Proposal: Add a splitBy / splitOn in Data.List

Elliot Cameron eacameron at
Fri Nov 2 11:38:45 UTC 2018

Despite these subtleties, I must confess I've often wanted to whip up a
quick script and been frustrated that these functions are missing from
base. For example using Haskell as a sed/awk alternative can be pleasant
*if* the functions you need are in base. What's more, in many years I've
only really wanted one or two versions of this.

What if we added the most flexible of versions and included only that? This
version would accept multicharacter delimiters, always throw them away, and
always produce a new entry in the result for every occurrence of the
delimiter. If you don't want the empty entries, you can filter. If you
don't want leading, you can dropWhile. If you want the delimiters back, you
can map. This seems like a nice trade-off for just being available in base.

On Fri, Nov 2, 2018, 1:51 AM Edward Kmett <ekmett at wrote:

> The main thing that prevented it from going into base is the number of
> subtleties about what precisely it means to properly "split" something.
> Most languages make fairly arbitrary calls on topics such as:
> * Do you split on list elements (e.g. ',') or list of elements, so you
> can multi-character delimiters ", "? What about multiple types of thing
> that are all delimiters, e.g. any whitespace character?
> * What do you do with the delimiters?
> * What happens with runs of delimiters?
> * What about initial or final runs of delimiters (e.g. leading spaces)?
> The end result was that a split package was written by Brent Yorgey back
> in 2008 or so that rather comprehensively covers the design space, and it
> was incorporated into the Haskell Platform.
> -Edward
> On Thu, Nov 1, 2018 at 1:34 PM Saurabh Nanda <saurabhnanda at>
> wrote:
>> This has certainly been discussed before. A quick Google search turned up
>> the following past discussions:
>>    -
>>    -
>> Is there anything blocking this discussion & implementation? Anything
>> that can be done to unblock it?
>> -- Saurabh.
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