 @@
 namespace PhpSchool\PhpWorkshop\ExerciseRunner\Factory;
 
 use PhpSchool\PhpWorkshop\CommandDefinition;
+use PhpSchool\PhpWorkshop\Exercise\CustomVerifyingExercise;
 use PhpSchool\PhpWorkshop\Exercise\ExerciseInterface;
 use PhpSchool\PhpWorkshop\Exercise\ExerciseType;
 use PhpSchool\PhpWorkshop\ExerciseRunner\CustomVerifyingRunner;
@@ -24,7 +25,7 @@ class CustomVerifyingRunnerFactory implements ExerciseRunnerFactoryInterface
      * @param ExerciseInterface $exercise
      * @return bool
      */
-    public function supports(ExerciseInterface $exercise)
+    public function supports(ExerciseInterface $exercise): bool
     {
         return $exercise->getType()->getValue() === self::$type;
     }
@@ -34,17 +35,17 @@ public function supports(ExerciseInterface $exercise)
      *
      * @param CommandDefinition $commandDefinition
      */
-    public function configureInput(CommandDefinition $commandDefinition)
+    public function configureInput(CommandDefinition $commandDefinition): void
     {
     }
 
     /**
      * Create and return an instance of the runner.
      *
-     * @param ExerciseInterface $exercise
+     * @param ExerciseInterface&CustomVerifyingExercise $exercise
      * @return ExerciseRunnerInterface
      */
-    public function create(ExerciseInterface $exercise)
+    public function create(ExerciseInterface $exercise): ExerciseRunnerInterface
     {
         return new CustomVerifyingRunner($exercise);
     }
diff --git a/src/ExerciseRunner/Factory/ExerciseRunnerFactoryInterface.php b/src/ExerciseRunner/Factory/ExerciseRunnerFactoryInterface.php
index 49b76d7c..683023d0 100644
--- a/src/ExerciseRunner/Factory/ExerciseRunnerFactoryInterface.php
+++ b/src/ExerciseRunner/Factory/ExerciseRunnerFactoryInterface.php
@@ -17,14 +17,14 @@ interface ExerciseRunnerFactoryInterface
      * @param ExerciseInterface $exercise
      * @return bool
      */
-    public function supports(ExerciseInterface $exercise);
+    public function supports(ExerciseInterface $exercise): bool;
 
     /**
      * Add any extra required arguments to the command.
      *
      * @param CommandDefinition $commandDefinition
      */
-    public function configureInput(CommandDefinition $commandDefinition);
+    public function configureInput(CommandDefinition $commandDefinition): void;
 
     /**
      * Create and return an instance of the runner.
@@ -32,5 +32,5 @@ public function configureInput(CommandDefinition $commandDefinition);
      * @param ExerciseInterface $exercise
      * @return ExerciseRunnerInterface
      */
-    public function create(ExerciseInterface $exercise);
+    public function create(ExerciseInterface $exercise): ExerciseRunnerInterface;
 }
diff --git a/src/ExerciseRunner/RunnerManager.php b/src/ExerciseRunner/RunnerManager.php
index 3e4a4d52..9596dfd0 100644
--- a/src/ExerciseRunner/RunnerManager.php
+++ b/src/ExerciseRunner/RunnerManager.php
@@ -20,7 +20,7 @@ class RunnerManager
     /**
      * @param ExerciseRunnerFactoryInterface $factory
      */
-    public function addFactory(ExerciseRunnerFactoryInterface $factory)
+    public function addFactory(ExerciseRunnerFactoryInterface $factory): void
     {
         $this->factories[] = $factory;
     }
@@ -29,7 +29,7 @@ public function addFactory(ExerciseRunnerFactoryInterface $factory)
      * @param ExerciseInterface $exercise
      * @param CommandDefinition $commandDefinition
      */
-    public function configureInput(ExerciseInterface $exercise, CommandDefinition $commandDefinition)
+    public function configureInput(ExerciseInterface $exercise, CommandDefinition $commandDefinition): void
     {
         $this->getFactory($exercise)->configureInput($commandDefinition);
     }
@@ -38,7 +38,7 @@ public function configureInput(ExerciseInterface $exercise, CommandDefinition $c
      * @param ExerciseInterface $exercise
      * @return ExerciseRunnerInterface
      */
-    public function getRunner(ExerciseInterface $exercise)
+    public function getRunner(ExerciseInterface $exercise): ExerciseRunnerInterface
     {
         return $this->getFactory($exercise)->create($exercise);
     }
@@ -48,7 +48,7 @@ public function getRunner(ExerciseInterface $exercise)
      * @return ExerciseRunnerFactoryInterface
      * @throws InvalidArgumentException
      */
-    private function getFactory(ExerciseInterface $exercise)
+    private function getFactory(ExerciseInterface $exercise): ExerciseRunnerFactoryInterface
     {
         foreach ($this->factories as $factory) {
             if ($factory->supports($exercise)) {
diff --git a/src/Factory/EventDispatcherFactory.php b/src/Factory/EventDispatcherFactory.php
index 320e9246..16d0303c 100644
--- a/src/Factory/EventDispatcherFactory.php
+++ b/src/Factory/EventDispatcherFactory.php
@@ -14,13 +14,12 @@
  */
 class EventDispatcherFactory
 {
-
     /**
      * @param ContainerInterface $container
      * @return EventDispatcher
      * @throws InvalidArgumentException
      */
-    public function __invoke(ContainerInterface $container)
+    public function __invoke(ContainerInterface $container): EventDispatcher
     {
         $dispatcher = new EventDispatcher($container->get(ResultAggregator::class));
 
@@ -47,10 +46,10 @@ public function __invoke(ContainerInterface $container)
     }
 
     /**
-     * @param array $listeners
-     * @return array
+     * @param array<int, array<string, array>> $listeners
+     * @return array<int, array>
      */
-    private function mergeListenerGroups(array $listeners)
+    private function mergeListenerGroups(array $listeners): array
     {
         $listeners = new Collection($listeners);
 
@@ -79,17 +78,17 @@ private function mergeListenerGroups(array $listeners)
 
     /**
      * @param string $eventName
-     * @param array $listeners
+     * @param array<int, ContainerListenerHelper|callable> $listeners
      * @param ContainerInterface $container
      * @param EventDispatcher $dispatcher
-     * @throws \PhpSchool\PhpWorkshop\Exception\InvalidArgumentException
+     * @throws InvalidArgumentException
      */
     private function attachListeners(
-        $eventName,
+        string $eventName,
         array $listeners,
         ContainerInterface $container,
         EventDispatcher $dispatcher
-    ) {
+    ): void {
         array_walk($listeners, function ($listener) use ($eventName, $dispatcher, $container) {
             if ($listener instanceof ContainerListenerHelper) {
                 if (!$container->has($listener->getService())) {
@@ -98,7 +97,7 @@ private function attachListeners(
                     );
                 }
 
-                return $dispatcher->listen($eventName, function (...$args) use ($container, $listener) {
+                $dispatcher->listen($eventName, function (...$args) use ($container, $listener) {
                     $service = $container->get($listener->getService());
 
                     if (!method_exists($service, $listener->getMethod())) {
@@ -109,6 +108,7 @@ private function attachListeners(
 
                     $service->{$listener->getMethod()}(...$args);
                 });
+                return;
             }
 
             if (!is_callable($listener)) {
@@ -116,8 +116,7 @@ private function attachListeners(
                     sprintf('Listener must be a callable or a container entry for a callable service.')
                 );
             }
-
-            return $dispatcher->listen($eventName, $listener);
+            $dispatcher->listen($eventName, $listener);
         });
     }
 }
diff --git a/src/Factory/MenuFactory.php b/src/Factory/MenuFactory.php
index b02f2382..e684816d 100644
--- a/src/Factory/MenuFactory.php
+++ b/src/Factory/MenuFactory.php
@@ -142,7 +142,7 @@ private function isExerciseDisabled(ExerciseInterface $exercise, UserState $user
      * @param EventDispatcher $eventDispatcher
      * @param ExerciseInterface $exercise
      */
-    private function dispatchExerciseSelectedEvent(EventDispatcher $eventDispatcher, ExerciseInterface $exercise)
+    private function dispatchExerciseSelectedEvent(EventDispatcher $eventDispatcher, ExerciseInterface $exercise): void
     {
         $eventDispatcher->dispatch(
             new Event(
diff --git a/src/Factory/ResultRendererFactory.php b/src/Factory/ResultRendererFactory.php
index df3fd791..fb9f89ac 100644
--- a/src/Factory/ResultRendererFactory.php
+++ b/src/Factory/ResultRendererFactory.php
@@ -12,21 +12,21 @@
 class ResultRendererFactory
 {
     /**
-     * @var array
+     * @var array<class-string, class-string>
      */
     private $mappings = [];
 
     /**
-     * @var array
+     * @var array<class-string, callable>
      */
     private $factories = [];
 
     /**
-     * @param string $resultClass
-     * @param string $rendererClass
-     * @param callable $factory
+     * @param class-string $resultClass
+     * @param class-string $rendererClass
+     * @param callable|null $factory
      */
-    public function registerRenderer($resultClass, $rendererClass, callable $factory = null)
+    public function registerRenderer(string $resultClass, string $rendererClass, callable $factory = null): void
     {
         if (!$this->isImplementationNameOfClass($resultClass, ResultInterface::class)) {
             throw new InvalidArgumentException();
@@ -47,7 +47,7 @@ public function registerRenderer($resultClass, $rendererClass, callable $factory
      * @param ResultInterface $result
      * @return ResultRendererInterface
      */
-    public function create(ResultInterface $result)
+    public function create(ResultInterface $result): ResultRendererInterface
     {
         $class = get_class($result);
         if (!isset($this->mappings[$class])) {
@@ -70,11 +70,17 @@ public function create(ResultInterface $result)
             );
         }
 
+        /** @var ResultRendererInterface $renderer */
         return $renderer;
     }
 
-    protected function isImplementationNameOfClass($implementationName, $className)
+    /**
+     * @param class-string $implementationName
+     * @param class-string $className
+     * @return bool
+     */
+    protected function isImplementationNameOfClass(string $implementationName, string $className): bool
     {
-        return is_string($implementationName) && is_subclass_of($implementationName, $className);
+        return is_subclass_of($implementationName, $className);
     }
 }
diff --git a/src/Input/Input.php b/src/Input/Input.php
index 9b2bec9b..192534f8 100644
--- a/src/Input/Input.php
+++ b/src/Input/Input.php
@@ -16,16 +16,15 @@ class Input
     private $appName;
 
     /**
-     * @var array
+     * @var array<string, string|null>
      */
     private $arguments = [];
 
     /**
-     * Input constructor.
-     * @param $appName
-     * @param array $arguments
+     * @param string $appName
+     * @param array<string, string|null> $arguments
      */
-    public function __construct($appName, array $arguments = [])
+    public function __construct(string $appName, array $arguments = [])
     {
         $this->appName = $appName;
         $this->arguments = $arguments;
@@ -34,7 +33,7 @@ public function __construct($appName, array $arguments = [])
     /**
      * @return string
      */
-    public function getAppName()
+    public function getAppName(): string
     {
         return $this->appName;
     }
@@ -43,17 +42,17 @@ public function getAppName()
      * @param string $name
      * @return bool
      */
-    public function hasArgument($name)
+    public function hasArgument(string $name): bool
     {
-        return isset($this->arguments[$name]);
+        return array_key_exists($name, $this->arguments);
     }
 
