Difference between revisions of "GroupBy proposal"

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m (GroupBy Proposal moved to GroupBy proposal)
(moved into List_function_suggestions)
 
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[[Category:Proposals]]
 
  +
see [[List function suggestions]].
 
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:
 
 
<haskell>
 
> groupBy (\a b -> a+1 == b) [1,2,3,4,6]
 
[[1,2],[3,4],[6]]
 
</haskell>
 
 
I propose a different implementation of groupBy which would result in the following:
 
 
<haskell>
 
> groupBy (\a b -> a+1 == b) [1,2,3,4,6]
 
[[1,2,3,4],[6]]
 
</haskell>
 
 
The following a naive implementation that was written for "workiness" instead of speed or space behavior:
 
 
<haskell>
 
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
 
</haskell>
 
 
I'm sure there are much nicer ways to implement this...
 

Latest revision as of 09:57, 6 September 2007

see List function suggestions.