[Haskell-cafe] Using fundeps to resolve polymorphic types to concrete types

[Bryan Donlan]

Hi,

Is there any theoretical reason that functional dependencies can't be used
to resolve a polymorphic type to a concrete type? For example:

> -- compile with -fglasgow-exts
> 
> class DeriveType a b | a -> b
> 
> data A = A
> data B = B
> 
> instance DeriveType A B
> 

> simpleNarrow :: DeriveType A b => b -> B
> simpleNarrow = id

Since 'b' is uniquely determined by the fundep in DeriveType, it seems that
this ought to work; ie, since the only type equation satisfying DeriveType A b
is B -> B, it should reduce to that before trying to fit its type against its
body.

The motivation is this case:

> data ComplexType a where
>     SomeConstructor :: DeriveType a b => a -> b -> ComplexType a
> 
> specialCaseFunc :: ComplexType A -> B
> specialCaseFunc (SomeConstructor _ b) = b

Essentially, if I have a data structure with two types used as fields, and
one uniquely determines the other, I'd like to use these instances to avoid
re-stating the implied one in the type equations, if possible.

Is there some theoretical reason for this not to work, or is it just a
limitation of GHC's current implementation? (Note, I'm testing with GHC 6.8.2,
so it's possible this might be fixed in trunk already...)

Thanks,

Bryan Donlan