Copyright | (c) Brent Yorgey Louis Wasserman 2008-2012 |
---|---|

License | BSD-style (see LICENSE) |

Maintainer | Brent Yorgey <byorgey@gmail.com> |

Stability | stable |

Portability | Haskell 2010 |

Safe Haskell | Safe |

Language | Haskell2010 |

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 flexible yet simple. See below for usage, examples, and detailed documentation of all exported functions. If you want to learn about the implementation, see Data.List.Split.Internals.

A git repository containing the source (including a module with over 40 QuickCheck properties) can be found at https://github.com/byorgey/split.

## Synopsis

- splitOn :: Eq a => [a] -> [a] -> [[a]]
- splitOneOf :: Eq a => [a] -> [a] -> [[a]]
- splitWhen :: (a -> Bool) -> [a] -> [[a]]
- endBy :: Eq a => [a] -> [a] -> [[a]]
- endByOneOf :: Eq a => [a] -> [a] -> [[a]]
- wordsBy :: (a -> Bool) -> [a] -> [[a]]
- linesBy :: (a -> Bool) -> [a] -> [[a]]
- chunksOf :: Int -> [e] -> [[e]]
- splitPlaces :: Integral a => [a] -> [e] -> [[e]]
- splitPlacesBlanks :: Integral a => [a] -> [e] -> [[e]]
- chop :: ([a] -> (b, [a])) -> [a] -> [b]
- divvy :: Int -> Int -> [a] -> [[a]]
- data Splitter a
- defaultSplitter :: Splitter a
- split :: Splitter a -> [a] -> [[a]]
- oneOf :: Eq a => [a] -> Splitter a
- onSublist :: Eq a => [a] -> Splitter a
- whenElt :: (a -> Bool) -> Splitter a
- dropDelims :: Splitter a -> Splitter a
- keepDelimsL :: Splitter a -> Splitter a
- keepDelimsR :: Splitter a -> Splitter a
- condense :: Splitter a -> Splitter a
- dropInitBlank :: Splitter a -> Splitter a
- dropFinalBlank :: Splitter a -> Splitter a
- dropInnerBlanks :: Splitter a -> Splitter a
- dropBlanks :: Splitter a -> Splitter a
- startsWith :: Eq a => [a] -> Splitter a
- startsWithOneOf :: Eq a => [a] -> Splitter a
- endsWith :: Eq a => [a] -> Splitter a
- endsWithOneOf :: Eq a => [a] -> Splitter a

# Getting started

To get started, you should take a look at the functions `splitOn`

,
`splitOneOf`

, `splitWhen`

, `endBy`

, `chunksOf`

, `splitPlaces`

,
and other functions listed in the next two sections. These
functions implement various common splitting operations, and one of
them will probably do the job 90% of the time. For example:

> splitOn "x" "axbxc" ["a","b","c"] > splitOn "x" "axbxcx" ["a","b","c",""] > endBy ";" "foo;bar;baz;" ["foo","bar","baz"] > splitWhen (<0) [1,3,-4,5,7,-9,0,2] [[1,3],[5,7],[0,2]] > splitOneOf ";.," "foo,bar;baz.glurk" ["foo","bar","baz","glurk"] > chunksOf 3 ['a'..'z'] ["abc","def","ghi","jkl","mno","pqr","stu","vwx","yz"]

If you want more flexibility, however, you can use the combinator library in terms of which these functions are defined. For more information, see the section labeled "Splitting Combinators".

The goal of this library is to be flexible yet simple. It does not implement any particularly sophisticated list-splitting methods, nor is it tuned for speed. If you find yourself wanting something more complicated or optimized, it probably means you should use a real parsing or regular expression library.

# Convenience functions

These functions implement some common splitting strategies. Note that all of the functions in this section drop delimiters from the final output, since that is a more common use case. If you wish to keep the delimiters somehow, see the "Splitting Combinators" section.

splitOn :: Eq a => [a] -> [a] -> [[a]] #

Split on the given sublist. Equivalent to

. For example:`split`

. `dropDelims`

. `onSublist`

splitOn ".." "a..b...c....d.." == ["a","b",".c","","d",""]

In some parsing combinator frameworks this is also known as
`sepBy`

.

