[Haskell-cafe] Fun with multi-parameter type classes

[Jon Cast]

> Hi,
> 
> I've been getting into Haskell over the past few months and have just
> come up against a bit of a brick wall when trying to encapsulate
> some of the data structures in my code nicely.  Basically what I
> want to have, is a type class where one of the intermediate values
> is opaque with respect to code using its implementations.  This is
> a simplified version of what I'm trying to accomplish:
> 
>    class Foo t where
>       encode :: String -> t
>       decode :: t -> String
> 
>    instance Foo Int where
>       encode = read
>       decode = show
> 
>    test = decode . encode
> 
> This currently fails, because the type checker insists on trying to
> figure out what its type should be - even though it shouldn't be
> needed.

The intermediate type /is/ needed---it's a (hidden) parameter to your
`encode' and `decode' functions.  Why do you think it shouldn't be?

<snip>

Jon Cast