[lec04] readd tests

src/test/scala/laziness/StreamSpec.scala

+package laziness
+
+import org.scalatest._
+import testutil.PendingIfUnimplemented
+
+class StreamSpec extends FlatSpec with Matchers with AppendedClues with PendingIfUnimplemented {
+
+  import Stream._
+
+  /*
+   * Note that equivalence checking on streams does not work.
+   * Therefore it is important, that your toList implementation works for most later tests.
+   */
+
+  "A stream" should "be convertable to a list" in {
+    Stream(1, 2, 3).toList shouldBe List(1, 2, 3)
+    Stream.empty.toList shouldBe List()
+  }
+
+  it should "have a take method that keeps only a certain number of elements" in {
+    ones.take(1).toList shouldBe List(1)
+
+    ones.take(23).toList should have size 23
+  }
+
+  it should "have a drop method that removes a certain number of elements from the start" in {
+    Stream(1, 2, 3, 4).drop(1).toList shouldBe List(2, 3, 4)
+
+    ones.drop(17).headOption should contain(1)
+  }
+
+  "the fibs method" should "be able to generate fibonacci numbers" in {
+    fibs.take(8).toList shouldBe List(0, 1, 1, 2, 3, 5, 8, 13)
+  }
+
+  "unfold" should "should generate streams from a function while stopping on None" in {
+    val alphabet = unfold('a')(s => if(s <= 'z') Some((s, (s+1).toChar)) else None)
+    alphabet.toList shouldBe ('a' to 'z').toList
+  }
+
+  "fibsViaUnfold" should "result in the same values as fibs without unfold" in {
+    fibsViaUnfold.take(10).toList shouldBe fibs.take(10).toList
+  }
+}