test: Multiple solution from engine

Author: kelvin-olaiya

src/main/scala/io/github/kelvindev15/prolog/engine/Solver.scala

@@ -14,6 +14,8 @@ trait Solver:
 
 object Solver:
   type Substitution = Map[Variable, Term]
+  object Substitution:
+    def apply(substitutions: (Variable, Term)*): Substitution = Map(substitutions*)
   enum Solution:
     case Yes(query: Struct, substitution: Substitution)
     case No(query: Struct)
@@ -23,3 +25,6 @@ object Solver:
 
   def solve(using solver: Solver = tuPrologSolver())(prologProgram: PrologProgram): Iterator[Solution] = 
     solver solve prologProgram
+
+  def lazySolve(using solver: Solver = tuPrologSolver())(prologProgram: PrologProgram): LazyList[Solution] =
+    solver lazySolve prologProgram

src/test/scala/io/github/kelvindev15/engine/TestPrologEngine.scala

@@ -42,4 +42,16 @@ class TestPrologEngine extends AnyFlatSpec with Matchers with TestUtils with Pro
 
   "The goal 'X = 2, Y = 3, 5 is X + Y'" should "give a Yes solution" in:
     expectOne[Solution.Yes]:
-      Solver solve (PrologProgram.emptyTheory withGoal &&(X `=` 2, Y `=` 3, 5 is X + Y))
+      Solver solve (PrologProgram.emptyTheory withGoal &&(X `=` 2, Y `=` 3, 5 is X + Y))
+
+  "All solution" should "be returned by the engine" in:
+    val fruit = "fruit"
+    val fruits = Seq("banana", "orange", "apple")
+    val query = fruit(X)
+    Solver lazySolve {
+      prolog:
+        staticTheory:
+          for f <- fruits do clause(fruit(f))
+        goal:
+          query
+    } expectSolutionsIn fruits.map(e => query.yes(X -> e))

src/test/scala/io/github/kelvindev15/engine/utils/TestUtils.scala

@@ -1,6 +1,8 @@
 package io.github.kelvindev15.engine.utils
 
-import io.github.kelvindev15.prolog.engine.Solver.Solution
+import io.github.kelvindev15.prolog.core.{Struct, Term, Variable}
+import io.github.kelvindev15.prolog.engine.Solver.Solution.Yes
+import io.github.kelvindev15.prolog.engine.Solver.{Solution, Substitution}
 import org.scalatest.matchers.should.Matchers
 
 import scala.reflect.ClassTag
@@ -14,4 +16,11 @@ trait TestUtils:
   def expectOne[T <: Solution](using tag: ClassTag[T])(solution: Iterator[Solution]): Unit =
     expect[T](solution)
     solution.hasNext shouldBe false
-    
+
+  extension(solutions: Iterable[Solution])
+    def expectSolutionsIn(expectedSolutions: Iterable[Solution]): Unit =
+      solutions should contain allElementsOf expectedSolutions.to(LazyList)
+      
+  extension(query: Struct)
+    def yes(substitutions: (Variable, Term)*): Yes = Yes(query, Substitution(substitutions*))
+    def no: Solution.No = Solution.No(query)