     /**
      * @param string $name
-     * @return string
+     * @return ?string
      * @throws InvalidArgumentException
      */
-    public function getArgument($name)
+    public function getArgument(string $name): ?string
     {
         if (!$this->hasArgument($name)) {
             throw new InvalidArgumentException(sprintf('Argument with name: "%s" does not exist', $name));
@@ -62,11 +61,27 @@ public function getArgument($name)
         return $this->arguments[$name];
     }
 
+    /**
+     * @param string $name
+     * @return string
+     * @throws InvalidArgumentException
+     */
+    public function getRequiredArgument(string $name): string
+    {
+        $arg = $this->getArgument($name);
+
+        if (null === $arg) {
+            throw new InvalidArgumentException(sprintf('Argument with name: "%s" is required', $name));
+        }
+
+        return $arg;
+    }
+
     /**
      * @param string $name
      * @param string $value
      */
-    public function setArgument($name, $value)
+    public function setArgument(string $name, string $value): void
     {
         $this->arguments[$name] = $value;
     }
diff --git a/src/Listener/CheckExerciseAssignedListener.php b/src/Listener/CheckExerciseAssignedListener.php
index a4169dff..d7750ddb 100644
--- a/src/Listener/CheckExerciseAssignedListener.php
+++ b/src/Listener/CheckExerciseAssignedListener.php
@@ -24,7 +24,7 @@ public function __construct(UserState $userState)
     /**
      * @param Event $event
      */
-    public function __invoke(Event $event)
+    public function __invoke(Event $event): void
     {
         /** @var CommandDefinition $command */
         $command = $event->getParameter('command');
diff --git a/src/Listener/CodePatchListener.php b/src/Listener/CodePatchListener.php
index 01bd57cb..b38e7d5c 100644
--- a/src/Listener/CodePatchListener.php
+++ b/src/Listener/CodePatchListener.php
@@ -34,11 +34,15 @@ public function __construct(CodePatcher $codePatcher)
     /**
      * @param ExerciseRunnerEvent $event
      */
-    public function patch(ExerciseRunnerEvent $event)
+    public function patch(ExerciseRunnerEvent $event): void
     {
         $fileName = $event->getInput()->getArgument('program');
 
-        $this->originalCode = file_get_contents($fileName);
+        if (null === $fileName) {
+            return;
+        }
+
+        $this->originalCode = (string) file_get_contents($fileName);
         file_put_contents(
             $fileName,
             $this->codePatcher->patch($event->getExercise(), $this->originalCode)
@@ -48,12 +52,18 @@ public function patch(ExerciseRunnerEvent $event)
     /**
      * @param ExerciseRunnerEvent $event
      */
-    public function revert(ExerciseRunnerEvent $event)
+    public function revert(ExerciseRunnerEvent $event): void
     {
         if (null === $this->originalCode) {
             throw new RuntimeException('Can only revert previously patched code');
         }
 
-        file_put_contents($event->getInput()->getArgument('program'), $this->originalCode);
+        $fileName = $event->getInput()->getArgument('program');
+
+        if (null === $fileName) {
+            return;
+        }
+
+        file_put_contents($fileName, $this->originalCode);
     }
 }
diff --git a/src/Listener/ConfigureCommandListener.php b/src/Listener/ConfigureCommandListener.php
index 8c551c35..cc032bf5 100644
--- a/src/Listener/ConfigureCommandListener.php
+++ b/src/Listener/ConfigureCommandListener.php
@@ -46,7 +46,7 @@ public function __construct(
     /**
      * @param Event $event
      */
-    public function __invoke(Event $event)
+    public function __invoke(Event $event): void
     {
         /** @var CommandDefinition $command */
         $command = $event->getParameter('command');
diff --git a/src/Listener/PrepareSolutionListener.php b/src/Listener/PrepareSolutionListener.php
index c5b58eec..0095fa4f 100644
--- a/src/Listener/PrepareSolutionListener.php
+++ b/src/Listener/PrepareSolutionListener.php
@@ -3,6 +3,7 @@
 namespace PhpSchool\PhpWorkshop\Listener;
 
 use PhpSchool\PhpWorkshop\Event\ExerciseRunnerEvent;
+use PhpSchool\PhpWorkshop\Exercise\ProvidesSolution;
 use RuntimeException;
 use Symfony\Component\Process\Process;
 
@@ -14,7 +15,7 @@ class PrepareSolutionListener
     /**
      * Locations for composer executable
      *
-     * @var array
+     * @var array<string>
      */
     private $composerLocations = [
         'composer',
@@ -26,9 +27,15 @@ class PrepareSolutionListener
     /**
      * @param ExerciseRunnerEvent $event
      */
-    public function __invoke(ExerciseRunnerEvent $event)
+    public function __invoke(ExerciseRunnerEvent $event): void
     {
-        $solution = $event->getExercise()->getSolution();
+        $exercise = $event->getExercise();
+
+        if (!$exercise instanceof ProvidesSolution) {
+            return;
+        }
+
+        $solution = $exercise->getSolution();
 
         if ($solution->hasComposerFile()) {
             //prepare composer deps
diff --git a/src/Listener/RealPathListener.php b/src/Listener/RealPathListener.php
index 5ee6eaa8..2d8b8207 100644
--- a/src/Listener/RealPathListener.php
+++ b/src/Listener/RealPathListener.php
@@ -17,16 +17,16 @@ class RealPathListener
     /**
      * @param ExerciseRunnerEvent $event
      */
-    public function __invoke(ExerciseRunnerEvent $event)
+    public function __invoke(ExerciseRunnerEvent $event): void
     {
         if (!$event->getInput()->hasArgument('program')) {
             return;
         }
 
-        $program = $event->getInput()->getArgument('program');
+        $program = $event->getInput()->getRequiredArgument('program');
 
         if (file_exists($program)) {
-            $event->getInput()->setArgument('program', realpath($program));
+            $event->getInput()->setArgument('program', (string) realpath($program));
         }
     }
 }
diff --git a/src/Listener/SelfCheckListener.php b/src/Listener/SelfCheckListener.php
index ace5a0e7..e0773a92 100644
--- a/src/Listener/SelfCheckListener.php
+++ b/src/Listener/SelfCheckListener.php
@@ -27,7 +27,7 @@ public function __construct(ResultAggregator $results)
     /**
      * @param Event $event
      */
-    public function __invoke(Event $event)
+    public function __invoke(Event $event): void
     {
         $exercise = $event->getParameter('exercise');
 
diff --git a/src/MenuItem/ResetProgress.php b/src/MenuItem/ResetProgress.php
index f7d0517f..20399f5a 100644
--- a/src/MenuItem/ResetProgress.php
+++ b/src/MenuItem/ResetProgress.php
@@ -27,13 +27,17 @@ public function __construct(UserStateSerializer $userStateSerializer)
     /**
      * @param CliMenu $menu
      */
-    public function __invoke(CliMenu $menu)
+    public function __invoke(CliMenu $menu): void
     {
         $this->userStateSerializer->serialize(new UserState());
 
-        $items = $menu
-            ->getParent()
-            ->getItems();
+        $parent = $menu->getParent();
+
+        if (!$parent) {
+            return;
+        }
+
+        $items = $parent->getItems();
 
         foreach ($items as $item) {
             $item->hideItemExtra();
diff --git a/src/NodeVisitor/FunctionVisitor.php b/src/NodeVisitor/FunctionVisitor.php
index 6889cfa9..6db8540d 100644
--- a/src/NodeVisitor/FunctionVisitor.php
+++ b/src/NodeVisitor/FunctionVisitor.php
@@ -12,27 +12,27 @@
 class FunctionVisitor extends NodeVisitorAbstract
 {
     /**
-     * @var array
+     * @var array<string>
      */
     private $requiredFunctions;
     /**
-     * @var array
+     * @var array<string>
      */
     private $bannedFunctions;
 
     /**
-     * @var array
+     * @var array<FuncCall>
      */
     private $requiredUsages = [];
 
     /**
-     * @var array
+     * @var array<FuncCall>
      */
     private $bannedUsages = [];
 
     /**
-     * @param array $requiredFunctions
-     * @param array $bannedFunctions
+     * @param array<string> $requiredFunctions
+     * @param array<string> $bannedFunctions
      */
     public function __construct(array $requiredFunctions, array $bannedFunctions)
     {
@@ -42,41 +42,44 @@ public function __construct(array $requiredFunctions, array $bannedFunctions)
 
     /**
      * @param Node $node
+     * @return null
      */
     public function leaveNode(Node $node)
     {
-        if ($node instanceof FuncCall) {
+        if ($node instanceof FuncCall && $node->name instanceof Node\Name) {
             $name = $node->name->__toString();
-            if (in_array($name, $this->requiredFunctions)) {
+            if (in_array($name, $this->requiredFunctions, true)) {
                 $this->requiredUsages[] = $node;
             }
 
-            if (in_array($name, $this->bannedFunctions)) {
+            if (in_array($name, $this->bannedFunctions, true)) {
                 $this->bannedUsages[] = $node;
             }
         }
+
+        return null;
     }
 
     /**
      * @return bool
      */
-    public function hasUsedBannedFunctions()
+    public function hasUsedBannedFunctions(): bool
     {
         return count($this->bannedUsages) > 0;
     }
 
     /**
-     * @return array
+     * @return array<FuncCall>
      */
-    public function getBannedUsages()
+    public function getBannedUsages(): array
     {
         return $this->bannedUsages;
     }
 
     /**
-     * @return array
+     * @return array<FuncCall>
      */
-    public function getRequiredUsages()
+    public function getRequiredUsages(): array
     {
         return $this->requiredUsages;
     }
@@ -84,10 +87,14 @@ public function getRequiredUsages()
     /**
      * @return bool
      */
-    public function hasMetFunctionRequirements()
+    public function hasMetFunctionRequirements(): bool
     {
         $metRequires = array_filter($this->requiredFunctions, function ($function) {
             foreach ($this->getRequiredUsages() as $usage) {
+                if (!$usage->name instanceof Node\Name) {
+                    continue;
+                }
+
                 if ($usage->name->__toString() === $function) {
                     return true;
                 }
@@ -99,12 +106,16 @@ public function hasMetFunctionRequirements()
     }
 
     /**
-     * @return array
+     * @return array<string>
      */
-    public function getMissingRequirements()
+    public function getMissingRequirements(): array
     {
         return array_filter($this->requiredFunctions, function ($function) {
             foreach ($this->getRequiredUsages() as $usage) {
+                if (!$usage->name instanceof Node\Name) {
+                    continue;
+                }
+
                 if ($usage->name->__toString() === $function) {
                     return false;
                 }
diff --git a/src/Output/OutputInterface.php b/src/Output/OutputInterface.php
index 92a22dd8..dbbc6d2d 100644
--- a/src/Output/OutputInterface.php
+++ b/src/Output/OutputInterface.php
@@ -15,40 +15,38 @@ interface OutputInterface
      *
      * @param string $error
      */
-    public function printError($error);
+    public function printError(string $error): void;
 
     /**
      * Write a string to the output.
      *
      * @param string $content
      */
-    public function write($content);
+    public function write(string $content): void;
 
     /**
      * Write an array of strings, each on a new line.
      *
-     * @param array $lines
+     * @param array<string> $lines
      */
-    public function writeLines(array $lines);
+    public function writeLines(array $lines): void;
 
     /**
      * Write a string terminated with a newline.
      *
      * @param string $line
      */
-    public function writeLine($line);
+    public function writeLine(string $line): void;
 
     /**
      * Write an empty line.
      */
-    public function emptyLine();
+    public function emptyLine(): void;
 
     /**
      * Write a line break.
-     *
-     * @return string
      */
-    public function lineBreak();
+    public function lineBreak(): void;
 
