Re: [Haskell-cafe] Help to write type-level function

On 27.02.2013 17:35, Dmitry Kulagin wrote:

Hi Aleksey,

Unfortunately, your solution does not work for me (ghc 7.6.2). I reduced the problem to:

-- | Type class for type equality. class TypeEq (a :: α) (b :: α) (eq :: Bool) | a b -> eq instance TypeEq a a True -- instance TypeEq a b False instance eq ~ False => TypeEq a b eq

You need to add pragma {-# LANGUAGE OverlappingInstances #-} to the file where instances defined. Without it GHC will complain about overlap and unlike other extensions won't recommend pragma