Your type annotation
g :: (F a b,D b (T r)) => (a,T r) g = f
is simply incorrect.
I must say I don't understand. I need a value of that type. In the original, "g" is actually a method in a class, and its definition is in an instance declaration. Its type is actually given, not annotated. For instance:
-- ghc -fglasgow-exts -fallow-undecidable-instances -c WeirdInsts.hs
module WeirdInsts where
{
data T r = MkT;class C t;
class D b t;
instance (C (T r)) => D b (T r); -- (1)
class F a b | a -> b;
f :: (F a b,D b t) => (a,t);
f = undefined; class G t where
{
g :: forall a b. (F a b,D b t) => (a,t);
}; instance G (T r) where
{
g = f;
};
}This exhibits the same behaviour.
-- Ashley Yakeley, Seattle WA
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