Re: [Haskell-cafe] Is this a useful higher-order function, or should I RTFM?

8 Dec 2004


      Steven Huwig wrote:
...
On Dec 6, 2004, at 11:05 PM, Tom Pledger wrote:
...
import Data.Char(isSpace)
   import Data.List(groupBy)
(op `on` f) x y = f x `op` f y
   wordsAndSpaces  = groupBy ((==) `on` isSpace)
`on` is a handy little function in this instance.
Does it have a technical name?
Perhaps, but I don't know it, and wanted to make the code sound OK when 
read aloud. :-)
...
Have you used it elsewhere?
Yes, a few times, mainly as (compare `on` fst), as in:

    -- Apply to xs the permutation that would stably sort ks
    unpermute ks xs
        = map snd (sortBy (compare `on` fst) (zip ks xs))

Regards,
Tom