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Rewrote case classes tour #715

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144 changes: 29 additions & 115 deletions tutorials/tour/_posts/2017-02-13-case-classes.md
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Original file line number Diff line number Diff line change
Expand Up @@ -9,135 +9,49 @@ categories: tour
num: 11
next-page: pattern-matching
previous-page: currying
prerequisite-knowledge: classes, basics, mutability
---

Scala supports the notion of _case classes_. Case classes are just regular classes that are:

* Immutable by default
* Decomposable through [pattern matching](pattern-matching.html)
* Compared by structural equality instead of by reference
* Succinct to instantiate and operate on

Here is an example for a Notification type hierarchy which consists of an abstract super class `Notification` and three concrete Notification types implemented with case classes `Email`, `SMS`, and `VoiceRecording`.
Case classes are like regular classes with a few key differences which we will go over. Case classes are good for modeling immutable data. In the next step of the tour, we'll see how they are useful in [pattern matching](pattern-matching.html).

## Defining a case class
A minimal case class requires the keywords `case class`, an identifier, and a parameter list (which may be empty):
```tut
abstract class Notification
case class Email(sourceEmail: String, title: String, body: String) extends Notification
case class SMS(sourceNumber: String, message: String) extends Notification
case class VoiceRecording(contactName: String, link: String) extends Notification
```
case class Book(isbn: String)

Instantiating a case class is easy: (Note that we don't need to use the `new` keyword)

```tut
val emailFromJohn = Email("[email protected]", "Greetings From John!", "Hello World!")
val frankenstein = Book("978-0486282114")
```
Notice how the keyword `new` was not used to instantiate the `Message` case class. This is because case classes have an `apply` method by default which takes care of object construction.

The constructor parameters of case classes are treated as public values and can be accessed directly.

```tut
val title = emailFromJohn.title
println(title) // prints "Greetings From John!"
When you create a case class with parameters, the parameters are public `val`s.
```
case class Message(sender: String, recipient: String, body: String)
val message1 = Message("[email protected]", "[email protected]", "Ça va ?")

With case classes, you cannot mutate their fields directly. (unless you insert `var` before a field, but doing so is generally discouraged).

```tut:fail
emailFromJohn.title = "Goodbye From John!" // This is a compilation error. We cannot assign another value to val fields, which all case classes fields are by default.
println(message1.sender) // prints [email protected]
message1.sender = "[email protected]" // this line does not compile
```
You can't reassign `message1.sender` because it is a `val` (i.e. immutable). It is possible to use `var`s in case classes but this is discouraged.

Instead, you make a copy using the `copy` method. As seen below, you can replace just some of the fields:

```tut
val editedEmail = emailFromJohn.copy(title = "I am learning Scala!", body = "It's so cool!")

println(emailFromJohn) // prints "Email([email protected],Greetings From John!,Hello World!)"
println(editedEmail) // prints "Email([email protected],I am learning Scala,It's so cool!)"
## Comparison
Case classes are compared by structure and not by reference:
```
case class Message(sender: String, recipient: String, body: String)

For every case class the Scala compiler generates an `equals` method which implements structural equality and a `toString` method. For instance:

```tut
val firstSms = SMS("12345", "Hello!")
val secondSms = SMS("12345", "Hello!")

if (firstSms == secondSms) {
println("They are equal!")
}

println("SMS is: " + firstSms)
val message2 = Message("[email protected]", "[email protected]", "Com va?")
val message3 = Message("[email protected]", "[email protected]", "Com va?")
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These examples are excellent!

val messagesAreTheSame = message2 == message3 // true
```
Even though `message2` and `message3` refer to different objects, the value of each object is equal.

will print

## Copying
You can create a deep copy of an instance of a case class simply by using the `copy` method. You can optionally change the constructor arguments.
```
They are equal!
SMS is: SMS(12345, Hello!)
case class Message(sender: String, recipient: String, body: String)
val message4 = Message("[email protected]", "[email protected]", "Me zo o komz gant ma amezeg")
val message5 = message4.copy(sender = message4.recipient, recipient = "[email protected]")
message5.sender // [email protected]
message5.recipient // [email protected]
message5.body // "Me zo o komz gant ma amezeg"
```

With case classes, you can utilize **pattern matching** to work with your data. Here's a function that prints out different messages depending on what type of Notification is received:

```tut
def showNotification(notification: Notification): String = {
notification match {
case Email(email, title, _) =>
"You got an email from " + email + " with title: " + title
case SMS(number, message) =>
"You got an SMS from " + number + "! Message: " + message
case VoiceRecording(name, link) =>
"you received a Voice Recording from " + name + "! Click the link to hear it: " + link
}
}

val someSms = SMS("12345", "Are you there?")
val someVoiceRecording = VoiceRecording("Tom", "voicerecording.org/id/123")

println(showNotification(someSms))
println(showNotification(someVoiceRecording))

// prints:
// You got an SMS from 12345! Message: Are you there?
// you received a Voice Recording from Tom! Click the link to hear it: voicerecording.org/id/123
```

Here's a more involved example using `if` guards. With the `if` guard, the pattern match branch will fail if the condition in the guard returns false.

```tut
def showNotificationSpecial(notification: Notification, specialEmail: String, specialNumber: String): String = {
notification match {
case Email(email, _, _) if email == specialEmail =>
"You got an email from special someone!"
case SMS(number, _) if number == specialNumber =>
"You got an SMS from special someone!"
case other =>
showNotification(other) // nothing special, delegate to our original showNotification function
}
}

val SPECIAL_NUMBER = "55555"
val SPECIAL_EMAIL = "[email protected]"
val someSms = SMS("12345", "Are you there?")
val someVoiceRecording = VoiceRecording("Tom", "voicerecording.org/id/123")
val specialEmail = Email("[email protected]", "Drinks tonight?", "I'm free after 5!")
val specialSms = SMS("55555", "I'm here! Where are you?")

println(showNotificationSpecial(someSms, SPECIAL_EMAIL, SPECIAL_NUMBER))
println(showNotificationSpecial(someVoiceRecording, SPECIAL_EMAIL, SPECIAL_NUMBER))
println(showNotificationSpecial(specialEmail, SPECIAL_EMAIL, SPECIAL_NUMBER))
println(showNotificationSpecial(specialSms, SPECIAL_EMAIL, SPECIAL_NUMBER))

// prints:
// You got an SMS from 12345! Message: Are you there?
// you received a Voice Recording from Tom! Click the link to hear it: voicerecording.org/id/123
// You got an email from special someone!
// You got an SMS from special someone!

```

When programming in Scala, it is recommended that you use case classes pervasively to model/group data as they help you to write more expressive and maintainable code:

* Immutability frees you from needing to keep track of where and when things are mutated
* Comparison-by-value allows you compare instances as if they are primitive values - no more uncertainty regarding whether instances of a class is compared by value or reference
* Pattern matching simplifies branching logic, which leads to less bugs and more readable code.


The recipient of `message4` is used as the sender of `message5` but the `body` of `message4` was copied directly.