Hello,
> Is it possible in Haskell + GHC extensions to use reflection
> techniques to determine typeclass membership? I'm thinking of things
> like the following:
>
I think the short answer is not in general; i.e. I don't think there is any
way to access the members of an arbitrary typeclass (but I'd love to be
proved wrong).
However, you could always explicitly list the members of a typeclass you are
interested in (this is similar to your Idea 2):
{-# OPTIONS_GHC -fglasgow-exts -fallow-undecidable-instances
-fallow-overlapping-instances #-}
class InEq a b | a -> b where inEq :: a -> Bool
instance TypeCast b HFalse => InEq a b where inEq _ = False
instance InEq Int HTrue where inEq _ = True
instance InEq a b => InEq [a] b where inEq _ = inEq (undefined :: a)
data HTrue
data HFalse
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
You can also do an arguably nicer, more flexible version of the previous by
recreating some of the typeclass machinery yourself; Oleg has several
examples of this such as: //okmij.org/ftp/Haskell/poly2.txt
-Jeff
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