Note that this is the right inverse of the `intercalate`

function
from Data.List, that is,

intercalate x . splitOn x === id

is the identity on
certain lists, but it is tricky to state the precise conditions
under which this holds. (For example, it is not enough to say
that `splitOn`

x . `intercalate`

x`x`

does not occur in any elements of the input list.
Working out why is left as an exercise for the reader.)

splitOneOf :: Eq a => [a] -> [a] -> [[a]] #

Split on any of the given elements. Equivalent to

. For example:`split`

. `dropDelims`

. `oneOf`

splitOneOf ";.," "foo,bar;baz.glurk" == ["foo","bar","baz","glurk"]

splitWhen :: (a -> Bool) -> [a] -> [[a]] #

Split on elements satisfying the given predicate. Equivalent to

. For example:`split`

. `dropDelims`

. `whenElt`

splitWhen (<0) [1,3,-4,5,7,-9,0,2] == [[1,3],[5,7],[0,2]]

endBy :: Eq a => [a] -> [a] -> [[a]] #

Split into chunks terminated by the given subsequence.
Equivalent to

. For example:`split`

. `dropFinalBlank`

. `dropDelims`

. `onSublist`

endBy ";" "foo;bar;baz;" == ["foo","bar","baz"]

Note also that the `lines`

function from Data.List is equivalent
to

.`endBy`

"\n"

endByOneOf :: Eq a => [a] -> [a] -> [[a]] #

Split into chunks terminated by one of the given elements.
Equivalent to

. For example:`split`

. `dropFinalBlank`

. `dropDelims`

. `oneOf`

endByOneOf ";," "foo;bar,baz;" == ["foo","bar","baz"]

wordsBy :: (a -> Bool) -> [a] -> [[a]] #

Split into "words", with word boundaries indicated by the given
predicate. Satisfies

; equivalent to `words`

=== wordsBy
`isSpace`

. For example:`split`

. `dropBlanks`

. `dropDelims`

. `whenElt`

wordsBy (=='x') "dogxxxcatxbirdxx" == ["dog","cat","bird"]

linesBy :: (a -> Bool) -> [a] -> [[a]] #

Split into "lines", with line boundaries indicated by the given
predicate. Satisfies

; equivalent to
`lines`

=== linesBy (=='\n')

. For example:`split`

. `dropFinalBlank`

. `dropDelims`

. `whenElt`

linesBy (=='x') "dogxxxcatxbirdxx" == ["dog","","","cat","bird",""]

# Other splitting methods

Other useful splitting methods which are not implemented using the combinator framework.

chunksOf :: Int -> [e] -> [[e]] #

splits a list into length-n pieces. The last
piece will be shorter if `chunksOf`

n`n`

does not evenly divide the length of
the list. If `n <= 0`

,

returns an infinite list
of empty lists. For example:`chunksOf`

n l

Note that

is `chunksOf`

n []`[]`

, not `[[]]`

. This is
intentional, and is consistent with a recursive definition of
`chunksOf`

; it satisfies the property that

chunksOf n xs ++ chunksOf n ys == chunksOf n (xs ++ ys)

whenever `n`

evenly divides the length of `xs`

.

splitPlaces :: Integral a => [a] -> [e] -> [[e]] #

Split a list into chunks of the given lengths. For example:

splitPlaces [2,3,4] [1..20] == [[1,2],[3,4,5],[6,7,8,9]] splitPlaces [4,9] [1..10] == [[1,2,3,4],[5,6,7,8,9,10]] splitPlaces [4,9,3] [1..10] == [[1,2,3,4],[5,6,7,8,9,10]]

If the input list is longer than the total of the given lengths, then the remaining elements are dropped. If the list is shorter than the total of the given lengths, then the result may contain fewer chunks than requested, and the last chunk may be shorter than requested.

splitPlacesBlanks :: Integral a => [a] -> [e] -> [[e]] #

Split a list into chunks of the given lengths. Unlike
`splitPlaces`

, the output list will always be the same length as
the first input argument. If the input list is longer than the
total of the given lengths, then the remaining elements are
dropped. If the list is shorter than the total of the given
lengths, then the last several chunks will be shorter than
requested or empty. For example:

splitPlacesBlanks [2,3,4] [1..20] == [[1,2],[3,4,5],[6,7,8,9]] splitPlacesBlanks [4,9] [1..10] == [[1,2,3,4],[5,6,7,8,9,10]] splitPlacesBlanks [4,9,3] [1..10] == [[1,2,3,4],[5,6,7,8,9,10],[]]

Notice the empty list in the output of the third example, which
differs from the behavior of `splitPlaces`

.

chop :: ([a] -> (b, [a])) -> [a] -> [b] #

A useful recursion pattern for processing a list to produce a new list, often used for "chopping" up the input list. Typically chop is called with some function that will consume an initial prefix of the list and produce a value and the rest of the list.