     /**
      * Write a title section. Should be decorated in a way which makes
@@ -56,5 +54,5 @@ public function lineBreak();
      *
      * @param string $title
      */
-    public function writeTitle($title);
+    public function writeTitle(string $title): void;
 }
diff --git a/src/Output/StdOutput.php b/src/Output/StdOutput.php
index 9149954d..95260f5b 100644
--- a/src/Output/StdOutput.php
+++ b/src/Output/StdOutput.php
@@ -29,7 +29,7 @@ public function __construct(Color $color, Terminal $terminal)
     /**
      * @param string $error
      */
-    public function printError($error)
+    public function printError(string $error): void
     {
         $length = strlen($error) + 2;
         echo "\n";
@@ -45,7 +45,7 @@ public function printError($error)
      *
      * @param string $title
      */
-    public function writeTitle($title)
+    public function writeTitle(string $title): void
     {
         echo sprintf("\n%s\n", $this->color->__invoke($title)->underline()->bold());
     }
@@ -55,7 +55,7 @@ public function writeTitle($title)
      *
      * @param string $content
      */
-    public function write($content)
+    public function write(string $content): void
     {
         echo $content;
     }
@@ -63,9 +63,9 @@ public function write($content)
     /**
      * Write an array of strings, each on a new line.
      *
-     * @param array $lines
+     * @param array<string> $lines
      */
-    public function writeLines(array $lines)
+    public function writeLines(array $lines): void
     {
         foreach ($lines as $line) {
             $this->writeLine($line);
@@ -77,7 +77,7 @@ public function writeLines(array $lines)
      *
      * @param string $line
      */
-    public function writeLine($line)
+    public function writeLine(string $line): void
     {
         echo sprintf("%s\n", $line);
     }
@@ -85,17 +85,15 @@ public function writeLine($line)
     /**
      * Write an empty line.
      */
-    public function emptyLine()
+    public function emptyLine(): void
     {
         echo "\n";
     }
 
     /**
      * Write a line break.
-     *
-     * @return string
      */
-    public function lineBreak()
+    public function lineBreak(): void
     {
         echo $this->color->__invoke(str_repeat('─', $this->terminal->getWidth()))->yellow();
     }
diff --git a/src/Patch.php b/src/Patch.php
index 40aa377b..b1083459 100644
--- a/src/Patch.php
+++ b/src/Patch.php
@@ -12,10 +12,10 @@
  * An insertion is a block of code that can be inserted at the top or bottom of
  * the students solution.
  */
-class Patch
+final class Patch
 {
     /**
-     * @var array
+     * @var array<CodeInsertion|Closure>
      */
     private $modifications = [];
 
@@ -23,9 +23,9 @@ class Patch
      * Add a new `CodeInsertion`. `Patch` is immutable so a new instance is returned.
      *
      * @param CodeInsertion $insertion
-     * @return static
+     * @return self
      */
-    public function withInsertion(CodeInsertion $insertion)
+    public function withInsertion(CodeInsertion $insertion): self
     {
         $new = clone $this;
         $new->modifications[] = $insertion;
@@ -36,9 +36,9 @@ public function withInsertion(CodeInsertion $insertion)
      * Add a new transformer (`Closure`). `Patch` is immutable so a new instance is returned.
      *
      * @param Closure $closure
-     * @return static
+     * @return self
      */
-    public function withTransformer(Closure $closure)
+    public function withTransformer(Closure $closure): self
     {
         $new = clone $this;
         $new->modifications[] = $closure;
@@ -48,9 +48,9 @@ public function withTransformer(Closure $closure)
     /**
      * Retrieve all the modifications including insertions (`CodeInsertion`'s) & transformers (`Closure`'s)
      *
-     * @return array
+     * @return array<CodeInsertion|Closure>
      */
-    public function getModifiers()
+    public function getModifiers(): array
     {
         return $this->modifications;
     }
diff --git a/src/Result/Cgi/CgiResult.php b/src/Result/Cgi/CgiResult.php
index 5b24dae9..8b1e0446 100644
--- a/src/Result/Cgi/CgiResult.php
+++ b/src/Result/Cgi/CgiResult.php
@@ -4,11 +4,14 @@
 
 use ArrayIterator;
 use IteratorAggregate;
+use PhpSchool\PhpWorkshop\Result\Cgi\ResultInterface;
 use PhpSchool\PhpWorkshop\Result\ResultGroupInterface;
 
 /**
  * A result which encompasses all the results for each individual request made during
  * the CGI verification process.
+ *
+ * @implements IteratorAggregate<int, ResultInterface>
  */
 class CgiResult implements ResultGroupInterface, IteratorAggregate
 {
@@ -18,12 +21,12 @@ class CgiResult implements ResultGroupInterface, IteratorAggregate
     private $name = 'CGI Program Runner';
 
     /**
-     * @var array
+     * @var ResultInterface[]
      */
     private $results = [];
 
     /**
-     * @param array $requestResults An array of results representing each request.
+     * @param array<ResultInterface> $requestResults An array of results representing each request.
      */
     public function __construct(array $requestResults = [])
     {
@@ -37,7 +40,7 @@ public function __construct(array $requestResults = [])
      *
      * @param ResultInterface $result
      */
-    public function add(ResultInterface $result)
+    public function add(ResultInterface $result): void
     {
         $this->results[] = $result;
     }
@@ -47,7 +50,7 @@ public function add(ResultInterface $result)
      *
      * @return string
      */
-    public function getCheckName()
+    public function getCheckName(): string
     {
         return $this->name;
     }
@@ -55,7 +58,7 @@ public function getCheckName()
     /**
      * @return bool
      */
-    public function isSuccessful()
+    public function isSuccessful(): bool
     {
         return count(
             array_filter($this->results, function ($result) {
@@ -65,9 +68,9 @@ public function isSuccessful()
     }
 
     /**
-     * @return ResultInterface
+     * @return ResultInterface[]
      */
-    public function getResults()
+    public function getResults(): array
     {
         return $this->results;
     }
@@ -75,9 +78,9 @@ public function getResults()
     /**
      * Get an iterator in order to `foreach` the results.
      *
-     * @return ArrayIterator
+     * @return ArrayIterator<int, ResultInterface>
      */
-    public function getIterator()
+    public function getIterator(): ArrayIterator
     {
         return new ArrayIterator($this->results);
     }
diff --git a/src/Result/Cgi/GenericFailure.php b/src/Result/Cgi/GenericFailure.php
index 1f2fcd28..b8353f9e 100644
--- a/src/Result/Cgi/GenericFailure.php
+++ b/src/Result/Cgi/GenericFailure.php
@@ -37,11 +37,11 @@ public function __construct(RequestInterface $request, $reason = null)
      *
      * @param RequestInterface $request The request that caused the failure.
      * @param string|null $reason The reason (if any) of the failure.
-     * @return static The result.
+     * @return self The result.
      */
     public static function fromRequestAndReason(RequestInterface $request, $reason)
     {
-        return new static($request, $reason);
+        return new self($request, $reason);
     }
 
     /**
@@ -49,11 +49,11 @@ public static function fromRequestAndReason(RequestInterface $request, $reason)
      *
      * @param RequestInterface $request The request that caused the failure.
      * @param CodeExecutionException $e The exception.
-     * @return static The result.
+     * @return self The result.
      */
     public static function fromRequestAndCodeExecutionFailure(RequestInterface $request, CodeExecutionException $e)
     {
-        return new static($request, $e->getMessage());
+        return new self($request, $e->getMessage());
     }
 
     /**
diff --git a/src/Result/Cgi/RequestFailure.php b/src/Result/Cgi/RequestFailure.php
index c3b20103..7777ad4f 100644
--- a/src/Result/Cgi/RequestFailure.php
+++ b/src/Result/Cgi/RequestFailure.php
@@ -26,12 +26,12 @@ class RequestFailure implements FailureInterface
     private $actualOutput;
     
     /**
-     * @var array
+     * @var array<string, string>
      */
     private $expectedHeaders;
     
     /**
-     * @var array
+     * @var array<string, string>
      */
     private $actualHeaders;
 
@@ -39,21 +39,21 @@ class RequestFailure implements FailureInterface
      * @param RequestInterface $request The request that caused the failure.
      * @param string $expectedOutput
      * @param string $actualOutput
-     * @param array $expectedHeaders
-     * @param array $actualHeaders
+     * @param array<string, string> $expectedHeaders
+     * @param array<string, string> $actualHeaders
      */
     public function __construct(
         RequestInterface $request,
-        $expectedOutput,
-        $actualOutput,
+        string $expectedOutput,
+        string $actualOutput,
         array $expectedHeaders,
         array $actualHeaders
     ) {
-        $this->request          = $request;
-        $this->expectedOutput   = $expectedOutput;
-        $this->actualOutput     = $actualOutput;
-        $this->expectedHeaders  = $expectedHeaders;
-        $this->actualHeaders    = $actualHeaders;
+        $this->request = $request;
+        $this->expectedOutput = $expectedOutput;
+        $this->actualOutput = $actualOutput;
+        $this->expectedHeaders = $expectedHeaders;
+        $this->actualHeaders = $actualHeaders;
     }
 
     /**
@@ -61,7 +61,7 @@ public function __construct(
      *
      * @return RequestInterface
      */
-    public function getRequest()
+    public function getRequest(): RequestInterface
     {
         return $this->request;
     }
@@ -71,7 +71,7 @@ public function getRequest()
      *
      * @return bool
      */
-    public function bodyDifferent()
+    public function bodyDifferent(): bool
     {
         return $this->expectedOutput !== $this->actualOutput;
     }
@@ -81,7 +81,7 @@ public function bodyDifferent()
      *
      * @return bool
      */
-    public function headersDifferent()
+    public function headersDifferent(): bool
     {
         return $this->expectedHeaders !== $this->actualHeaders;
     }
@@ -91,7 +91,7 @@ public function headersDifferent()
      *
      * @return bool
      */
-    public function headersAndBodyDifferent()
+    public function headersAndBodyDifferent(): bool
     {
         return $this->bodyDifferent() && $this->headersDifferent();
     }
@@ -101,7 +101,7 @@ public function headersAndBodyDifferent()
      *
      * @return string
      */
-    public function getExpectedOutput()
+    public function getExpectedOutput(): string
     {
         return $this->expectedOutput;
     }
@@ -111,7 +111,7 @@ public function getExpectedOutput()
      *
      * @return string
      */
-    public function getActualOutput()
+    public function getActualOutput(): string
     {
         return $this->actualOutput;
     }
@@ -119,9 +119,9 @@ public function getActualOutput()
     /**
      * Get the array of expected headers.
      *
-     * @return array
+     * @return array<string, string>
      */
-    public function getExpectedHeaders()
+    public function getExpectedHeaders(): array
     {
         return $this->expectedHeaders;
     }
@@ -129,9 +129,9 @@ public function getExpectedHeaders()
     /**
      * Get the array of actual headers.
      *
-     * @return array
+     * @return array<string, string>
      */
-    public function getActualHeaders()
+    public function getActualHeaders(): array
     {
         return $this->actualHeaders;
     }
@@ -141,7 +141,7 @@ public function getActualHeaders()
      *
      * @return string
      */
-    public function getCheckName()
+    public function getCheckName(): string
     {
         return 'Request Failure';
     }
diff --git a/src/Result/Cli/CliResult.php b/src/Result/Cli/CliResult.php
index 049b1e7d..3a6ff39c 100644
--- a/src/Result/Cli/CliResult.php
+++ b/src/Result/Cli/CliResult.php
@@ -9,6 +9,8 @@
 /**
  * A result which encompasses all the results for each individual execution made during
  * the CLI verification process.
+ *
+ * @implements IteratorAggregate<int, ResultInterface>
  */
 class CliResult implements ResultGroupInterface, IteratorAggregate
 {
@@ -18,12 +20,12 @@ class CliResult implements ResultGroupInterface, IteratorAggregate
     private $name = 'CLI Program Runner';
 
