Hello,
please consider the following code:
> {-# 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:
> FDGADT.hs:12:11:
> Couldn't match expected type `b' against inferred type `()'
> `b' is a rigid type variable bound by
> the type signature for `fun' at FDGADT.hs:11:16
> In the expression: ()
> In the definition of `fun': fun GADT = ()
What’s the reason for this? Is there a workaround? Does this work in 6.8.3
or 6.10.1?
Thank you in advance.
Best wishes,
Wolfgang
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