Problem with functional dependencies

[Lennart Augustsson]

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
>
> but if we infer the type (HasFoo Int b => Int -> b)
> for f's RHS, we can then "improve" it using the instance
> decl to (HasFoo Int Bool => Int -> Bool), and now the signature
> isn't a substitution insance of the type of the RHS.

I definitely want it to be legal.  I have examples where this is immensly useful.


--

        -- Lennart