{-|
This module contains a collection of orderings of dubious utility, inspired by discussion on the haskell-cafe mailing-list.
-}
module Acme.MiscOrder (
Contrarian(..),
LuckySortOrder(..),
Perverse(..),
Fiendish(..)
) where
import Control.Monad
import System.Random
import System.IO.Unsafe(unsafePerformIO)
reverseOrdering :: Ordering -> Ordering
reverseOrdering LT = GT
reverseOrdering EQ = EQ
reverseOrdering GT = LT
-- | This newtype inverts the result of the (==) function.
newtype Contrarian a = Contrarian a
instance Eq a => Eq (Contrarian a) where
(==) (Contrarian x) (Contrarian y) = (/=) x y
-- | This newtype is perfect to transform any sorting algorithm into a lucky sort!
newtype LuckySortOrder a = LuckySortOrder a deriving (Eq)
instance Ord a => Ord (LuckySortOrder a) where
(<=) (LuckySortOrder x) (LuckySortOrder y) = True
{-|
This newtype gives a random result for any two inequal values.
A warning though: given that I didn't manage to memoize a polymorphic function for use as a typeclass instance, its Ord instance can give different results on different calls with the same arguments.
-}
newtype Fiendish a = Fiendish a deriving (Eq, Show)
instance Eq a => Ord (Fiendish a) where
compare x y
| x == y = EQ
| unsafePerformIO randomIO = LT
| otherwise = GT
-- | This newtype does respects the equality relation lawsâ€¦by treating any two values as equal
newtype Perverse a = Perverse a
instance Eq (Perverse a) where
(==) _ _ = True
instance Ord (Perverse a) where
compare _ _ = EQ