Re: [Haskell] compares :: Ord a => a -> a -> Ordering -> Ordering

15 Feb 2006


      On Wed, Feb 15, 2006 at 01:17:43AM +0000, Ben Rudiak-Gould wrote:
...
I just realized that the class Ord should have an additional method:
class Eq a => Ord a where
    compares :: a -> a -> Ordering -> Ordering
    compares x y d = case compare x y of { EQ -> d ; o -> o }
    ...
This would make writing Ord instances much easier:
instance (Ord a, Ord b, Ord c, Ord d) => Ord (a,b,c,d) where
    compares (a1,b1,c1,d1) (a2,b2,c2,d2) =
      compares a1 a2 . compares b1 b2 . compares c1 c2 . compares d1 d2
This does the same thing:

import Data.Monoid

instance (Ord a, Ord b, Ord c, Ord d) => Ord (a,b,c,d) where
	compare (a1,b1,c1,d1) (a2,b2,c2,d2) =
	compare a1 a2 `mappend` compare b1 b2 `mappend`
		compare c1 c2 `mappend` compare d1 d2

Re: [Haskell] compares :: Ord a => a -> a -> Ordering -> Ordering

Ross Paterson