Merge pull request #649 from ceedubs/arb-streamingt

Make Arbitrary StreamingT more arbitrary

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

@@ -56,25 +56,35 @@ object arbitrary extends ArbitraryInstances0 {
   implicit def streamingArbitrary[A](implicit A: Arbitrary[A]): Arbitrary[Streaming[A]] =
     Arbitrary(Gen.listOf(A.arbitrary).map(Streaming.fromList(_)))
 
-  // TODO: it would be better to do this recursively, i.e. more like:
-  //
-  // Gen.oneOf(
-  //   for { a <- arbitrary[A]; s <- arbitrary[F[StreamingT[F, A]]] } yield cons(a, s),
-  //   for { s <- arbitrary[F[StreamingT[F, A]]] } yield wait(s),
-  //   const(StreamingT.empty[F, A]))
-  //
-  // However, getting this right with Scalacheck (and avoiding SOEs) is
-  // somewhat fiddly, so this will have to do for now.
-  //
+  def emptyStreamingTGen[F[_], A]: Gen[StreamingT[F, A]] =
+    Gen.const(StreamingT.empty[F, A])
+
+  def streamingTGen[F[_], A](maxDepth: Int)(implicit F: Monad[F], A: Arbitrary[A]): Gen[StreamingT[F, A]] = {
+    if (maxDepth <= 1)
+      emptyStreamingTGen[F, A]
+    else {
+      Gen.oneOf(
+        // Empty
+        emptyStreamingTGen[F, A],
+        // Wait
+        streamingTGen[F, A](maxDepth - 1).map(s =>
+          StreamingT.wait(F.pure(s))),
+        // Cons
+        for {
+          a <- A.arbitrary
+          s <- streamingTGen[F, A](maxDepth - 1)
+        } yield StreamingT.cons(a, F.pure(s))
+      )
+    }
+  }
+
   // The max possible size of a StreamingT instance (n) will result in
   // instances of up to n^3 in length when testing flatMap
   // composition. The current value (8) could result in streams of up
   // to 512 elements in length. Thus, since F may not be stack-safe,
   // we want to keep n relatively small.
   implicit def streamingTArbitrary[F[_], A](implicit F: Monad[F], A: Arbitrary[A]): Arbitrary[StreamingT[F, A]] =
-    Arbitrary(for {
-      as <- Gen.listOf(A.arbitrary).map(_.take(8))
-    } yield StreamingT.fromList(as))
+    Arbitrary(streamingTGen[F, A](8))
 
   implicit def writerArbitrary[L:Arbitrary, V:Arbitrary]: Arbitrary[Writer[L, V]] =
     writerTArbitrary[Id, L, V]