     /**
-     * @var array
+     * @var ResultInterface[]
      */
     private $results = [];
 
     /**
-     * @param array $requestResults An array of results representing each request.
+     * @param array<ResultInterface> $requestResults An array of results representing each request.
      */
     public function __construct(array $requestResults = [])
     {
@@ -37,7 +39,7 @@ public function __construct(array $requestResults = [])
      *
      * @param ResultInterface $result
      */
-    public function add(ResultInterface $result)
+    public function add(ResultInterface $result): void
     {
         $this->results[] = $result;
     }
@@ -47,7 +49,7 @@ public function add(ResultInterface $result)
      *
      * @return string
      */
-    public function getCheckName()
+    public function getCheckName(): string
     {
         return $this->name;
     }
@@ -55,7 +57,7 @@ public function getCheckName()
     /**
      * @return bool
      */
-    public function isSuccessful()
+    public function isSuccessful(): bool
     {
         return count(
             array_filter($this->results, function ($result) {
@@ -65,9 +67,9 @@ public function isSuccessful()
     }
 
     /**
-     * @return ResultInterface
+     * @return ResultInterface[]
      */
-    public function getResults()
+    public function getResults(): array
     {
         return $this->results;
     }
@@ -75,9 +77,9 @@ public function getResults()
     /**
      * Get an iterator in order to `foreach` the results.
      *
-     * @return ArrayIterator
+     * @return ArrayIterator<int, ResultInterface>
      */
-    public function getIterator()
+    public function getIterator(): ArrayIterator
     {
         return new ArrayIterator($this->results);
     }
diff --git a/src/Result/Cli/GenericFailure.php b/src/Result/Cli/GenericFailure.php
index a6224574..bdaf3ae9 100644
--- a/src/Result/Cli/GenericFailure.php
+++ b/src/Result/Cli/GenericFailure.php
@@ -12,7 +12,7 @@
 class GenericFailure extends Failure implements FailureInterface
 {
     /**
-     * @var ArrayObject
+     * @var ArrayObject<string>
      */
     private $args;
 
@@ -22,7 +22,7 @@ class GenericFailure extends Failure implements FailureInterface
     private static $name = 'CLI Program Runner';
 
     /**
-     * @param ArrayObject $args The arguments that caused the failure.
+     * @param ArrayObject<string> $args The arguments that caused the failure.
      * @param string|null $reason The reason (if any) of the failure.
      */
     public function __construct(ArrayObject $args, $reason = null)
@@ -34,31 +34,31 @@ public function __construct(ArrayObject $args, $reason = null)
     /**
      * Named constructor, for added code legibility.
      *
-     * @param ArrayObject $args The arguments that caused the failure.
+     * @param ArrayObject<string> $args The arguments that caused the failure.
      * @param string|null $reason The reason (if any) of the failure.
-     * @return static The result.
+     * @return self The result.
      */
-    public static function fromArgsAndReason(ArrayObject $args, $reason)
+    public static function fromArgsAndReason(ArrayObject $args, ?string $reason): self
     {
-        return new static($args, $reason);
+        return new self($args, $reason);
     }
 
     /**
      * Static constructor to create from a `PhpSchool\PhpWorkshop\Exception\CodeExecutionException` exception.
      *
-     * @param ArrayObject $args The arguments that caused the failure.
+     * @param ArrayObject<string> $args The arguments that caused the failure.
      * @param CodeExecutionException $e The exception.
-     * @return static The result.
+     * @return self The result.
      */
-    public static function fromArgsAndCodeExecutionFailure(ArrayObject $args, CodeExecutionException $e)
+    public static function fromArgsAndCodeExecutionFailure(ArrayObject $args, CodeExecutionException $e): self
     {
-        return new static($args, $e->getMessage());
+        return new self($args, $e->getMessage());
     }
 
     /**
-     * @return ArrayObject
+     * @return ArrayObject<string>
      */
-    public function getArgs()
+    public function getArgs(): ArrayObject
     {
         return $this->args;
     }
diff --git a/src/Result/Cli/RequestFailure.php b/src/Result/Cli/RequestFailure.php
index fd549e01..22db1fa4 100644
--- a/src/Result/Cli/RequestFailure.php
+++ b/src/Result/Cli/RequestFailure.php
@@ -11,7 +11,7 @@
 class RequestFailure implements FailureInterface
 {
     /**
-     * @var ArrayObject
+     * @var ArrayObject<string>
      */
     private $args;
 
@@ -26,36 +26,36 @@ class RequestFailure implements FailureInterface
     private $actualOutput;
 
     /**
-     * @param ArrayObject $args The arguments that caused the failure.
+     * @param ArrayObject<string> $args The arguments that caused the failure.
      * @param string $expectedOutput The expected output.
      * @param string $actualOutput The actual output.
      */
-    public function __construct(ArrayObject $args, $expectedOutput, $actualOutput)
+    public function __construct(ArrayObject $args, string $expectedOutput, string $actualOutput)
     {
-        $this->args             = $args;
-        $this->expectedOutput   = $expectedOutput;
-        $this->actualOutput     = $actualOutput;
+        $this->args = $args;
+        $this->expectedOutput = $expectedOutput;
+        $this->actualOutput = $actualOutput;
     }
 
     /**
      * Named constructor, for added code legibility.
      *
-     * @param ArrayObject $args The arguments that caused the failure.
+     * @param ArrayObject<string> $args The arguments that caused the failure.
      * @param string $expectedOutput The expected result.
      * @param string $actualOutput The actual output.
-     * @return static The result.
+     * @return self The result.
      */
-    public static function fromArgsAndOutput(ArrayObject $args, $expectedOutput, $actualOutput)
+    public static function fromArgsAndOutput(ArrayObject $args, string $expectedOutput, string $actualOutput): self
     {
-        return new static($args, $expectedOutput, $actualOutput);
+        return new self($args, $expectedOutput, $actualOutput);
     }
 
     /**
      * Get the arguments that caused the failure.
      *
-     * @return ArrayObject
+     * @return ArrayObject<string>
      */
-    public function getArgs()
+    public function getArgs(): ArrayObject
     {
         return $this->args;
     }
@@ -65,7 +65,7 @@ public function getArgs()
      *
      * @return string
      */
-    public function getExpectedOutput()
+    public function getExpectedOutput(): string
     {
         return $this->expectedOutput;
     }
@@ -75,7 +75,7 @@ public function getExpectedOutput()
      *
      * @return string
      */
-    public function getActualOutput()
+    public function getActualOutput(): string
     {
         return $this->actualOutput;
     }
@@ -85,7 +85,7 @@ public function getActualOutput()
      *
      * @return string
      */
-    public function getCheckName()
+    public function getCheckName(): string
     {
         return 'Request Failure';
     }
diff --git a/src/Result/Cli/ResultInterface.php b/src/Result/Cli/ResultInterface.php
index d4945878..08135cfc 100644
--- a/src/Result/Cli/ResultInterface.php
+++ b/src/Result/Cli/ResultInterface.php
@@ -12,7 +12,7 @@ interface ResultInterface extends \PhpSchool\PhpWorkshop\Result\ResultInterface
     /**
      * Get the arguments associated with this result.
      *
-     * @return ArrayObject
+     * @return ArrayObject<string>
      */
-    public function getArgs();
+    public function getArgs(): ArrayObject;
 }
diff --git a/src/Result/Cli/Success.php b/src/Result/Cli/Success.php
index 1b995afd..9ab24e26 100644
--- a/src/Result/Cli/Success.php
+++ b/src/Result/Cli/Success.php
@@ -10,7 +10,7 @@
 class Success implements SuccessInterface
 {
     /**
-     * @var ArrayObject
+     * @var ArrayObject<string>
      */
     private $args;
 
@@ -20,7 +20,7 @@ class Success implements SuccessInterface
     private $name = 'CLI Program Runner';
 
     /**
-     * @param ArrayObject $args The arguments for this success.
+     * @param ArrayObject<string> $args The arguments for this success.
      */
     public function __construct(ArrayObject $args)
     {
@@ -32,7 +32,7 @@ public function __construct(ArrayObject $args)
      *
      * @return string
      */
-    public function getCheckName()
+    public function getCheckName(): string
     {
         return $this->name;
     }
@@ -40,9 +40,9 @@ public function getCheckName()
     /**
      * Get the arguments for this success.
      *
-     * @return ArrayObject
+     * @return ArrayObject<string>
      */
-    public function getArgs()
+    public function getArgs(): ArrayObject
     {
         return $this->args;
     }
diff --git a/src/Result/ComparisonFailure.php b/src/Result/ComparisonFailure.php
index 942bdf77..8ea5efea 100644
--- a/src/Result/ComparisonFailure.php
+++ b/src/Result/ComparisonFailure.php
@@ -38,11 +38,11 @@ public function __construct($name, $expectedValue, $actualValue)
      * @param string $name
      * @param string $expectedValue
      * @param string $actualValue
-     * @return static
+     * @return self
      */
     public static function fromNameAndValues($name, $expectedValue, $actualValue)
     {
-        return new static($name, $expectedValue, $actualValue);
+        return new self($name, $expectedValue, $actualValue);
     }
 
     /**
diff --git a/src/Result/Failure.php b/src/Result/Failure.php
index b0be16c6..88a9830d 100644
--- a/src/Result/Failure.php
+++ b/src/Result/Failure.php
@@ -28,10 +28,10 @@ class Failure implements FailureInterface
      * @param string $name The name of the check that produced this result.
      * @param string|null $reason The reason (if any) of the failure.
      */
-    public function __construct($name, $reason = null)
+    public function __construct(string $name, string $reason = null)
     {
-        $this->name     = $name;
-        $this->reason   = $reason;
+        $this->name = $name;
+        $this->reason = $reason;
     }
 
     /**
@@ -39,11 +39,11 @@ public function __construct($name, $reason = null)
      *
      * @param string $name The name of the check that produced this result.
      * @param string|null $reason The reason (if any) of the failure.
-     * @return static The result.
+     * @return self The result.
      */
-    public static function fromNameAndReason($name, $reason)
+    public static function fromNameAndReason(string $name, ?string $reason): self
     {
-        return new static($name, $reason);
+        return new self($name, $reason);
     }
     
     /**
@@ -51,11 +51,11 @@ public static function fromNameAndReason($name, $reason)
      *
      * @param CheckInterface $check The check instance.
      * @param string $reason The reason (if any) of the failure.
-     * @return static The result.
+     * @return self The result.
      */
-    public static function fromCheckAndReason(CheckInterface $check, $reason)
+    public static function fromCheckAndReason(CheckInterface $check, string $reason): self
     {
-        return new static($check->getName(), $reason);
+        return new self($check->getName(), $reason);
     }
 
     /**
@@ -63,11 +63,11 @@ public static function fromCheckAndReason(CheckInterface $check, $reason)
      *
      * @param string $name The name of the check that produced this result.
      * @param CodeExecutionException $e The exception.
-     * @return static The result.
+     * @return self The result.
      */
-    public static function fromNameAndCodeExecutionFailure($name, CodeExecutionException $e)
+    public static function fromNameAndCodeExecutionFailure(string $name, CodeExecutionException $e): self
     {
-        return new static($name, $e->getMessage());
+        return new self($name, $e->getMessage());
     }
 
