Proposal (Trac ticket #3671): Add takeRec, genericTakeRec and spanRec to Data.List

[Yitzchak Gale]

Ian Lynagh wrote:
>>>    breaks (',' ==) "123,456,,78" == ["123", "456", "", "78"]

I wrote:
>> runs = groupBy . on (==)
>> breaks p = filter (not . p . head) . runs p

> Except that has slightly different semantics than Ian's:
> breaks (',' ==) "123,456,,78" == ["123","456","78"]

A combinator approach to Ian's semantics:

spans p = map (takeWhile p) . takeWhile (not . null) . iterate (drop 1
. dropWhile p)
breaks p = spans $ not . p

Here you have to put the "not null" step in the middle. I wonder
if that interferes with the fusion.

-Yitz