Justus Sagemüller leftaroundabout

leftaroundabout / Enum.hs

Created April 1, 2012 23:22 — forked from depp/Enum.hs

Demonstration of how incorrect (Enum x, Enum y) => Enum (x, y) instance can be used for Enum a => Enum (a,a)

	{-# LANGUAGE FlexibleInstances #-}

	-- See http://stackoverflow.com/questions/9967790/instance-in-haskell

	instance (Enum a) => Enum (a, a) where
	fromEnum (x,y) =
	k^2 + 2*j + if permuted then 1 else 0
	where
	k = max (fromEnum x) (fromEnum y)
	j = min (fromEnum x) (fromEnum y)