Subject: Add Data.Eq.equating to match Data.Ord.comparing?
Newsgroups: gmane.comp.lang.haskell.libraries
Date: 2007-09-20 10:26:48 GMT (34 weeks, 17 hours and 3 minutes ago)
I recently wanted to group a list of pairs by the second item in the pairs. So I tried writing > x = groupBy (comparing snd) pairs But this threw a type error because groupBy expects its first argument to return a Bool and comparing returns an Ordering. So I had to write > x = groupBy ((== EQ) . comparing snd) pairs Which is clunky. What I needed was a function in Data.Eq such as > equating :: Eq a => (b -> a) -> b -> b -> Bool This could be generalised to a function such as > pairWise :: (b -> b -> c) -> (a -> b) -> a -> a -> c > pairWise pair project x y = pair (project x) (project y) Then we can write > comparing = pairWise compare > equating = pairWise (==) Should I submit a patch to add this? And is "pairWise" the right name? Paul.