RE: GADT problems

15 Sep 2008

      | > | (case undefined of Foo GadtValue -> ()) :: () -- is rigid
...
|
| But the first compiles fine, so it seems that the scrutinee doesn't have
| to always be rigid?

Not for me! Either with 6.8.3 or HEAD.  What compiler are you using?

Simon

$ ghc-6.8.3 -c Neil.hs

Neil.hs:17:12:
    GADT pattern match in non-rigid context for `GadtValue'
      Solution: add a type signature
    In the pattern: GadtValue
    In the pattern: Foo GadtValue
    In a case alternative: Foo GadtValue -> ()
bash-3.2$ ~/builds-04/validate-HEAD/ghc/stage1-inplace/ghc -c Neil.hs

Neil.hs:17:12:
    GADT pattern match in non-rigid context for `GadtValue'
      Solution: add a type signature
    In the pattern: GadtValue
    In the pattern: Foo GadtValue
    In a case alternative: Foo GadtValue -> ()
bash-3.2$

{-# LANGUAGE GADTs, ScopedTypeVariables #-}
module Test where

data E x = E x

data Foo a where
  Foo :: Gadt a -> Foo a

data Gadt a where
  GadtValue :: a -> Gadt (E a)

g :: Int -> ()
g x = case undefined of
        Foo GadtValue -> ()

RE: GADT problems

Simon Peyton-Jones