     /**
@@ -77,11 +77,14 @@ public static function fromNameAndCodeExecutionFailure($name, CodeExecutionExcep
      * @param CheckInterface $check The check that attempted to parse the solution.
      * @param ParseErrorException $e The parse exception.
      * @param string $file The absolute path to the solution.
-     * @return static The result.
+     * @return self The result.
      */
-    public static function fromCheckAndCodeParseFailure(CheckInterface $check, ParseErrorException $e, $file)
-    {
-        return new static(
+    public static function fromCheckAndCodeParseFailure(
+        CheckInterface $check,
+        ParseErrorException $e,
+        string $file
+    ): self {
+        return new self(
             $check->getName(),
             sprintf('File: "%s" could not be parsed. Error: "%s"', $file, $e->getMessage())
         );
@@ -92,7 +95,7 @@ public static function fromCheckAndCodeParseFailure(CheckInterface $check, Parse
      *
      * @return string
      */
-    public function getCheckName()
+    public function getCheckName(): string
     {
         return $this->name;
     }
@@ -102,7 +105,7 @@ public function getCheckName()
      *
      * @return string|null
      */
-    public function getReason()
+    public function getReason(): ?string
     {
         return $this->reason;
     }
diff --git a/src/Result/FunctionRequirementsFailure.php b/src/Result/FunctionRequirementsFailure.php
index 66944f9a..2ed33dfd 100644
--- a/src/Result/FunctionRequirementsFailure.php
+++ b/src/Result/FunctionRequirementsFailure.php
@@ -13,33 +13,33 @@ class FunctionRequirementsFailure implements FailureInterface
     use ResultTrait;
     
     /**
-     * @var array
+     * @var array<int, string>
      */
     private $bannedFunctions;
 
     /**
-     * @var array
+     * @var array<int, string>
      */
     private $missingFunctions;
 
     /**
      * @param CheckInterface $check The check that produced this result.
-     * @param array $bannedFunctions A list of functions that were used, but were banned.
-     * @param array $missingFunctions A list of functions that were not used, but were required.
+     * @param array<int, string> $bannedFunctions A list of functions that were used, but were banned.
+     * @param array<int, string> $missingFunctions A list of functions that were not used, but were required.
      */
     public function __construct(CheckInterface $check, array $bannedFunctions, array $missingFunctions)
     {
-        $this->check            = $check;
-        $this->bannedFunctions  = $bannedFunctions;
+        $this->check = $check;
+        $this->bannedFunctions = $bannedFunctions;
         $this->missingFunctions = $missingFunctions;
     }
 
     /**
      * Get the list of functions that were used, but were banned.
      *
-     * @return array
+     * @return array<int, string>
      */
-    public function getBannedFunctions()
+    public function getBannedFunctions(): array
     {
         return $this->bannedFunctions;
     }
@@ -47,9 +47,9 @@ public function getBannedFunctions()
     /**
      * Get the list of functions that were not used, but were required.
      *
-     * @return array
+     * @return array<int, string>
      */
-    public function getMissingFunctions()
+    public function getMissingFunctions(): array
     {
         return $this->missingFunctions;
     }
diff --git a/src/Result/Success.php b/src/Result/Success.php
index 9c1bfcfb..846acb9a 100644
--- a/src/Result/Success.php
+++ b/src/Result/Success.php
@@ -17,7 +17,7 @@ class Success implements SuccessInterface
     /**
      * @param string $name The name of the check that produced this result.
      */
-    public function __construct($name)
+    public function __construct(string $name)
     {
         $this->name = $name;
     }
@@ -26,11 +26,11 @@ public function __construct($name)
      * Static constructor to create from an instance of `PhpSchool\PhpWorkshop\Check\CheckInterface`.
      *
      * @param CheckInterface $check The check instance.
-     * @return static The result.
+     * @return self The result.
      */
     public static function fromCheck(CheckInterface $check)
     {
-        return new static($check->getName());
+        return new self($check->getName());
     }
 
     /**
@@ -38,7 +38,7 @@ public static function fromCheck(CheckInterface $check)
      *
      * @return string
      */
-    public function getCheckName()
+    public function getCheckName(): string
     {
         return $this->name;
     }
diff --git a/src/ResultAggregator.php b/src/ResultAggregator.php
index a3753bb9..dadeb624 100644
--- a/src/ResultAggregator.php
+++ b/src/ResultAggregator.php
@@ -12,6 +12,8 @@
 /**
  * This class is a container to hold all the results produced
  * throughout the verification of a students solution.
+ *
+ * @implements IteratorAggregate<int, ResultInterface>
  */
 class ResultAggregator implements IteratorAggregate
 {
@@ -25,7 +27,7 @@ class ResultAggregator implements IteratorAggregate
      *
      * @param ResultInterface $result
      */
-    public function add(ResultInterface $result)
+    public function add(ResultInterface $result): void
     {
         $this->results[] = $result;
     }
@@ -36,7 +38,7 @@ public function add(ResultInterface $result)
      *
      * @return bool
      */
-    public function isSuccessful()
+    public function isSuccessful(): bool
     {
         return count(
             array_filter($this->results, function ($result) {
@@ -51,9 +53,9 @@ public function isSuccessful()
     /**
      * Get an iterator in order to `foreach` the results.
      *
-     * @return ArrayIterator
+     * @return ArrayIterator<int, ResultInterface>
      */
-    public function getIterator()
+    public function getIterator(): ArrayIterator
     {
         return new ArrayIterator($this->results);
     }
diff --git a/src/ResultRenderer/Cgi/RequestFailureRenderer.php b/src/ResultRenderer/Cgi/RequestFailureRenderer.php
index efa91440..9cf2d1f8 100644
--- a/src/ResultRenderer/Cgi/RequestFailureRenderer.php
+++ b/src/ResultRenderer/Cgi/RequestFailureRenderer.php
@@ -30,7 +30,7 @@ public function __construct(RequestFailure $result)
      * @param ResultsRenderer $renderer
      * @return string
      */
-    public function render(ResultsRenderer $renderer)
+    public function render(ResultsRenderer $renderer): string
     {
         $output = '';
         if ($this->result->headersDifferent()) {
@@ -61,12 +61,12 @@ public function render(ResultsRenderer $renderer)
     }
 
     /**
-     * @param array $headers
+     * @param array<string, string> $headers
      * @param ResultsRenderer $renderer
      * @param bool $actual
      * @return string
      */
-    private function headers(array $headers, ResultsRenderer $renderer, $actual = true)
+    private function headers(array $headers, ResultsRenderer $renderer, $actual = true): string
     {
         $indent = false;
         $output = '';
diff --git a/src/ResultRenderer/CgiResultRenderer.php b/src/ResultRenderer/CgiResultRenderer.php
index 020c2f0f..cf6b6266 100644
--- a/src/ResultRenderer/CgiResultRenderer.php
+++ b/src/ResultRenderer/CgiResultRenderer.php
@@ -38,7 +38,7 @@ public function __construct(CgiResult $result, RequestRenderer $requestRenderer)
      * @param ResultsRenderer $renderer
      * @return string
      */
-    public function render(ResultsRenderer $renderer)
+    public function render(ResultsRenderer $renderer): string
     {
         $results = array_filter($this->result->getResults(), function (ResultInterface $result) {
             return $result instanceof FailureInterface;
diff --git a/src/ResultRenderer/CliResultRenderer.php b/src/ResultRenderer/CliResultRenderer.php
index 643d2454..23a08110 100644
--- a/src/ResultRenderer/CliResultRenderer.php
+++ b/src/ResultRenderer/CliResultRenderer.php
@@ -30,7 +30,7 @@ public function __construct(CliResult $result)
      * @param ResultsRenderer $renderer
      * @return string
      */
-    public function render(ResultsRenderer $renderer)
+    public function render(ResultsRenderer $renderer): string
     {
         $results = array_filter($this->result->getResults(), function (ResultInterface $result) {
             return $result instanceof FailureInterface;
diff --git a/src/ResultRenderer/ComparisonFailureRenderer.php b/src/ResultRenderer/ComparisonFailureRenderer.php
index 8c14c16f..752f097f 100644
--- a/src/ResultRenderer/ComparisonFailureRenderer.php
+++ b/src/ResultRenderer/ComparisonFailureRenderer.php
@@ -11,7 +11,7 @@
 class ComparisonFailureRenderer implements ResultRendererInterface
 {
     /**
-     * @var RequestFailure
+     * @var ComparisonFailure
      */
     private $result;
 
diff --git a/src/ResultRenderer/FailureRenderer.php b/src/ResultRenderer/FailureRenderer.php
index 403e1913..e141a6e7 100644
--- a/src/ResultRenderer/FailureRenderer.php
+++ b/src/ResultRenderer/FailureRenderer.php
@@ -28,8 +28,8 @@ public function __construct(Failure $result)
      * @param ResultsRenderer $renderer
      * @return string
      */
-    public function render(ResultsRenderer $renderer)
+    public function render(ResultsRenderer $renderer): string
     {
-        return $renderer->center($this->result->getReason()) . "\n";
+        return $renderer->center((string) $this->result->getReason()) . "\n";
     }
 }
diff --git a/src/ResultRenderer/ResultsRenderer.php b/src/ResultRenderer/ResultsRenderer.php
index 8a0feb4e..827bccfe 100644
--- a/src/ResultRenderer/ResultsRenderer.php
+++ b/src/ResultRenderer/ResultsRenderer.php
@@ -5,6 +5,7 @@
 use Colors\Color;
 use Kadet\Highlighter\Formatter\CliFormatter;
 use Kadet\Highlighter\KeyLighter;
+use PhpSchool\PhpWorkshop\Result\FailureInterface;
 use PhpSchool\Terminal\Terminal;
 use PhpSchool\CliMenu\Util\StringUtil;
 use PhpSchool\PhpWorkshop\Exercise\ProvidesSolution;
@@ -91,7 +92,7 @@ public function render(
         ExerciseInterface $exercise,
         UserState $userState,
         OutputInterface $output
-    ) {
+    ): void {
         $successes  = [];
         $failures   = [];
         foreach ($results as $result) {
@@ -104,7 +105,7 @@ public function render(
                 $failures[] = [$result, sprintf(' ✗ Check: %s', $result->getCheckName())];
             }
         }
-
+        /** @var array<int, array{0: FailureInterface, 1: string}> $failures */
         $output->emptyLine();
         $output->writeLine($this->center($this->style('*** RESULTS ***', ['magenta', 'bold'])));
         $output->emptyLine();
@@ -122,13 +123,14 @@ public function render(
         }
 
         if ($results->isSuccessful()) {
-            return $this->renderSuccessInformation($exercise, $userState, $output);
+            $this->renderSuccessInformation($exercise, $userState, $output);
+            return;
         }
         $this->renderErrorInformation($failures, $longest, $exercise, $output);
     }
 
     /**
-     * @param array $failures
+     * @param array<int, array{0: FailureInterface, 1: string}> $failures
      * @param int $padLength
      * @param ExerciseInterface $exercise
      * @param OutputInterface $output
@@ -138,8 +140,8 @@ private function renderErrorInformation(
         $padLength,
         ExerciseInterface $exercise,
         OutputInterface $output
-    ) {
-        foreach ($failures as list ($failure, $message)) {
+    ): void {
+        foreach ($failures as [$failure, $message]) {
             $output->writeLine($this->center($this->style(str_repeat(' ', $padLength), ['bg_red'])));
             $output->writeLine($this->center($this->style(\mb_str_pad($message, $padLength), ['bg_red'])));
             $output->writeLine($this->center($this->style(str_repeat(' ', $padLength), ['bg_red'])));
@@ -170,7 +172,7 @@ private function renderSuccessInformation(
         ExerciseInterface $exercise,
         UserState $userState,
         OutputInterface $output
-    ) {
+    ): void {
         $output->lineBreak();
         $output->emptyLine();
         $output->emptyLine();
@@ -213,7 +215,7 @@ private function renderSuccessInformation(
      * @param string $string
      * @return string string
      */
-    public function center($string)
+    public function center(string $string): string
     {
         $stringHalfLength = mb_strlen(StringUtil::stripAnsiEscapeSequence($string)) / 2;
         $widthHalfLength  = ceil($this->terminal->getWidth() / 2);
@@ -223,15 +225,15 @@ public function center($string)
             $start = 0;
         }
 
