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

[Tom Pledger]

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