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

Tom Pledger tpledger at ihug.co.nz
Mon Dec 6 23:05:03 EST 2004

Steven Huwig wrote:

> 1) Did I miss something in the Prelude or standard library that gives
>    me this functionality, or something close to it?


> 3) The 3-tuple output of unravel looks ugly to me, but I can't think
>    of an alternative. For the case where there is an equal number of
>    p-groups and not-p-groups, we need to know which side to start the
>    zipWith. Does anyone have a better way? 

Here's another way to get something similar.

    import Data.Char(isSpace)
    import Data.List(groupBy)

    (op `on` f) x y = f x `op` f y
    wordsAndSpaces  = groupBy ((==) `on` isSpace)

It has a couple of advantages: laziness, and being reversible by good 
ol' concat.

It's a slight nuisance that you have to use isSpace *again* to get your 
bearings, if you use wordsAndSpaces in a mapWords function.


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