-        return str_repeat(' ', $start) . $string;
+        return str_repeat(' ', (int) $start) . $string;
     }
 
     /**
      * @param OutputInterface $output
-     * @param $string
-     * @param array $style
+     * @param string $string
+     * @param array<string> $style
      */
-    private function fullWidthBlock(OutputInterface $output, $string, array $style)
+    private function fullWidthBlock(OutputInterface $output, string $string, array $style): void
     {
         $stringLength     = mb_strlen(StringUtil::stripAnsiEscapeSequence($string));
         $stringHalfLength = $stringLength / 2;
@@ -243,9 +245,9 @@ private function fullWidthBlock(OutputInterface $output, $string, array $style)
             $this->style(
                 sprintf(
                     '%s%s%s',
-                    str_repeat(' ', $start),
+                    str_repeat(' ', (int) $start),
                     $string,
-                    str_repeat(' ', $this->terminal->getWidth() - $stringLength - $start)
+                    str_repeat(' ', (int) ($this->terminal->getWidth() - $stringLength - $start))
                 ),
                 $style
             )
@@ -258,12 +260,12 @@ private function fullWidthBlock(OutputInterface $output, $string, array $style)
      * Can be any of: black, red, green, yellow, blue, magenta, cyan, white, bold, italic, underline.
      *
      * @param string $string
-     * @param array|string $colourOrStyle A single style as a string or multiple styles as an array.
+     * @param array<string>|string $colourOrStyle A single style as a string or multiple styles as an array.
      *
      * @return string
      *
      */
-    public function style($string, $colourOrStyle)
+    public function style(string $string, $colourOrStyle): string
     {
         if (is_array($colourOrStyle)) {
             $this->color->__invoke($string);
@@ -283,7 +285,7 @@ public function style($string, $colourOrStyle)
      * @param ResultInterface $result The result.
      * @return string The string representation of the result.
      */
-    public function renderResult(ResultInterface $result)
+    public function renderResult(ResultInterface $result): string
     {
         return $this->resultRendererFactory->create($result)->render($this);
     }
@@ -293,7 +295,7 @@ public function renderResult(ResultInterface $result)
      *
      * @return string
      */
-    public function lineBreak()
+    public function lineBreak(): string
     {
         return $this->style(str_repeat('─', $this->terminal->getWidth()), 'yellow');
     }
diff --git a/src/Solution/DirectorySolution.php b/src/Solution/DirectorySolution.php
index 0d8e93bc..bdfdb052 100644
--- a/src/Solution/DirectorySolution.php
+++ b/src/Solution/DirectorySolution.php
@@ -12,14 +12,13 @@
  */
 class DirectorySolution implements SolutionInterface
 {
-
     /**
      * @var string
      */
     private $entryPoint;
 
     /**
-     * @var SolutionFile[]
+     * @var array<SolutionFile>
      */
     private $files = [];
 
@@ -35,18 +34,18 @@ class DirectorySolution implements SolutionInterface
      *
      * @param string $directory The directory to search for files.
      * @param string $entryPoint The relative path from the directory of the entry point file.
-     * @param array  $exclusions An array of file names to exclude from the folder.
+     * @param array<string> $exclusions An array of file names to exclude from the folder.
      * @throws InvalidArgumentException If the entry point does not exist in the folder.
      */
-    public function __construct($directory, $entryPoint, array $exclusions = [])
+    public function __construct(string $directory, string $entryPoint, array $exclusions = [])
     {
-        $directory  = realpath(rtrim($directory, '/'));
+        $directory  = (string) realpath(rtrim($directory, '/'));
         $entryPoint = ltrim($entryPoint, '/');
 
         $dir  = new RecursiveDirectoryIterator($directory, RecursiveDirectoryIterator::SKIP_DOTS);
         $iter = new RecursiveIteratorIterator(
             new RecursiveCallbackFilterIterator($dir, function (\SplFileInfo $current) use ($exclusions) {
-                return !in_array($current->getBasename(), $exclusions);
+                return !in_array($current->getBasename(), $exclusions, true);
             }),
             RecursiveIteratorIterator::SELF_FIRST
         );
@@ -77,13 +76,13 @@ public function __construct($directory, $entryPoint, array $exclusions = [])
      * Static constructor to build an instance from a directory.
      *
      * @param string $directory The directory to search for files.
-     * @param array  $exclusions An array of file names to exclude from the folder.
+     * @param array<string> $exclusions An array of file names to exclude from the folder.
      * @param string $entryPoint The relative path from the directory of the entry point file.
-     * @return static
+     * @return self
      */
-    public static function fromDirectory($directory, array $exclusions = [], $entryPoint = 'solution.php')
+    public static function fromDirectory(string $directory, array $exclusions = [], $entryPoint = 'solution.php'): self
     {
-        return new static($directory, $entryPoint, array_merge($exclusions, ['composer.lock', 'vendor']));
+        return new self($directory, $entryPoint, array_merge($exclusions, ['composer.lock', 'vendor']));
     }
 
     /**
@@ -92,7 +91,7 @@ public static function fromDirectory($directory, array $exclusions = [], $entryP
      *
      * @return string
      */
-    public function getEntryPoint()
+    public function getEntryPoint(): string
     {
         return $this->entryPoint;
     }
@@ -100,9 +99,9 @@ public function getEntryPoint()
     /**
      * Get all the files which are contained with the solution.
      *
-     * @return SolutionFile[]
+     * @return array<SolutionFile>
      */
-    public function getFiles()
+    public function getFiles(): array
     {
         return $this->files;
     }
@@ -112,7 +111,7 @@ public function getFiles()
      *
      * @return string
      */
-    public function getBaseDirectory()
+    public function getBaseDirectory(): string
     {
         return $this->baseDirectory;
     }
@@ -122,7 +121,7 @@ public function getBaseDirectory()
      *
      * @return bool
      */
-    public function hasComposerFile()
+    public function hasComposerFile(): bool
     {
         return file_exists(sprintf('%s/composer.lock', $this->baseDirectory));
     }
diff --git a/src/Solution/SingleFileSolution.php b/src/Solution/SingleFileSolution.php
index 9c579c15..7611c983 100644
--- a/src/Solution/SingleFileSolution.php
+++ b/src/Solution/SingleFileSolution.php
@@ -9,7 +9,6 @@
  */
 class SingleFileSolution implements SolutionInterface
 {
-
     /**
      * @var SolutionFile
      */
@@ -19,21 +18,21 @@ class SingleFileSolution implements SolutionInterface
      * @param string $file The absolute path of the reference solution.
      * @throws InvalidArgumentException If the file does not exist.
      */
-    public function __construct($file)
+    public function __construct(string $file)
     {
-        $this->file = SolutionFile::fromFile(realpath($file));
+        $this->file = SolutionFile::fromFile((string) realpath($file));
     }
 
     /**
      * Static constructor to build an instance from an absolute file path.
      *
      * @param string $file The absolute path of the reference solution.
-     * @return static
+     * @return self
      * @throws InvalidArgumentException If the file does not exist.
      */
-    public static function fromFile($file)
+    public static function fromFile(string $file): self
     {
-        return new static($file);
+        return new self($file);
     }
 
     /**
@@ -42,7 +41,7 @@ public static function fromFile($file)
      *
      * @return string
      */
-    public function getEntryPoint()
+    public function getEntryPoint(): string
     {
         return $this->file->__toString();
     }
@@ -50,7 +49,7 @@ public function getEntryPoint()
     /**
      * Get all the files which are contained with the solution.
      *
-     * @return SolutionFile[]
+     * @return array<SolutionFile>
      */
     public function getFiles(): array
     {
@@ -62,7 +61,7 @@ public function getFiles(): array
      *
      * @return string
      */
-    public function getBaseDirectory()
+    public function getBaseDirectory(): string
     {
         return $this->file->getBaseDirectory();
     }
@@ -73,7 +72,7 @@ public function getBaseDirectory()
      *
      * @return bool
      */
-    public function hasComposerFile()
+    public function hasComposerFile(): bool
     {
         return false;
     }
diff --git a/src/Solution/SolutionFile.php b/src/Solution/SolutionFile.php
index b052ab38..47f5d988 100644
--- a/src/Solution/SolutionFile.php
+++ b/src/Solution/SolutionFile.php
@@ -24,7 +24,7 @@ class SolutionFile
      * @param string $baseDirectory The base directory of the solution.
      * @throws InvalidArgumentException If the file does not exist.
      */
-    public function __construct($relativePath, $baseDirectory)
+    public function __construct(string $relativePath, string $baseDirectory)
     {
         $this->relativePath  = trim($relativePath, '/');
         $this->baseDirectory = rtrim($baseDirectory, '/');
@@ -39,11 +39,11 @@ public function __construct($relativePath, $baseDirectory)
      * Will assume the base directory should be the immediate parent of the file.
      *
      * @param string $file
-     * @return static
+     * @return self
      */
-    public static function fromFile($file)
+    public static function fromFile(string $file): self
     {
-        return new static(basename($file), dirname($file));
+        return new self(basename($file), dirname($file));
     }
 
     /**
@@ -51,7 +51,7 @@ public static function fromFile($file)
      *
      * @return string
      */
-    private function getAbsolutePath()
+    private function getAbsolutePath(): string
     {
         return sprintf('%s/%s', $this->baseDirectory, $this->relativePath);
     }
@@ -61,7 +61,7 @@ private function getAbsolutePath()
      *
      * @return string
      */
-    public function getRelativePath()
+    public function getRelativePath(): string
     {
         return $this->relativePath;
     }
@@ -71,7 +71,7 @@ public function getRelativePath()
      *
      * @return string
      */
-    public function getBaseDirectory()
+    public function getBaseDirectory(): string
     {
         return $this->baseDirectory;
     }
@@ -81,9 +81,9 @@ public function getBaseDirectory()
      *
      * @return string
      */
-    public function getContents()
+    public function getContents(): string
     {
-        return file_get_contents($this->getAbsolutePath());
+        return (string) file_get_contents($this->getAbsolutePath());
     }
 
     /**
@@ -91,7 +91,7 @@ public function getContents()
      *
      * @return string
      */
-    public function getExtension()
+    public function getExtension(): string
     {
         return pathinfo($this->getRelativePath(), PATHINFO_EXTENSION);
     }
@@ -101,7 +101,7 @@ public function getExtension()
      *
      * @return string
      */
-    public function __toString()
+    public function __toString(): string
     {
         return $this->getAbsolutePath();
     }
diff --git a/src/Solution/SolutionInterface.php b/src/Solution/SolutionInterface.php
index 7301e1ef..5df660c1 100644
--- a/src/Solution/SolutionInterface.php
+++ b/src/Solution/SolutionInterface.php
@@ -13,26 +13,26 @@ interface SolutionInterface
      *
      * @return string
      */
-    public function getEntryPoint();
+    public function getEntryPoint(): string;
 
     /**
      * Get all the files which are contained with the solution.
      *
-     * @return SolutionFile[]
+     * @return array<SolutionFile>
      */
-    public function getFiles();
+    public function getFiles(): array;
 
