> > {-# OPTIONS_GHC -fglasgow-exts #-}
> > module Main where
> >
> > data Foo a b where
> > Foo :: Int -> Foo a b
> >
> > data Patch a b where
> > PP :: Foo a b -> Patch a b
> > Lis :: PL a b -> Patch a b
> >
> > data PL a b where
> > U :: Patch a b -> PL a b
> > Nil :: PL x x
> > (:-) :: PL c d -> PL d e -> PL c e
> >
> > data Pair alpha omega where
> > (:.) :: Patch a i -> Patch i o -> Pair a o
> >
> > foo :: Pair a b -> Maybe (Pair a b)
> > foo (Lis (U x :- y) :. Lis Nil) = Just (PP x :. Lis y)
>
> ^^^^
>
> Oddly enough, the code *does* typecheck if we change the above line to
>
> foo (Lis (U a :- b) :. Lis Nil) = Just (Lis (U a) :. Lis b)
>
> which differs only in here............ ^^^^^^^^
Looks to me as if ghc is right to complain. PP takes a Foo, but U takes
a Patch. Therefore the x that's matched on the lhs is a Patch, not a Foo.
> test.lhs:27:44:
> Couldn't match `Foo a b' against `Patch a1 d'
> Expected type: Foo a b
> Inferred type: Patch a1 d
> In the first argument of `PP', namely `x'
> In the first argument of `(:.)', namely `PP x'
> Glasgow Haskell Compiler, Version 6.4, for Haskell 98, compiled by GHC
> version 6.4
FWIW, same error with ghc-6.5 from a few weeks ago.
Cheers,
Andres
_______________________________________________
Glasgow-haskell-bugs mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
