@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
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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