On Wed, 2007-03-21 at 11:48 +0100, Bertram Felgenhauer wrote:
Duncan Coutts wrote:
So can anyone break this hypothesis?
nub . sort = map head . group . sort
Just make Eq and Ord instances that are completely unrelated, say
I had to compile the code and go thorough it to convince myself, but yes as you say I'd fallen into the trap of 'believing' <= and == etc. I was assuming that sort would put 'equal' elements next to each other in the result but of course with compare x y == EQ not being the same as x == y that's not the case. Sigh. Thanks for the example. I want type classes with real property conditions. I don't mind if they're checked or not, but I want to be able to break people's code if they don't obey the properties and optimise the code for the rest of us. class Ord a => OrdReallyTruely a where ... then I want to use ghc rules that match on the subset of Ord types that are instances of OrdReallyTruely and be able to do the appropriate transformations. forall (xs :: OrdReallyTruely a => [a]). nub (sort xs) = map head (group (sort xs)) Duncan