[Haskell-cafe] A small (?) problem with type families

[Andy Gimblett]

Ack. I've just realised that P/Q is not a functional dependency.  I  
need to use a multi-parameter type class there.  So my question is  
probably completely pointless - sorry!

Thanks anyway,

-Andy

On 13 Nov 2009, at 20:26, Andy Gimblett wrote:

> Hi all,
>
> This email is literate Haskell.  I'm trying to use type families to
> express some dependencies between type classes, and I'm running into
> trouble, I think because I'm producing chains of dependencies which
> the checker can't resolve...  Here's a minimised version of the state
> I've got myself into.  :-)
>
> > {-# LANGUAGE FlexibleContexts #-}
> > {-# LANGUAGE TypeFamilies #-}
>
> > module Families where
>
> First a type family where the type Y is functionally dependent on
> the type X, and we have a function from Y to ().
>
> > class X a where
> >   type Y a
> >   enact :: Y a -> ()
>
> Now another type family, where the type Q is functionally dependent
> on the type P, _and_ it must also be an instance of the X
> class.
>
> > class (X (Q s)) => P s where
> >   type Q s
>
> (Perhaps there's a better way to express that dependency?)
>
> Now a function which takes a value whose type is an instance of the Y
> depending on the Q depending on the P.  (Phew!)  The function just
> tries to call enact on that value.
>
> > bar :: P s => Y (Q s) -> ()
> > bar w = enact w
>
> The error we get is:
>
> src/Families.lhs:35:16:
>    Couldn't match expected type `Y a' against inferred type `Y (Q s)'
>    In the first argument of `enact', namely `w'
>    In the expression: enact w
>    In the definition of `bar': bar w = enact w
>
> Presumably this way I'm chaining type dependencies is flawed.  Any
> suggestions on how to improve it, and/or what to read to understand
> what I'm dealing with better?  (So far I've read "Fun with type
> functions V2", but that's about it, and I admit I didn't grok it all.)
>
> Thanks!
>
> -Andy
>
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