[lec02] Scala 3 migration

src/main/scala/datastructures/List.scala

 package datastructures
 
-sealed trait List[+A]:
-  def head = this match
+enum List[+A]:
+  case Nil
+  case Cons(_head: A, _tail: List[A])
+
+  def head: A = this match
     case Nil => sys.error("head of empty list")
     case Cons(a, _) => a
 
+  /** removes the first element of a list and returns the rest */
   def tail: List[A] = ???
 
+  /** returns all but the last element of a list */
   def init: List[A] = ???
 
+  /** replaces the first element of a list */
   def setHead[AA >: A](head: AA): List[AA] = ???
 
+  /** recurses through the list, combining elements with the given function
+   *  Uncomment the annotation to enable checking for tail-recursiveness */
+  //@annotation.tailrec
   def foldLeft[B](z: B)(f: (B, A) => B): B = ???
-case object Nil extends List[Nothing]
-case class Cons[+A](elem: A, rest: List[A]) extends List[A]
 
 object List:
+  /** construct a list by passing elements
+   *
+   *  Remember: `apply` makes the object behave like a function,
+   *    you can call it as `List(elem1, elem2,...)`
+   **/
   def apply[A](as: A*): List[A] =
     if as.isEmpty then Nil
     else Cons(as.head, apply(as.tail: _*))

src/test/scala/datastructures/ListSpec.scala

@@ -12,13 +12,13 @@ class ListSpec extends AnyFlatSpec with Matchers with AppendedClues with Pending
 
   "A list" should "have a tail function that removes the first element" in {
     numbers.tail shouldBe List(2, 3, 4, 5)
-    single.tail shouldBe Nil
+    single.tail shouldBe List.Nil
     a[RuntimeException] should be thrownBy List().tail
   }
 
   it should "have a init function that removes the last element" in {
     numbers.init shouldBe List(1, 2, 3, 4)
-    single.init shouldBe Nil
+    single.init shouldBe List.Nil
     a[RuntimeException] should be thrownBy List().init
   }
 
@@ -31,4 +31,4 @@ class ListSpec extends AnyFlatSpec with Matchers with AppendedClues with Pending
     strings.foldLeft("X")(_ + _) should not be "Xcba" withClue ", this looks like a right fold with flipped args"
     strings.foldLeft("X")(_ + _) shouldBe "Xabc"
   }
-}
\ No newline at end of file
+}