groupWhen – a groupBy that compares consecutive values

groupBy has a minor problem: It always uses the first value of a group to decide whether a new value belongs to this group or the next. In several cases it would be more useful if it would take the last value of a group, thus always comparing consecutive values.

Example code:

groupWhen :: (a -> a -> Bool) -> [a] -> [[a]]
groupWhen _ []    = []
groupWhen _ [a]   = [[a]]
groupWhen f (a:l) = if f a (head c) then (a:c):r
                                    else [a]:c:r
  where (c:r) = groupWhen f l

Uses:

groupWhen (\a b -> b - a < 5) [1,2,4,10,14,16,18] -- Finding holes in a increasing series, e.g. log time stamps (my real use case)

or

groupWhen (<) [1,2,3,2,10,12,10,11] -- Group into strictly increasing sublists

Note that for transitive and symetrical comparision functions f (such as (==)), groupBy f == groupWhen f.

It should probably go to Data.List

Trac metadata

Trac field	Value
Version	6.6.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries (other)
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Operating system	Unknown
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