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
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