Ganesh Sittampalam:
The following program doesn't compile in latest GHC HEAD, although it does if I remove the signature on foo'. Is this expected?
Yes, unfortunately, this is expected, although it is very unintuitive. This is for the following reason. Let's alpha-rename the signatures and use explicit foralls for clarity: foo :: forall a. Id a -> Id a foo' :: forall b. Id b -> Id b GHC will try to match (Id a) against (Id b). As Id is a type synonym family, it would *not* be valid to derive (a ~ b) from this. After all, Id could have the same result for different argument types. (That's not the case for your one instance, but maybe in another module, there are additional instances for Id, where that is the case.) Now, as GHC cannot show that a and b are the same, it can also not show that (Id a) and (Id b) are the same. It does look odd when you use the same type variable in both signatures, especially as Haskell allows you to leave out the quantifiers, but the 'a' in the signature of foo and the 'a' in the signatures of foo' are not the same thing; they just happen to have the same name. BTW, here is the equivalent problem using FDs: class IdC a b | a -> b instance IdC Int Int bar :: IdC a b => b -> b bar = id bar' :: IdC a b => b -> b bar' = bar Given that this is a confusing issue, I am wondering whether we could improve matters by giving a better error message, or an additional hint in the message. Do you have any suggestion regarding what sort of message might have helped you? Manuel
{-# LANGUAGE TypeFamilies #-} module Test7 where
type family Id a
type instance Id Int = Int
foo :: Id a -> Id a foo = id
foo' :: Id a -> Id a foo' = foo _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe