Copyright  (c) Brent Yorgey Louis Wasserman 20082012 

License  BSDstyle (see LICENSE) 
Maintainer  Brent Yorgey <byorgey@gmail.com> 
Stability  stable 
Portability  Haskell 2010 
Safe Haskell  Safe 
Language  Haskell2010 
Implementation module for Data.List.Split, a combinator library for splitting lists. See the Data.List.Split documentation for more description and examples.
 data Splitter a = Splitter {}
 defaultSplitter :: Splitter a
 newtype Delimiter a = Delimiter [a > Bool]
 matchDelim :: Delimiter a > [a] > Maybe ([a], [a])
 data DelimPolicy
 data CondensePolicy
 data EndPolicy
 data Chunk a
 type SplitList a = [Chunk a]
 fromElem :: Chunk a > [a]
 isDelim :: Chunk a > Bool
 isText :: Chunk a > Bool
 splitInternal :: Delimiter a > [a] > SplitList a
 postProcess :: Splitter a > SplitList a > SplitList a
 doDrop :: DelimPolicy > SplitList a > SplitList a
 doCondense :: CondensePolicy > SplitList a > SplitList a
 insertBlanks :: CondensePolicy > SplitList a > SplitList a
 insertBlanks' :: CondensePolicy > SplitList a > SplitList a
 doMerge :: DelimPolicy > SplitList a > SplitList a
 mergeLeft :: SplitList a > SplitList a
 mergeRight :: SplitList a > SplitList a
 dropInitial :: EndPolicy > SplitList a > SplitList a
 dropFinal :: EndPolicy > SplitList a > SplitList 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
 splitOneOf :: Eq a => [a] > [a] > [[a]]
 splitOn :: 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]]
 build :: ((a > [a] > [a]) > [a] > [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]]
Types and utilities
A splitting strategy.
Splitter  

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 informationpreserving 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.
A delimiter is a list of predicates on elements, matched by some contiguous subsequence of a list.
matchDelim :: Delimiter a > [a] > Maybe ([a], [a]) #
Try to match a delimiter at the start of a list, either failing or decomposing the list into the portion which matched the delimiter and the remainder.
data DelimPolicy #
What to do with delimiters?
data CondensePolicy #
What to do with multiple consecutive delimiters?
Condense  Condense into a single delimiter. 
DropBlankFields  Keep consecutive delimiters separate, but don't insert blank chunks in between them. 
KeepBlankFields  Insert blank chunks between consecutive delimiters. 
What to do with a blank chunk at either end of the list (i.e. when the list begins or ends with a delimiter).
Tag chunks as delimiters or text.
type SplitList a = [Chunk a] #
Internal representation of a split list that tracks which pieces are delimiters and which aren't.
Implementation
splitInternal :: Delimiter a > [a] > SplitList a #
Given a delimiter to use, split a list into an internal representation with chunks tagged as delimiters or text. This transformation is lossless; in particular,
concatMap
fromElem
(splitInternal
d l) == l.
postProcess :: Splitter a > SplitList a > SplitList a #
Given a split list in the internal tagged representation, produce
a new internal tagged representation corresponding to the final
output, according to the strategy defined by the given
Splitter
.
doDrop :: DelimPolicy > SplitList a > SplitList a #
Drop delimiters if the DelimPolicy
is Drop
.
doCondense :: CondensePolicy > SplitList a > SplitList a #
Condense multiple consecutive delimiters into one if the
CondensePolicy
is Condense
.
insertBlanks :: CondensePolicy > SplitList a > SplitList a #
Insert blank chunks between any remaining consecutive delimiters
(unless the condense policy is DropBlankFields
), and at the
beginning or end if the first or last element is a delimiter.
insertBlanks' :: CondensePolicy > SplitList a > SplitList a #
Insert blank chunks between consecutive delimiters.
doMerge :: DelimPolicy > SplitList a > SplitList a #
Merge delimiters into adjacent chunks according to the DelimPolicy
.
mergeLeft :: SplitList a > SplitList a #
Merge delimiters with adjacent chunks to the right (yes, that's not a typo: the delimiters should end up on the left of the chunks, so they are merged with chunks to their right).
mergeRight :: SplitList a > SplitList a #
Merge delimiters with adjacent chunks to the left.
dropInitial :: EndPolicy > SplitList a > SplitList a #
Drop an initial blank chunk according to the given EndPolicy
.
dropFinal :: EndPolicy > SplitList a > SplitList a #
Drop a final blank chunk according to the given EndPolicy
.
Combinators
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 delimiters.
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
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
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?"]
Convenience functions
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"]
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
xx
does not occur in any elements of the input list.
Working out why is left as an exercise for the reader.)
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
build :: ((a > [a] > [a]) > [a] > [a]) > [a] #
Standard build function, specialized to building lists.
Usually build is given the rank2 type
build :: (forall b. (a > b > b) > b > b) > [a]
but since we only use it when (b ~ [a])
, we give it the more
restricted type signature in order to avoid needing a
nonHaskell2010 extension.
Note that the 0.1.4.3 release of this package did away with a
custom build
implementation in favor of importing one from
GHC.Exts, which was (reportedly) faster for some applications.
However, in the interest of simplicity and complete Haskell2010
compliance as split
is being included in the Haskel Platform,
version 0.2.1.0 has gone back to defining build
manually. This
is in line with split
's design philosophy of having efficiency
as a nongoal.
chunksOf :: Int > [e] > [[e]] #
splits a list into lengthn pieces. The last
piece will be shorter if chunksOf
nn
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