For example, many common Prelude functions can be implemented in
terms of `chop`

:

group :: (Eq a) => [a] -> [[a]] group = chop (\ xs@(x:_) -> span (==x) xs) words :: String -> [String] words = filter (not . null) . chop (span (not . isSpace) . dropWhile isSpace)

divvy :: Int -> Int -> [a] -> [[a]] #

Divides up an input list into a set of sublists, according to `n`

and `m`

input specifications you provide. Each sublist will have `n`

items, and the
start of each sublist will be offset by `m`

items from the previous one.

divvy 5 5 [1..20] == [[1,2,3,4,5],[6,7,8,9,10],[11,12,13,14,15],[16,17,18,19,20]]

In the case where a source list's trailing elements do no fill an entire sublist, those trailing elements will be dropped.

divvy 5 2 [1..10] == [[1,2,3,4,5],[3,4,5,6,7],[5,6,7,8,9]]

As an example, you can generate a moving average over a list of prices:

type Prices = [Float] type AveragePrices = [Float] average :: [Float] -> Float average xs = sum xs / (fromIntegral $ length xs) simpleMovingAverage :: Prices -> AveragePrices simpleMovingAverage priceList = map average divvyedPrices where divvyedPrices = divvy 20 1 priceList

# Splitting combinators

The core of the library is the `Splitter`

type, which represents a
particular list-splitting strategy. All of the combinators revolve
around constructing or transforming `Splitter`

objects; once a
suitable `Splitter`

has been created, it can be run with the
`split`

function. For example:

> split (dropBlanks . condense $ whenElt (<0)) [1,2,4,-5,-6,4,9,-19,-30] [[1,2,4],[-5,-6],[4,9],[-19,-30]]

defaultSplitter :: Splitter a #

The default splitting strategy: keep delimiters in the output as separate chunks, don't condense multiple consecutive delimiters into one, keep initial and final blank chunks. Default delimiter is the constantly false predicate.

Note that `defaultSplitter`

should normally not be used; use
`oneOf`

, `onSublist`

, or `whenElt`

instead, which are the same as
the `defaultSplitter`

with just the delimiter overridden.

The `defaultSplitter`

strategy with any delimiter gives a
maximally information-preserving splitting strategy, in the sense
that (a) taking the `concat`

of the output yields the original
list, and (b) given only the output list, we can reconstruct a
`Splitter`

which would produce the same output list again given
the original input list. This default strategy can be overridden
to allow discarding various sorts of information.

split :: Splitter a -> [a] -> [[a]] #

Split a list according to the given splitting strategy. This is
how to "run" a `Splitter`

that has been built using the other
combinators.

## Basic strategies

All these basic strategies have the same parameters as the
`defaultSplitter`

except for the delimiter.

oneOf :: Eq a => [a] -> Splitter a #

A splitting strategy that splits on any one of the given elements. For example:

split (oneOf "xyz") "aazbxyzcxd" == ["aa","z","b","x","","y","","z","c","x","d"]

onSublist :: Eq a => [a] -> Splitter a #

A splitting strategy that splits on the given list, when it is encountered as an exact subsequence. For example:

split (onSublist "xyz") "aazbxyzcxd" == ["aazb","xyz","cxd"]

Note that splitting on the empty list is a special case, which splits just before every element of the list being split. For example:

split (onSublist "") "abc" == ["","","a","","b","","c"] split (dropDelims . dropBlanks $ onSublist "") "abc" == ["a","b","c"]

However, if you want to break a list into singleton elements like
this, you are better off using

, or better yet,
`chunksOf`

1

.`map`

(:[])

whenElt :: (a -> Bool) -> Splitter a #

A splitting strategy that splits on any elements that satisfy the given predicate. For example:

split (whenElt (<0)) [2,4,-3,6,-9,1] == [[2,4],[-3],[6],[-9],[1]]

