[lec02] Add templates

src/main/scala/datastructures/List.scala

+package datastructures
+
+sealed trait List[+A] {
+  def head = this match {
+    case Nil => sys.error("head of empty list")
+    case Cons(a, _) => a
+  }
+
+  def tail: List[A] = ???
+
+  def init: List[A] = ???
+
+  def setHead[AA >: A](head: AA): List[AA] = ???
+
+  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 {
+  def apply[A](as: A*): List[A] =
+    if (as.isEmpty) Nil
+    else Cons(as.head, apply(as.tail: _*))
+}

src/main/scala/datastructures/Parametricity.scala

+package datastructures
+
+object Parametricity {
+  // with our compiler settings from build.sbt, there are even less
+  // possibilities to do something wrong here without the compiler complaining
+  def para[A,B,C](a: A, b: B)(f: (A,B) => C): C = ???
+}

src/test/scala/datastructures/ListSpec.scala

+package datastructures
+
+import org.scalatest._
+import testutil.PendingIfUnimplemented
+
+class ListSpec extends FlatSpec with Matchers with AppendedClues with PendingIfUnimplemented {
+  val numbers = List(1 to 5: _*)
+  val single = List(1)
+  val strings = List("a", "b", "c")
+
+  "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
+    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
+    a[RuntimeException] should be thrownBy List().init
+  }
+
+  it should "have a setHead method, which replaces the first element" in {
+    numbers.setHead(8) shouldBe List(8, 2, 3, 4, 5)
+    a[RuntimeException] should be thrownBy List().tail
+  }
+
+  it should "have a left fold" in {
+    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

src/test/scala/testutil/PendingIfUnimplemented.scala

+package testutil
+
+import org.scalatest._
+
+trait PendingIfUnimplemented extends TestSuiteMixin { this: TestSuite =>
+  abstract override def withFixture(test: NoArgTest): Outcome = {
+    super.withFixture(test) match {
+      case Failed(_: NotImplementedError) => Pending
+      case other => other
+    }
+  }
+}