2008/3/21 Krzysztof Skrzętnicki <[EMAIL PROTECTED]>:
> ...
> 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-extensions.html#instance-rules
http://www.haskell.org/ghc/docs/latest/html/users_guide/type-class-extensions.html#undecidable-instances
regards,
Bas
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