[Haskell-cafe] Type Classes in Haskell - how can I make GHC make a choice of types, when the type chosen doesn't matter?

[Chris Dew]

@Stephen Tetley - The stream class exists simply to allow for the
creation of a ->> operator which can be used to 'Stream' data through
multiple pure and IO functions, on the way to some form of output.
It's probably not a great idea, as there are more idiomatic solutions
in Haskell - I'm sure someone will mention arrows.

I want the result of (->>) to be what the following function requires,
either an 'a' or and 'IO a'.  This is too unconstrained if the
following function is flexible in it's input.  (e.g. another
application of (->>)).   Hence my original problem.

a and b have are related, but not in a way I know how to express in
Haskell.  They are constrained to: a == b || IO a == b || a == IO b. c
and d have a similar constraint.

Could you suggest how these constraints could be expressed in the
Haskell type system?

Thanks,

Chris.

On 14 April 2011 14:28, Stephen Tetley <stephen.tetley at gmail.com> wrote:
> Hi Chris
>
> What does the Stream class *do* though?
>
> class Stream a b c d where
>    (->>) :: a ->  (b ->  c) ->  d
>
> Even with Neil's change its still quite unusual:
>
> class Stream a b c where
>  (->>) :: a -> (b -> c) -> c
>
> In the first formulation there is an input of type a, a function (b ->
> c) and a result of a completely different type d.
>
> In Neil's class the function relates to the type of the answer but not
> to the input.
>
> The "difficult" type classes in Haskell - Applicative, Monad, and
> Arrows / Category - are related to some degree to fairly standard
> combinators on functions. But they generalize the combinators to
> operate on other types than the function type (->). As there isn't a
> relation between input and output, I don't quite see how the Stream
> type could start as a combinator.
>
> Best wishes
>
> Stephen
>
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