Re: [Haskell-cafe] Beginners problem with the type system

On 22. Oct 2007, at 20:16 , Stefan O'Rear wrote:

This type signature means that 'port' is return type overloaded - it can return ANY kind of port, and the caller gets to choose. Which I don't think is what you want.

True.

(Requires type family extension in GHC 6.8; an equivalent formulation using the older (2000) functional dependancies is possible)

Thanks for mentioning functional dependencies. I tied together the C and the P types like this:

class Channel c p | c -> p where port :: c a -> IO (p a)

data C a = C (P a)

instance Channel C P where port (C p) = return p

Which works nicely. Thanks, Henrik