[Haskell-cafe] warn-incomplete-patterns and GADTs

[Ryan Ingram]

See these bugs:
http://hackage.haskell.org/trac/ghc2/ticket/366
http://hackage.haskell.org/trac/ghc2/ticket/595

  -- ryan


On Fri, Aug 27, 2010 at 6:40 AM, Tom Nielsen <tanielsen at gmail.com> wrote:
> Hi,
>
> is warn-incomplete-patterns (in GHC 6.10.3) less clever than it could be?
>
> {-# OPTIONS_GHC -fglasgow-exts #-}
> {-# OPTIONS_GHC -fwarn-incomplete-patterns  #-}
> module Vec where
>
> data Z
> data S a
>
> data Vec n a where
>   VNil :: Vec Z a
>   VCons :: a -> Vec m a -> Vec (S m) a
>
> instance Eq a => Eq (Vec n a) where
>   VNil == VNil = True
>   VCons x vx == VCons y vy = x==y && vx == vy
>
> give the warning:
>    Warning: Pattern match(es) are non-exhaustive
>             In the definition of `==':
>                 Patterns not matched:
>                     VNil (VCons _ _)
>                     (VCons _ _) VNil
>
> but of course VNil and VCons can never have the same type.
>
> Tom
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