interaction of GADTs and data families: a bug?

[José Pedro Magalhães]

Hi Sebastian,

Is this perhaps another instance of #3851?
http://hackage.haskell.org/trac/ghc/ticket/3851


Cheers,
Pedro

On Thu, Apr 15, 2010 at 14:10, Sebastian Fischer <
sebf at informatik.uni-kiel.de> wrote:

> Dear GHC experts,
>
> Certain behaviour when using
>
>    {-# LANGUAGE GADTs, TypeFamilies #-}
>
> surprises me. The following is accepted by GHC 6.12.1:
>
>    data GADT a where
>      BoolGADT :: GADT Bool
>
>    foo :: GADT a -> a -> Int
>    foo BoolGADT True = 42
>
> But the following is not:
>
>    data family DataFam a
>    data instance DataFam Bool where
>      BoolDataFam :: DataFam Bool
>
>    fffuuuu :: DataFam a -> a -> Int
>    fffuuuu BoolDataFam True = 42
>
> GHC 6.12.1 throws the following error (GHC 6.10.4 panics):
>
>     Couldn't match expected type `a' against inferred type `Bool'
>       `a' is a rigid type variable bound by
>           the type signature for `fffuuuu' at gadtDataFam.hs:13:19
>     In the pattern: BoolDataFam
>     In the definition of `fffuuuu': fffuuuu BoolDataFam True = 42
>
> I expect that `fffuuuu` should be accepted just like `foo`. Do I expect too
> much?
>
> Cheers,
> Sebastian
>
> --
> Underestimating the novelty of the future is a time-honored tradition.
> (D.G.)
>
>
>
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