Problem with functional dependencies
Jeffrey R. Lewis
Thu, 21 Dec 2000 00:59:29 -0800
Simon Peyton-Jones wrote:
> I think you can simplify the example. Given
> class HasFoo a b | a -> b where
> foo :: a -> b
> instance HasFoo Int Bool where ...
> Is this legal?
> f :: HasFoo Int b => Int -> b
> f x = foo x
> You might think so, since
> HasFoo Int b => Int -> b
> is a substitution instance of
> HasFoo a b => a -> b
This is the step where the reasoning goes wrong. The functional dependency tells you that `b' isn't really a free variable, since it is dependent on `a'. If you substitute for `a', you can't expect `b' to remain unconstrained.
Hugs complains that the inferred type for `f' is not general enough. It's right to complain, but the real problem is that the signature is too general. Asimilar situation arises if you try to declare an instance `HasFoo Int b', but in this case, hugs complains that the instance is more general than the dependency allows. A useful thing to do would be to check for this sort of thing in signatures as well, so that the more appropriate error message can be given.