Difference between revisions of "GroupBy proposal"
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BrettGiles (talk  contribs) m (GroupBy Proposal moved to GroupBy proposal) 
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Revision as of 02:30, 29 June 2007
I was a bit annoyed when I found out that Data.List.groupBy compared each new element to the first element of a group instead of the last added element.
I guess the current way it is easier to implement, but severely hinders its usefulness:
> groupBy (\a b > a+1 == b) [1,2,3,4,6]
[[1,2],[3,4],[6]]
I propose a different implementation of groupBy which would result in the following:
> groupBy (\a b > a+1 == b) [1,2,3,4,6]
[[1,2,3,4],[6]]
The following a naive implementation that was written for "workiness" instead of speed or space behavior:
groupBy' :: (a > a > Bool) > [a] > [[a]]
groupBy' _ [] = []
groupBy' f (x:xs) = gb f xs [[x]]
where gb f (x:xs) ((a:as):bs) = gb f xs $ if f a x then ((x:a:as):bs)
else ([x]:(a:as):bs)
gb _ [] as = reverse . map reverse $ as
I'm sure there are much nicer ways to implement this...