## Strategy transformers

Functions for altering splitting strategy parameters.

dropDelims :: Splitter a -> Splitter a #

Drop delimiters from the output (the default is to keep them). For example,

split (oneOf ":") "a:b:c" == ["a", ":", "b", ":", "c"] split (dropDelims $ oneOf ":") "a:b:c" == ["a", "b", "c"]

keepDelimsL :: Splitter a -> Splitter a #

Keep delimiters in the output by prepending them to adjacent chunks. For example:

split (keepDelimsL $ oneOf "xyz") "aazbxyzcxd" == ["aa","zb","x","y","zc","xd"]

keepDelimsR :: Splitter a -> Splitter a #

Keep delimiters in the output by appending them to adjacent chunks. For example:

split (keepDelimsR $ oneOf "xyz") "aazbxyzcxd" == ["aaz","bx","y","z","cx","d"]

condense :: Splitter a -> Splitter a #

Condense multiple consecutive delimiters into one. For example:

split (condense $ oneOf "xyz") "aazbxyzcxd" == ["aa","z","b","xyz","c","x","d"] split (dropDelims $ oneOf "xyz") "aazbxyzcxd" == ["aa","b","","","c","d"] split (condense . dropDelims $ oneOf "xyz") "aazbxyzcxd" == ["aa","b","c","d"]

dropInitBlank :: Splitter a -> Splitter a #

Don't generate a blank chunk if there is a delimiter at the beginning. For example:

split (oneOf ":") ":a:b" == ["",":","a",":","b"] split (dropInitBlank $ oneOf ":") ":a:b" == [":","a",":","b"]

dropFinalBlank :: Splitter a -> Splitter a #

Don't generate a blank chunk if there is a delimiter at the end. For example:

split (oneOf ":") "a:b:" == ["a",":","b",":",""] split (dropFinalBlank $ oneOf ":") "a:b:" == ["a",":","b",":"]

dropInnerBlanks :: Splitter a -> Splitter a #

Don't generate blank chunks between consecutive delimiters. For example:

split (oneOf ":") "::b:::a" == ["",":","",":","b",":","",":","",":","a"] split (dropInnerBlanks $ oneOf ":") "::b:::a" == ["", ":",":","b",":",":",":","a"]

## Derived combinators

Combinators which can be defined in terms of other combinators, but are provided for convenience.

dropBlanks :: Splitter a -> Splitter a #

Drop all blank chunks from the output, and condense consecutive
delimiters into one. Equivalent to

. For example:`dropInitBlank`

. `dropFinalBlank`

. `condense`

split (oneOf ":") "::b:::a" == ["",":","",":","b",":","",":","",":","a"] split (dropBlanks $ oneOf ":") "::b:::a" == ["::","b",":::","a"]

startsWith :: Eq a => [a] -> Splitter a #

Make a strategy that splits a list into chunks that all start
with the given subsequence (except possibly the first).
Equivalent to

.
For example:`dropInitBlank`

. `keepDelimsL`

. `onSublist`

split (startsWith "app") "applyapplicativeapplaudapproachapple" == ["apply","applicative","applaud","approach","apple"]

startsWithOneOf :: Eq a => [a] -> Splitter a #

Make a strategy that splits a list into chunks that all start
with one of the given elements (except possibly the first).
Equivalent to

. For
example:`dropInitBlank`

. `keepDelimsL`

. `oneOf`

split (startsWithOneOf ['A'..'Z']) "ACamelCaseIdentifier" == ["A","Camel","Case","Identifier"]

endsWith :: Eq a => [a] -> Splitter a #

Make a strategy that splits a list into chunks that all end with
the given subsequence, except possibly the last. Equivalent to

. For example:`dropFinalBlank`

. `keepDelimsR`

. `onSublist`

split (endsWith "ly") "happilyslowlygnarlylily" == ["happily","slowly","gnarly","lily"]

endsWithOneOf :: Eq a => [a] -> Splitter a #

Make a strategy that splits a list into chunks that all end with
one of the given elements, except possibly the last. Equivalent
to

. For example:`dropFinalBlank`

. `keepDelimsR`

. `oneOf`

split (condense $ endsWithOneOf ".,?! ") "Hi, there! How are you?" == ["Hi, ","there! ","How ","are ","you?"]