Hi, I think the problem is that when you give functions in where clauses type signatures, type parameters like "a" are bound by forall. So the "a" in h is a different one to in "g". You might have more luck if you enable -XScopedTypeVariables , and give a top-level type signature explicitly binding a and b: "forall a b. Stream a -> …" On Mon, 15 Aug 2016 at 13:04 David Banas wrote:

Hi all,

I wonder if anyone can help me find my error, in this code:

It’s note clear to me why x :: a is an error, except that maybe the type of f is not being correctly inferred? Am I, maybe, not allowed to use pattern matching, on the right side of ‘=‘?

Thanks, -db

data Stream a = Cons a (Stream a)

newtype StreamMap a b = SM (Stream a -> Stream b)

instance Arrow StreamMap where pure f = SM g where g (Cons x xs) = Cons (f x) (g xs) SM f >>> SM g = SM (g . f) first (SM g) = SM h -- h :: Stream (a, c) -> Stream (b, c) where h :: Stream (a, c) -> Stream (b, c) h (Cons (x, y) zs) = Cons ((f x), y) (h zs) g :: Stream a -> Stream b g (Cons x xs) = Cons (f x) (g xs)

Couldn't match expected type ‘b1’ with actual type ‘a’ ‘a’ is a rigid type variable bound by the type signature for g :: interactive:IHaskell152.Stream a -> interactive:IHaskell152.Stream b1 at :10:17 ‘b1’ is a rigid type variable bound by the type signature for g :: interactive:IHaskell152.Stream a -> interactive:IHaskell152.Stream b1 at :10:17 Relevant bindings include xs :: interactive:IHaskell152.Stream a (bound at :11:27) x :: a (bound at :11:20) g :: interactive:IHaskell152.Stream a -> interactive:IHaskell152.Stream b1 (bound at :11:12) In the first argument of ‘f’, namely ‘x’ In the first argument of ‘IHaskell152.Cons’, namely ‘(f x)’

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.