[GHC] #1408: groupWhen – a groupBy that compares consecutive values

[GHC]

#1408: groupWhen – a groupBy that compares consecutive values
----------------------------------------------------------+-----------------
  Reporter:  Joachim Breitner <mail at joachim-breitner.de>  |          Owner:         
      Type:  feature request                              |         Status:  new    
  Priority:  normal                                       |      Milestone:         
 Component:  libraries (other)                            |        Version:  6.6.1  
  Severity:  normal                                       |       Keywords:         
Difficulty:  Easy (1 hr)                                  |             Os:  Unknown
  Testcase:                                               |   Architecture:  Unknown
----------------------------------------------------------+-----------------
`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

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1408>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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