Fix Streaming and StreamingT dropWhile functions

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

@@ -501,10 +501,11 @@ sealed abstract class Streaming[A] extends Product with Serializable { lhs =>
    * If no elements satisfy `f`, an empty stream will be returned.
    *
    * For example:
-   *
-   *   Streaming(1, 2, 3, 4, 5, 6, 7).takeWhile(n => n != 4)
-   *
-   * Will result in: Streaming(1, 2, 3)
+   * {{{
+   * scala> val s = Streaming(1, 2, 3, 4, 5, 6, 7)
+   * scala> s.takeWhile(n => n != 4).toList
+   * res0: List[Int] = List(1, 2, 3)
+   * }}}
    */
   def takeWhile(f: A => Boolean): Streaming[A] =
     this match {
@@ -523,16 +524,17 @@ sealed abstract class Streaming[A] extends Product with Serializable { lhs =>
    * If no elements satisfy `f`, the current stream will be returned.
    *
    * For example:
-   *
-   *   Streaming(1, 2, 3, 4, 5, 6, 7).takeWhile(n => n != 4)
-   *
-   * Will result in: Streaming(4, 5, 6, 7)
+   * {{{
+   * scala> val s = Streaming(1, 2, 3, 4, 5, 6, 7)
+   * scala> s.dropWhile(n => n != 4).toList
+   * res0: List[Int] = List(4, 5, 6, 7)
+   * }}}
    */
   def dropWhile(f: A => Boolean): Streaming[A] =
     this match {
       case Empty() => Empty()
       case Wait(lt) => Wait(lt.map(_.dropWhile(f)))
-      case Cons(a, lt) => if (f(a)) Empty() else Cons(a, lt.map(_.dropWhile(f)))
+      case s @ Cons(a, lt) => if (f(a)) Wait(lt.map(_.dropWhile(f))) else s
     }
 
   /**

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

@@ -222,10 +222,12 @@ sealed abstract class StreamingT[F[_], A] extends Product with Serializable { lh
    * If no elements satisfy `f`, an empty stream will be returned.
    *
    * For example:
-   *
-   *   StreamingT[List, Int](1, 2, 3, 4, 5, 6, 7).takeWhile(n => n != 4)
-   *
-   * Will result in: StreamingT[List, Int](1, 2, 3)
+   * {{{
+   * scala> import cats.std.list._
+   * scala> val s = StreamingT[List, Int](1, 2, 3, 4, 5, 6, 7)
+   * scala> s.takeWhile(n => n != 4).toList.flatten
+   * res0: List[Int] = List(1, 2, 3)
+   * }}}
    */
   def takeWhile(f: A => Boolean)(implicit ev: Functor[F]): StreamingT[F, A] =
     this match {
@@ -244,14 +246,16 @@ sealed abstract class StreamingT[F[_], A] extends Product with Serializable { lh
    * If no elements satisfy `f`, the current stream will be returned.
    *
    * For example:
-   *
-   *   StreamingT[List, Int](1, 2, 3, 4, 5, 6, 7).dropWhile(n => n != 4)
-   *
-   * Will result in: StreamingT[List, Int](4, 5, 6, 7)
+   * {{{
+   * scala> import cats.std.list._
+   * scala> val s = StreamingT[List, Int](1, 2, 3, 4, 5, 6, 7)
+   * scala> s.dropWhile(n => n != 4).toList.flatten
+   * res0: List[Int] = List(4, 5, 6, 7)
+   * }}}
    */
   def dropWhile(f: A => Boolean)(implicit ev: Functor[F]): StreamingT[F, A] =
     this match {
-      case Cons(a, ft) => if (f(a)) Empty() else Cons(a, ft.map(_.dropWhile(f)))
+      case s @ Cons(a, ft) => if (f(a)) Wait(ft.map(_.dropWhile(f))) else s
       case Wait(ft) => Wait(ft.map(_.dropWhile(f)))
       case Empty() => Empty()
     }

laws/src/main/scala/cats/laws/discipline/Arbitrary.scala

@@ -11,6 +11,12 @@ import org.scalacheck.Arbitrary.{arbitrary => getArbitrary}
  */
 object arbitrary extends ArbitraryInstances0 {
 
+  // A special function1Arbitrary for testing operations like dropWhile specifically
+  // in the context of Int => Boolean. Once scalacheck supports better Function1 arbitrary
+  // instances this can be removed.
+  implicit def function1Arbitrary: Arbitrary[(Int) => Boolean] =
+    Arbitrary(getArbitrary[Int].map(x => (input) => input < x))
+
   implicit def constArbitrary[A, B](implicit A: Arbitrary[A]): Arbitrary[Const[A, B]] =
     Arbitrary(A.arbitrary.map(Const[A, B]))