Re: [Haskell-cafe] GADTs and pattern matching

Francesco Mazzoli writes:

I have stumbled upon a strange annoyance:

{-# LANGUAGE GADTs #-}

Hi Francesco, I think you'll find that the 'annoyance' is nothing to do with GADTs. I suggest you take the type signature off of foo1, and see what type ghc infers for it. It isn't :: a -> Foo a -> Int.

data Foo v where Foo :: Foo (Maybe v)

-- This doesn't work -- **** foo1 :: a -> Foo a -> Int foo1 Nothing Foo = undefined foo1 (Just x) Foo = undefined

...

The first definition fails with the error

Couldn't match expected type `a' with actual type `Maybe t0' ... In the pattern: Nothing

Yep, that message explains what's going on well enough for me.