     /**
      * Get the absolute path to the directory containing the solution.
      *
      * @return string
      */
-    public function getBaseDirectory();
+    public function getBaseDirectory(): string;
 
     /**
      * Whether or not the solution has a `composer.json` & `composer.lock` file.
      *
      * @return bool
      */
-    public function hasComposerFile();
+    public function hasComposerFile(): bool;
 }
diff --git a/src/UserState.php b/src/UserState.php
index ff4543ab..33b1b0d2 100644
--- a/src/UserState.php
+++ b/src/UserState.php
@@ -2,20 +2,21 @@
 
 namespace PhpSchool\PhpWorkshop;
 
+use PhpSchool\PhpWorkshop\Exception\ExerciseNotAssignedException;
+
 /**
  * This class represents the current state of the user. Which exercises she/he has completed and
  * which is the current exercise being attempted.
  */
 class UserState
 {
-
     /**
-     * @var string
+     * @var string|null
      */
     private $currentExercise;
 
     /**
-     * @var array
+     * @var array<string>
      */
     private $completedExercises;
 
@@ -23,10 +24,10 @@ class UserState
      * Take an array of completed exercises (the exercise names) and a string containing the current
      * exercise.
      *
-     * @param array $completedExercises An array of exercise names.
-     * @param string $currentExercise Can be null in-case the student did not start an exercise yet.
+     * @param array<string> $completedExercises An array of exercise names.
+     * @param string|null $currentExercise Can be null in-case the student did not start an exercise yet.
      */
-    public function __construct(array $completedExercises = [], $currentExercise = null)
+    public function __construct(array $completedExercises = [], string $currentExercise = null)
     {
         $this->currentExercise = $currentExercise;
         $this->completedExercises = $completedExercises;
@@ -37,9 +38,9 @@ public function __construct(array $completedExercises = [], $currentExercise = n
      *
      * @param string $exercise
      */
-    public function addCompletedExercise($exercise)
+    public function addCompletedExercise(string $exercise): void
     {
-        if (!in_array($exercise, $this->completedExercises)) {
+        if (!in_array($exercise, $this->completedExercises, true)) {
             $this->completedExercises[] = $exercise;
         }
     }
@@ -49,7 +50,7 @@ public function addCompletedExercise($exercise)
      *
      * @param string $exercise
      */
-    public function setCurrentExercise($exercise)
+    public function setCurrentExercise(string $exercise): void
     {
         $this->currentExercise = $exercise;
     }
@@ -59,17 +60,21 @@ public function setCurrentExercise($exercise)
      *
      * @return string
      */
-    public function getCurrentExercise()
+    public function getCurrentExercise(): string
     {
+        if (null === $this->currentExercise) {
+            throw ExerciseNotAssignedException::notAssigned();
+        }
+
         return $this->currentExercise;
     }
 
     /**
      * Get an array of the completed exercises (the exercise names).
      *
-     * @return array
+     * @return array<string>
      */
-    public function getCompletedExercises()
+    public function getCompletedExercises(): array
     {
         return $this->completedExercises;
     }
@@ -79,7 +84,7 @@ public function getCompletedExercises()
      *
      * @return bool
      */
-    public function isAssignedExercise()
+    public function isAssignedExercise(): bool
     {
         return null !== $this->currentExercise;
     }
@@ -90,8 +95,8 @@ public function isAssignedExercise()
      * @param string $exercise
      * @return bool
      */
-    public function completedExercise($exercise)
+    public function completedExercise(string $exercise): bool
     {
-        return in_array($exercise, $this->completedExercises);
+        return in_array($exercise, $this->completedExercises, true);
     }
 }
diff --git a/src/UserStateSerializer.php b/src/UserStateSerializer.php
index afcd5524..e2a5a7a0 100644
--- a/src/UserStateSerializer.php
+++ b/src/UserStateSerializer.php
@@ -52,9 +52,9 @@ public function __construct($saveFileDirectory, $workshopName, ExerciseRepositor
      * Save the students state for this workshop to disk.
      *
      * @param UserState $state
-     * @return int
+     * @return void
      */
-    public function serialize(UserState $state)
+    public function serialize(UserState $state): void
     {
         $saveFile = sprintf('%s/%s', $this->path, static::SAVE_FILE);
 
@@ -64,10 +64,10 @@ public function serialize(UserState $state)
 
         $data[$this->workshopName] = [
             'completed_exercises'   => $state->getCompletedExercises(),
-            'current_exercise'      => $state->getCurrentExercise(),
+            'current_exercise'      => $state->isAssignedExercise() ? $state->getCurrentExercise() : null,
         ];
 
-        return file_put_contents($saveFile, json_encode($data));
+        file_put_contents($saveFile, json_encode($data));
     }
 
     /**
@@ -75,7 +75,7 @@ public function serialize(UserState $state)
      *
      * @return UserState
      */
-    public function deSerialize()
+    public function deSerialize(): UserState
     {
         $legacySaveFile = sprintf('%s/%s', $this->path, static::LEGACY_SAVE_FILE);
         if (file_exists($legacySaveFile)) {
@@ -136,7 +136,7 @@ public function deSerialize()
      * @param string $legacySaveFile
      * @return null|UserState
      */
-    private function migrateData($legacySaveFile)
+    private function migrateData(string $legacySaveFile): ?UserState
     {
         $data = $this->readJson($legacySaveFile);
 
@@ -171,15 +171,15 @@ private function migrateData($legacySaveFile)
 
     /**
      * @param string $filePath
-     * @return array|null
+     * @return array<mixed>|null
      */
-    private function readJson($filePath)
+    private function readJson($filePath): ?array
     {
         if (!file_exists($filePath)) {
             return null;
         }
 
-        $data = file_get_contents($filePath);
+        $data = (string) file_get_contents($filePath);
 
         if (trim($data) === "") {
             return null;
@@ -197,7 +197,7 @@ private function readJson($filePath)
     /**
      * Remove the file
      */
-    private function wipeFile()
+    private function wipeFile(): void
     {
         @unlink($this->path);
     }
diff --git a/src/Utils/ArrayObject.php b/src/Utils/ArrayObject.php
index b22990e9..62424202 100644
--- a/src/Utils/ArrayObject.php
+++ b/src/Utils/ArrayObject.php
@@ -7,22 +7,26 @@
 use IteratorAggregate;
 
 /**
+ *
  * Utility collection class.
+ *
+ * @template T
+ * @implements IteratorAggregate<int, T>
  */
 class ArrayObject implements IteratorAggregate, Countable
 {
 
     /**
-     * @var array
+     * @var array<T>
      */
     private $array;
 
     /**
      * Accepts an array of items.
      *
-     * @param array $array
+     * @param array<T> $array
      */
-    public function __construct(array $array = [])
+    final public function __construct(array $array = [])
     {
         $this->array = $array;
     }
@@ -34,11 +38,20 @@ public function __construct(array $array = [])
      * @param callable $callback
      * @return static
      */
-    public function map(callable $callback)
+    public function map(callable $callback): self
     {
         return new static(array_map($callback, $this->array));
     }
 
+    /**
+     * @param callable $callback
+     * @return static
+     */
+    public function filter(callable $callback): self
+    {
+        return new static(array_filter($this->array, $callback));
+    }
+
     /**
      * Run a callable over each item in the array and flatten the results by one level returning a new instance of
      * `ArrayObject` with the flattened items.
@@ -46,7 +59,7 @@ public function map(callable $callback)
      * @param callable $callback
      * @return static
      */
-    public function flatMap(callable $callback)
+    public function flatMap(callable $callback): self
     {
         return $this->map($callback)->collapse();
     }
@@ -57,7 +70,7 @@ public function flatMap(callable $callback)
      *
      * @return static
      */
-    public function collapse()
+    public function collapse(): self
     {
         $results = [];
 
@@ -75,8 +88,8 @@ public function collapse()
     /**
      * Reduce the items to a single value.
      *
-     * @param  callable  $callback
-     * @param  mixed     $initial
+     * @param callable $callback
+     * @param mixed $initial
      * @return mixed
      */
     public function reduce(callable $callback, $initial = null)
@@ -87,7 +100,7 @@ public function reduce(callable $callback, $initial = null)
     /**
      * @return static
      */
-    public function keys()
+    public function keys(): self
     {
         return new static(array_keys($this->array));
     }
@@ -98,7 +111,7 @@ public function keys()
      * @param string $glue
      * @return string
      */
-    public function implode($glue)
+    public function implode(string $glue): string
     {
         return implode($glue, $this->array);
     }
@@ -106,10 +119,10 @@ public function implode($glue)
     /**
      * Add a new item on to the beginning of the collection. A new instance is returned.
      *
-     * @param mixed $value
+     * @param T $value
      * @return static
      */
-    public function prepend($value)
+    public function prepend($value): self
     {
         return new static(array_merge([$value], $this->array));
     }
@@ -117,10 +130,10 @@ public function prepend($value)
     /**
      * Add a new item to the end of the collection. A new instance is returned.
      *
-     * @param mixed $value
+     * @param T $value
      * @return static
      */
-    public function append($value)
+    public function append($value): self
     {
         return new static(array_merge($this->array, [$value]));
     }
@@ -128,11 +141,11 @@ public function append($value)
     /**
      * Get an item at the given key.
      *
-     * @param mixed $key
+     * @param string $key
      * @param mixed $default
-     * @return mixed
+     * @return T|mixed
      */
-    public function get($key, $default = null)
+    public function get(string $key, $default = null)
     {
         if (isset($this->array[$key])) {
             return $this->array[$key];
@@ -144,11 +157,11 @@ public function get($key, $default = null)
     /**
      * Set the item at a given offset and return a new instance.
      *
-     * @param mixed $key
-     * @param mixed $value
+     * @param string $key
+     * @param T $value
      * @return static
      */
-    public function set($key, $value)
+    public function set(string $key, $value): self
     {
         $items = $this->array;
         $items[$key] = $value;
@@ -158,9 +171,9 @@ public function set($key, $value)
     /**
      * Return an iterator containing all the items. Allows to `foreach` over.
      *
-     * @return ArrayIterator
+     * @return ArrayIterator<int, T>
      */
-    public function getIterator()
+    public function getIterator(): ArrayIterator
     {
         return new ArrayIterator($this->array);
     }
@@ -168,9 +181,9 @@ public function getIterator()
     /**
      * Get all the items in the array.
      *
-     * @return array
+     * @return array<T>
      */
-    public function getArrayCopy()
+    public function getArrayCopy(): array
     {
         return $this->array;
     }
@@ -180,7 +193,7 @@ public function getArrayCopy()
      *
      * @return int
      */
-    public function count()
+    public function count(): int
     {
         return count($this->array);
     }
@@ -188,7 +201,7 @@ public function count()
     /**
      * @return bool
      */
-    public function isEmpty()
+    public function isEmpty(): bool
     {
         return $this->array === [];
     }
diff --git a/src/Utils/Collection.php b/src/Utils/Collection.php
index a4dc642e..2bb60621 100644
--- a/src/Utils/Collection.php
+++ b/src/Utils/Collection.php
@@ -4,6 +4,9 @@
 
