Haskell platform proposal: split package

Brent Yorgey byorgey at seas.upenn.edu
Fri Jul 20 22:35:57 CEST 2012

Hello everyone,

This is a proposal for the split package [1] to be included in the
next major release of the Haskell platform.

Everyone is invited to review this proposal, following the standard
procedure [2] for proposing and reviewing packages.
Review comments should be sent to the libraries mailing list by August
20 (arbitrarily chosen; there's plenty of time before the October 1
deadline [3]). The Haskell Platform wiki will be kept up-to-date with
the results of the review process:


[1] http://hackage.haskell.org/package/split
[2] http://trac.haskell.org/haskell-platform/wiki/AddingPackages 
[3] http://trac.haskell.org/haskell-platform/wiki/ReleaseTimetable


Proposal author and package maintainer: 
  Brent Yorgey <byorgey at cis.upenn.edu>


The Data.List.Split module contains a wide range of strategies for
splitting lists with respect to some sort of delimiter, mostly
implemented through a unified combinator interface. The goal is to be
a flexible yet simple alternative to the standard 'split' function
found in some other mainstream languages.

Documentation and tarball from the hackage page:


Development repo:

  darcs get http://code.haskell.org/~byorgey/code/split


Splitting a list into chunks based on some sort of delimiter(s) is a
common need, and is provided in the standard libraries of several
mainstream languages (e.g. Python [4], Ruby [5], Java [6]).  Haskell
beginners routinely ask whether such a function exists in the standard
libraries.  For a long time, the answer was no.  Adding such a
function to Haskell's standard libraries has been proposed multiple
times over the years, but consensus was never reached on the design of
such a function. (See, e.g. [7, 8, 9].) 

[4] http://docs.python.org/py3k/library/stdtypes.html?highlight=split#str.split
[5] http://www.ruby-doc.org/core-1.9.3/String.html#method-i-split
[6] http://docs.oracle.com/javase/7/docs/api/java/lang/String.html#split(java.lang.String)
[7] http://www.haskell.org/pipermail/libraries/2006-July/005504.html
[8] http://www.haskell.org/pipermail/libraries/2006-October/006072.html
[9] http://www.haskell.org/pipermail/libraries/2008-January/008922.html

In December 2008 the split package was released, implementing not just
a single split method, but a wide range of splitting strategies.

Since then the split package has gained wide acceptance, with almost
95 reverse dependencies [10], putting it in the top 40 for number of
reverse dependencies on Hackage.

[10] http://packdeps.haskellers.com/reverse/split 

The package is quite stable. Since the 0.1.4 release in April 2011
only very minor updates have been made.  It has a large suite of
QuickCheck properties [11]; to my recollection no bugs have ever been

[11] http://code.haskell.org/~byorgey/code/split/Properties.hs


For a detailed description of the package API and example usage, see
the Haddock documentation:


Design decisions

Most of the library is based around a (rather simple) combinator
interface.  Combinators are used to build up configuration records
(recording options such as whether to keep delimiters, whether to keep
blank segments, etc).  A configuration record is finally handed off to
a function which performs a generic maximally-information-preserving
splitting algorithm and then does various postprocessing steps (based
on the configuration) to selectively throw information away.  It is
probably not the fastest way to implement these methods, but speed is
explicitly not a design goal: the aim is to provide a reasonably wide
range of splitting strategies which can be used simply.  Blazing speed
(or more complex processing), when needed, can be obtained from a
proper parsing package.

Open issues

Use of GHC.Exts

At the request of a user, the release switched from defining
its own version of the standard 'build' function, to importing it from
GHC.Exts.  This allows GHC to do more optimization, resulting in
reported speedups to uses of splitEvery, splitPlaces, and
splitPlacesBlanks.  However, this makes the library GHC-specific.  If
any reviewers think this is an issue I would be willing to go back to
defining build by hand, or use CPP macros to select between build
implementations based on the compiler.

Missing strategies

The specific way that the generic splitting algorithm is implemented
does preclude some imaginable splitting strategies.  For example, a
few years ago I tried adding a strategy that used a predicate on pairs
of elements, splitting down the middle of any pairs that satisfy the
predicate, but gave up because it simply did not fit.

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