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Make Arbitrary StreamingT more arbitrary #649

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Nov 15, 2015
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Make Arbitrary StreamingT more arbitrary #649

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36 changes: 23 additions & 13 deletions laws/src/main/scala/cats/laws/discipline/Arbitrary.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -56,25 +56,35 @@ object arbitrary {
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 writerTArbitrary[F[_], L, V](implicit F: Arbitrary[F[(L, V)]]): Arbitrary[WriterT[F, L, V]] =
Arbitrary(F.arbitrary.map(WriterT(_)))
Expand Down