 /**
  * ArrayObject is not a good name...
+ *
+ * @template T
+ * @extends ArrayObject<T>
  */
 class Collection extends ArrayObject
 {
diff --git a/src/WorkshopType.php b/src/WorkshopType.php
index b6b138b4..6cf6c93e 100644
--- a/src/WorkshopType.php
+++ b/src/WorkshopType.php
@@ -7,6 +7,8 @@
 /**
  * @method static WorkshopType STANDARD()
  * @method static WorkshopType TUTORIAL()
+ *
+ * @extends Enum<int>
  */
 class WorkshopType extends Enum
 {
@@ -16,7 +18,7 @@ class WorkshopType extends Enum
     /**
      * @return bool
      */
-    public function isTutorialMode()
+    public function isTutorialMode(): bool
     {
         return $this->getValue() === static::TUTORIAL;
     }
diff --git a/src/functions.php b/src/functions.php
index 2496b3c4..d792efab 100644
--- a/src/functions.php
+++ b/src/functions.php
@@ -9,7 +9,7 @@
      * @param int $padType
      * @return string
      */
-    function mb_str_pad($input, $padLength, $padString = ' ', $padType = STR_PAD_RIGHT)
+    function mb_str_pad(string $input, int $padLength, string $padString = ' ', int $padType = STR_PAD_RIGHT): string
     {
         $diff = strlen($input) - mb_strlen($input);
         return str_pad($input, $padLength + $diff, $padString, $padType);
@@ -22,9 +22,9 @@ function mb_str_pad($input, $padLength, $padString = ' ', $padType = STR_PAD_RIG
      * @param string $string
      * @return string
      */
-    function camel_case_to_kebab_case($string)
+    function camel_case_to_kebab_case(string $string): string
     {
-        return preg_replace_callback('/[A-Z]/', function ($matches) {
+        return (string) preg_replace_callback('/[A-Z]/', function ($matches) {
             return '-' . strtolower($matches[0]);
         }, $string);
     }
diff --git a/test/CodeInsertionTest.php b/test/CodeInsertionTest.php
index cbcd49da..b385ca71 100644
--- a/test/CodeInsertionTest.php
+++ b/test/CodeInsertionTest.php
@@ -14,12 +14,6 @@ public function testInvalidType(): void
         new CodeInsertion('notatype', '');
     }
 
-    public function testInvalidCode(): void
-    {
-        $this->expectException(InvalidArgumentException::class);
-        new CodeInsertion(CodeInsertion::TYPE_BEFORE, new \stdClass());
-    }
-
     public function testGetters(): void
     {
         $mod = new CodeInsertion(CodeInsertion::TYPE_BEFORE, '<?php codez');
diff --git a/test/Command/VerifyCommandTest.php b/test/Command/VerifyCommandTest.php
index 8ea59b8f..7e691f78 100644
--- a/test/Command/VerifyCommandTest.php
+++ b/test/Command/VerifyCommandTest.php
@@ -59,7 +59,7 @@ public function testVerifyAddsCompletedExerciseAndReturnsCorrectCodeOnSuccess():
         $renderer = $this->createMock(ResultsRenderer::class);
 
         $results = new ResultAggregator();
-        $results->add(new Success($this->check));
+        $results->add(Success::fromCheck($this->check));
 
         $dispatcher = $this->createMock(ExerciseDispatcher::class);
 
@@ -106,7 +106,7 @@ public function testVerifyDoesNotAddCompletedExerciseAndReturnsCorrectCodeOnFail
         $renderer = $this->createMock(ResultsRenderer::class);
 
         $results = new ResultAggregator();
-        $results->add(new Failure($this->check, 'cba'));
+        $results->add(Failure::fromCheckAndReason($this->check, 'cba'));
 
         $dispatcher = $this->createMock(ExerciseDispatcher::class);
 
diff --git a/test/Factory/ResultRendererFactoryTest.php b/test/Factory/ResultRendererFactoryTest.php
index 6088f325..110dad9a 100644
--- a/test/Factory/ResultRendererFactoryTest.php
+++ b/test/Factory/ResultRendererFactoryTest.php
@@ -33,28 +33,6 @@ public function testRegisterRendererRequiresResultRendererInterface(): void
         $factory->registerRenderer($resultClass, $rendererClass);
     }
 
-    public function testRegisterRendererRequiresResultClassToBeString(): void
-    {
-        $this->expectException(InvalidArgumentException::class);
-
-        $resultClass = $this->createMock(ResultInterface::class);
-        $rendererClass = get_class($this->createMock(ResultRendererInterface::class));
-        $factory = new ResultRendererFactory();
-        
-        $factory->registerRenderer($resultClass, $rendererClass);
-    }
-
-    public function testRegisterRendererRequiresRendererClassToBeString(): void
-    {
-        $this->expectException(InvalidArgumentException::class);
-
-        $resultClass = get_class($this->createMock(ResultInterface::class));
-        $rendererClass = $this->createMock(ResultRendererInterface::class);
-        $factory = new ResultRendererFactory();
-        
-        $factory->registerRenderer($resultClass, $rendererClass);
-    }
-
     public function testCreateRequiresMappingToClassName(): void
     {
         $this->expectException(RuntimeException::class);
diff --git a/test/ResultAggregatorTest.php b/test/ResultAggregatorTest.php
index e34da8d4..77eceff6 100644
--- a/test/ResultAggregatorTest.php
+++ b/test/ResultAggregatorTest.php
@@ -32,10 +32,10 @@ public function testIsSuccessful(): void
         $resultAggregator = new ResultAggregator();
         $this->assertTrue($resultAggregator->isSuccessful());
 
-        $resultAggregator->add(new Success($this->check));
+        $resultAggregator->add(Success::fromCheck($this->check));
         $this->assertTrue($resultAggregator->isSuccessful());
 
-        $resultAggregator->add(new Failure($this->check, 'nope'));
+        $resultAggregator->add(Failure::fromCheckAndReason($this->check, 'nope'));
         $this->assertFalse($resultAggregator->isSuccessful());
     }
 
@@ -58,8 +58,8 @@ public function testIsSuccessfulWithResultGroups(): void
     public function testIterator(): void
     {
         $results = [
-            new Success($this->check),
-            new Failure($this->check, 'nope')
+            new Success('Some Check'),
+            new Failure('Some Check', 'nope')
         ];
         
         $resultAggregator = new ResultAggregator();
diff --git a/test/ResultRenderer/FailureRendererTest.php b/test/ResultRenderer/FailureRendererTest.php
index 6b1959af..9884f07f 100644
--- a/test/ResultRenderer/FailureRendererTest.php
+++ b/test/ResultRenderer/FailureRendererTest.php
@@ -10,7 +10,7 @@ class FailureRendererTest extends AbstractResultRendererTest
 {
     public function testRender(): void
     {
-        $failure = new Failure($this->createMock(CheckInterface::class), 'Something went wrong');
+        $failure = new Failure('My check', 'Something went wrong');
         $renderer = new FailureRenderer($failure);
         $this->assertEquals("               Something went wrong\n", $renderer->render($this->getRenderer()));
     }
diff --git a/test/UserStateSerializerTest.php b/test/UserStateSerializerTest.php
index c2980a41..75a6e50c 100644
--- a/test/UserStateSerializerTest.php
+++ b/test/UserStateSerializerTest.php
@@ -66,8 +66,7 @@ public function testSerializeEmptySate(): void
             ]
         ]);
 
-        $res = $serializer->serialize($state);
-        $this->assertTrue($res > 0);
+        $serializer->serialize($state);
         $this->assertSame($expected, file_get_contents($this->tmpFile));
     }
 
@@ -86,8 +85,8 @@ public function testSerialize(): void
             ]
         ]);
 
-        $res = $serializer->serialize($state);
-        $this->assertTrue($res > 0);
+        $serializer->serialize($state);
+
         $this->assertSame($expected, file_get_contents($this->tmpFile));
     }
 
@@ -96,7 +95,7 @@ public function testDeserializeNonExistingFile(): void
         mkdir($this->tmpDir, 0777, true);
         $serializer = new UserStateSerializer($this->tmpDir, $this->workshopName, $this->exerciseRepository);
         $state = $serializer->deSerialize();
-        $this->assertNull($state->getCurrentExercise());
+        $this->assertFalse($state->isAssignedExercise());
         $this->assertEmpty($state->getCompletedExercises());
     }
 
@@ -106,7 +105,7 @@ public function testDeserializeEmptyFile(): void
         file_put_contents($this->tmpFile, '');
         $serializer = new UserStateSerializer($this->tmpDir, $this->workshopName, $this->exerciseRepository);
         $state = $serializer->deSerialize();
-        $this->assertNull($state->getCurrentExercise());
+        $this->assertFalse($state->isAssignedExercise());
         $this->assertEmpty($state->getCompletedExercises());
     }
 
@@ -116,7 +115,7 @@ public function testDeserializeNonValidJson(): void
         file_put_contents($this->tmpFile, 'yayayayayanotjson');
         $serializer = new UserStateSerializer($this->tmpDir, $this->workshopName, $this->exerciseRepository);
         $state = $serializer->deSerialize();
-        $this->assertNull($state->getCurrentExercise());
+        $this->assertFalse($state->isAssignedExercise());
         $this->assertEmpty($state->getCompletedExercises());
     }
 
@@ -131,7 +130,10 @@ public function testDeserialize(array $data, array $expected): void
         $state = $serializer->deSerialize();
 
         $this->assertEquals($expected['completed_exercises'], $state->getCompletedExercises());
-        $this->assertEquals($expected['current_exercise'], $state->getCurrentExercise());
+        $this->assertEquals(
+            $expected['current_exercise'],
+            $state->isAssignedExercise() ? $state->getCurrentExercise() : null
+        );
 
         if (file_exists($this->tmpFile)) {
             unlink($this->tmpFile);
@@ -256,7 +258,7 @@ public function testOldDataWillNotBeMigratedWhenNotInCorrectWorkshop(): void
         $state = $serializer->deSerialize();
 
         $this->assertEquals([], $state->getCompletedExercises());
-        $this->assertEquals(null, $state->getCurrentExercise());
+        $this->assertFalse($state->isAssignedExercise());
 
         $this->assertFileExists($oldSave);
         $this->assertEquals($oldData, json_decode(file_get_contents($oldSave), true));
diff --git a/test/UserStateTest.php b/test/UserStateTest.php
index 60db1b10..166dc629 100644
--- a/test/UserStateTest.php
+++ b/test/UserStateTest.php
@@ -2,6 +2,7 @@
 
 namespace PhpSchool\PhpWorkshopTest;
 
+use PhpSchool\PhpWorkshop\Exception\ExerciseNotAssignedException;
 use PHPUnit\Framework\TestCase;
 use PhpSchool\PhpWorkshop\UserState;
 
@@ -12,7 +13,16 @@ public function testWithNoCurrentExercisesOrCompleted(): void
         $state = new UserState();
         $this->assertFalse($state->isAssignedExercise());
         $this->assertEmpty($state->getCompletedExercises());
-        $this->assertNull($state->getCurrentExercise());
+    }
+
+    public function testGetCurrentExerciseThrowsExceptionIfNonAssigned(): void
+    {
+        $this->expectException(ExerciseNotAssignedException::class);
+        $this->expectExceptionMessage('Student has no exercise assigned');
+
+        $state = new UserState();
+        $this->assertFalse($state->isAssignedExercise());
+        $state->getCurrentExercise();
     }
 
     public function testWithCurrentExerciseButNoCompleted(): void
@@ -36,7 +46,6 @@ public function testWithCompletedExerciseButNoCurrent(): void
         $state = new UserState(['exercise1']);
         $this->assertFalse($state->isAssignedExercise());
         $this->assertSame(['exercise1'], $state->getCompletedExercises());
-        $this->assertNull($state->getCurrentExercise());
     }
 
     public function testAddCompletedExercise(): void
@@ -56,7 +65,6 @@ public function testSetCompletedExercise(): void
     {
         $state = new UserState(['exercise1']);
         $this->assertFalse($state->isAssignedExercise());
-        $this->assertNull($state->getCurrentExercise());
         $this->assertSame(['exercise1'], $state->getCompletedExercises());
 
         $state->setCurrentExercise('exercise2');