type instance Id Int = Int
foo :: Id a -> Id a foo = id
foo' :: Id a -> Id a foo' = foo Is this expected?
Yes, unfortunately, this is expected, although it is very unintuitive. This is for the following reason.
Huh? This sounds very wrong to me, simply because foo and foo' have the very same type.
Type systems reject programs that don't go wrong. It's hard to understand on the basis of such a single program why it should be rejected. The problem is decidability. There is no algorithm that accepts all well-behaved programs and rejects all ill-behaved programs. There probably is an algorithm that accepts a particular program. So far we haven't found an algorithm that accepts this example, that is decidable and sufficiently general to cover many other useful cases. This is our motivation for rejecting this program. Consider it a challenge to find a better algorithm in the design space. Cheers, Tom -- Tom Schrijvers Department of Computer Science K.U. Leuven Celestijnenlaan 200A B-3001 Heverlee Belgium tel: +32 16 327544 e-mail: tom.schrijvers@cs.kuleuven.be url: http://www.cs.kuleuven.be/~toms/