Re: GADTs and functional dependencies

On Tue, Sep 23, 2008 at 9:36 AM, Wolfgang Jeltsch wrote:

Am Dienstag, 23. September 2008 18:19 schrieben Sie:

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On Tue, Sep 23, 2008 at 6:07 PM, Wolfgang Jeltsch

wrote:

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Hello,

please consider the following code:

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{-# LANGUAGE GADTs, MultiParamTypeClasses, FunctionalDependencies #-}

data GADT a where

GADT :: GADT ()

class Class a b | a -> b

instance Class () ()

fun :: (Class a b) => GADT a -> b fun GADT = ()

I'd expect this to work but unfortunately, using GHC 6.8.2, it fails with the following message:

bear in mind that the only instance you defined is

instance Class () ()

which doesn't involve your GADT at all.

This is correct. (It's only a trimmed-down example, after all.)

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Maybe you meant something like:

instance Class (GADT a) ()

No, I didn't.

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Moreover, your fun cannot typecheck, regardless of using MPTC or GADTs. The unit constructor, (), has type () and not b.

Pattern matching against the data constructor GADT specializes a to (). Since Class uses a functional dependency, it is clear that b has to be (). So it should typecheck. At least, I want it to. ;-)

In the signature: fun :: (Class a b) => GADT a -> b What is there to determine the type a? It's a phantom in the definition of GADT, so it can unify arbitrarily for us, but there is nothing in your program to cause it to unify a certain way. Based on what you're hoping for, I think you would need: class Class a b | b -> a But, then think about how the type checker looks at fun, which returns (). Now we see that b should be (), but () is not the same as b. That is, () does not generalize to b, even though an arbitrary b could specialize to (). Did some other version of ghc accept this code? Jason