Re: Proposal #2659: Add sortOn and friends to Data.List

How about capturing the pattern in higher order functions? if we define:

import Data.Function import Data.List

data Cache b a = Cache b a fromCache (Cache _ a) = a toCache f a = Cache (f a) a cache (Cache b _) = b

caching f g = map fromCache . g . map (toCache f)

then sortOn becomes:

sortOn2 f = caching f (sortBy (compare `on` cache))

and with the instances

instance Ord b => Ord (Cache b a) where compare x y = compare (cache x) (cache y)

instance Eq b => Eq (Cache b a) where x == y = cache x == cache y

it gets even simpler:

sortOn3 f = caching f sort

It could be that I missed the point, but I'd much rather not see so many functions added to base without a strong case for it. Cheers, JP.