Hi, Andrew's "problem" is not a typing problem. The problem is due to the way type classes are implemented. Note that Oleg's "solution" imposes the exact some improvement conditions as the FD in Andrew's code. However, Oleg manipulates the instance declaration and the instance body, so that the dictionary-passing translation won't fail. Martin oleg@pobox.com writes:
Andrew Bromage wrote:
module FD where
class C from to | from -> to where g :: from -> to h :: to -> from
instance C Char Bool where g c = c == 'T' h b = if b then 'T' else 'F'
--f :: (C Char a) => Char -> a f c = g c
Indeed, functional dependencies are the way to close a class that is _visible_ to the typechecker (unlike the export directive that has no such significance, at least to the inference algorithm).
The problem is that we wish to give 'f' a signature, like the one in the commented code, but we cannot use the type variable 'a'. We must use Bool -- which is silly as the typechecker would have figures it out anyway.
This is a pretty serious problem when you're doing typeclass metaprogramming. In this case, the type you're trying to find is Bool. In general, it might be a large complex type, generated by the class C as a typeclass metaprogram, which you don't want to put in the type signature for maintainability reasons.
Here's the way to solve the problem -- which is actually the problem with the sequencing of FD resolution and other actions. The following are the changes:
instance (TypeCast Bool a, TypeCast a Bool) => C Char a where g c = typeCast $ c == 'T' h b = if (typeCast b) then 'T' else 'F'
f :: (C Char a) => Char -> a f c = g c
-- snipped verbatim from the HList library class TypeCast a b | a -> b, b->a where typeCast :: a -> b class TypeCast' t a b | t a -> b, t b -> a where typeCast' :: t->a->b class TypeCast'' t a b | t a -> b, t b -> a where typeCast'' :: t->a->b instance TypeCast' () a b => TypeCast a b where typeCast x = typeCast' () x instance TypeCast'' t a b => TypeCast' t a b where typeCast' = typeCast'' instance TypeCast'' () a a where typeCast'' _ x = x
The reason for TypeCast existence is to suitably delay the FD resolution. TypeCast has permitted a lot of code that seemed impossible.
_______________________________________________ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell