Re: [Haskell-cafe] Type constraints for class instances

It compiles, but it doesn't work when i try to use it on lists. There are some bugs in that code, but I need some time to fix it. 2008/3/21 Bas van Dijk :

2008/3/21 Krzysztof Skrzętnicki :

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I'd like to write the following code:

instance (Ord a) => YOrd a where ycmp x y = case x `compare` y of LT -> (x,y) GT -> (y,x) EQ -> (x,y)

But i get an error "Undecidable instances" for any type [a]. Does anyone know the way to solve this?

The module compiles fine when you add the following pragma's to your module:

{-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE UndecidableInstances #-}

See: http://www.haskell.org/ghc/docs/latest/html/users_guide/type-class-extension... http://www.haskell.org/ghc/docs/latest/html/users_guide/type-class-extension...

regards,

Bas