id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
1408	groupWhen – a groupBy that compares consecutive values	Joachim Breitner <mail@…>		"`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"	feature request	closed	normal		libraries/base	7.0.2	wontfix		ghc@… rturk@…	Unknown/Multiple	Unknown/Multiple	None/Unknown	Easy (less than 1 hour)