On Thu, Apr 15, 2010 at 14:10, Sebastian Fischer <sebf@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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