Re: Problem with functional dependencies

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