Feature #44

groupByLeader / groupByTail

Added by mail-joachim-breitner-de - 613 days ago. Updated 210 days ago.

Status :Assigned Start :
Priority :Normal Due date :
Assigned to :John Goerzen % Done :

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Target version :-
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Description

Hi.

I find these functions very useful and I alsways get them wrong the first time. Do you think they should go into Data.List?

groupByLeader :: (a -> Bool) -> [a] -> a
groupByLeader p = groupBy (\_ l -> not (p l))

groupByLeader :: (a -> Bool) -> [a] -> a
groupByLast p = groupBy (\l _ -> not (p l))

This is very handy when parsing data with segments that are marked either by a certain head, or a cartain tail, such as the output of pdftk dump_data, or ini files, or debian control files, or ... well, there are plenty.

History

02/22/2007 11:02 AM - John Goerzen

  • Status changed from New to Assigned

Could you write up a Haddock comment for these, including examples? If do, I think they would make sense being added to Data.List.Utils.

02/22/2007 12:35 PM - mail-joachim-breitner-de -

Ok, here is my shot at it, although most real-life examples would probably be too large for the documentation.

groupByLeader
Splits the list in a list of sublists, starting a new sublist with every element for which the predicate is true.

Example:
groupByLeader (\n -> n mod 10 0) [2,4,6,8,10,12,14,17,18,20,22,24] -> [[2,3,4,6,8],[10,12,14,17,18],[20,22,24]]

groupByLast
Splits the list in a list of sublists, finishing each sublist with an element for which the predicate is true.

Example:
groupByLeader (\n -> n mod 10 0) [2,4,6,8,10,12,14,17,18,20,22,24] -> [[2,3,4,6,8,10],[12,14,17,18,20],[22,24]]

I can’t think of a more useful one-line-example, sorry.

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