[Haskell-cafe] Type constraints for class instances

Luke Palmer lrpalmer at gmail.com
Fri Mar 21 01:47:48 EDT 2008

First I'd like to say that this is a very clever idea.  Thanks for the
exposition. :-)

2008/3/21 Krzysztof Skrzętnicki <gtener at gmail.com>:
>  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?

This type of superclassing is not supported by the typeclass system.
Finding systems where it
is supported is a point of much lost sleep on my part.

The typical way to solve this is rather disappointing: wrap it in a newtype:

    newtype YOrdWrap a = YOrdWrap { yordUnwrap :: a }
    instance (Ord a) => YOrd (YOrdWrap a) where
        ycmp (YOrdWrap x) (YOrdWrap y) = ycmpWrap x y

And then apply the constructor whenever you want to use a "normal" type.


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