Merge pull request #683 from non/bug/fix-streaming-thunk

Fix bug with Streaming#thunk.

core/src/main/scala/cats/data/Streaming.scala

@@ -815,11 +815,11 @@ object Streaming extends StreamingInstances {
    * Continually return the result of a thunk.
    *
    * This method only differs from `continually` in that the thunk may
-   * not be pure. The stream is memoized to ensure that repeated
-   * traversals produce the same results.
+   * not be pure. Thus, repeated traversals may produce different
+   * results.
    */
   def thunk[A](f: () => A): Streaming[A] =
-    knot(s => Cons(f(), s), memo = true)
+    knot(s => Cons(f(), s), memo = false)
 
   /**
    * Produce an infinite stream of values given an initial value and a

tests/src/test/scala/cats/tests/StreamingTests.scala

@@ -250,6 +250,16 @@ class AdHocStreamingTests extends CatsSuite {
     }
   }
 
+  test("thunk is evaluated for each item") {
+    // don't want the stream to be too big
+    implicit val arbInt = Arbitrary(Gen.choose(-10, 20))
+    forAll { (start: Int, end: Int) =>
+      var i = start - 1
+      val stream = Streaming.thunk{ () => i += 1; i}.takeWhile(_ <= end)
+      stream.toList should === ((start to end).toList)
+    }
+  }
+
   test("interval") {
     // we don't want this test to take a really long time
     implicit val arbInt: Arbitrary[Int] = Arbitrary(Gen.choose(-10, 20))