[Haskell-cafe] Re: Could FDs help usurp an ATs syntactic restriction?

[Nicolas Frisby]

Perhaps this ticket is related?

 http://hackage.haskell.org/trac/ghc/ticket/714

On Thu, Dec 4, 2008 at 9:38 PM, Nicolas Frisby <nicolas.frisby at gmail.com> wrote:
> From the error below, I'm inferring that the RHS of the associated
> type definition can only contain type variables from the instance
> head, not the instance context. I didn't explicitly see this
> restriction when reading the GHC/Type_families entry.
>
> Could perhaps the "a b -> bn" functional dependency of the TypeEq
> class lift this restriction for bn? This isn't my ball park, but that
> idea has my hopes up :).
>
> <haskell>
> {-# LANGUAGE TypeFamilies #-}
>
> import TypeEq
>
> -- Attempting to encapsulate TypeEq behind an associated type.
>
> class EQ a b where
>  type BN a b
>
> instance TypeEq a b bn => EQ a b where
>  type BN a b = bn
> </haskell>
>
> results in an error
>
> <ghci>
>  /tmp/Test.hs:9:16: Not in scope: type variable `bn'
>  Failed, modules loaded: none.
> </ghci>
>