A splitting strategy.

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.

A delimiter can either be a predicate on elements, or a list of elements to be matched as a subsequence.

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.

What to do with delimiters?

What to do with multiple 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.

Internal representation of a split list that tracks which pieces are delimiters and which aren't.

Untag a Chunk.

Test whether a Chunk is a delimiter.

Test whether a Chunk is text.

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.

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.

Drop delimiters if the DelimPolicy is Drop.

Condense multiple consecutive delimiters into one if the CondensePolicy is Condense.

Insert blank chunks between any remaining consecutive delimiters, and at the beginning or end if the first or last element is a delimiter.

Insert blank chunks between consecutive delimiters.

Merge delimiters into adjacent chunks according to the DelimPolicy.

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).

Merge delimiters with adjacent chunks to the left.

Drop an initial blank chunk according to the given EndPolicy.

Drop a final blank chunk according to the given EndPolicy.

Split a list according to the given splitting strategy. This is how to "run" a Splitter that has been built using the other combinators.

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"]

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 splitEvery 1, or better yet, map (:[]).

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]]

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"]

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

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

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 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"]

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"]

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",":"]

Drop all blank chunks from the output. Equivalent to dropInitBlank . dropFinalBlank . condense. For example:

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

Make a strategy that splits a list into chunks that all start with the given subsequence (except possibly the first). Equivalent to dropInitBlank . keepDelimsL . onSublist. For example:

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

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

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

Make a strategy that splits a list into chunks that all end with the given subsequence, except possibly the last. Equivalent to dropFinalBlank . keepDelimsR . onSublist. For example:

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

Make a strategy that splits a list into chunks that all end with one of the given elements, except possibly the last. Equivalent to dropFinalBlank . keepDelimsR . oneOf. For example:

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

Split on any of the given elements. Equivalent to split . dropDelims . oneOf. For example:

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

Split on the given sublist. Equivalent to split . dropDelims . onSublist. For example:

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

Split on elements satisfying the given predicate. Equivalent to split . dropDelims . whenElt. For example:

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

A synonym for splitOn.

A synonym for splitOneOf.

Split into chunks terminated by the given subsequence. Equivalent to split . dropFinalBlank . dropDelims . onSublist. For example:

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

Note also that the lines function from Data.List is equivalent to endBy "\n".

Split into chunks terminated by one of the given elements. Equivalent to split . dropFinalBlank . dropDelims . oneOf.

A synonym for sepBy / splitOn.

Note that this is the right inverse of the intercalate function from Data.List, that is, intercalate x . unintercalate x == id. It is also the case that unintercalate x . intercalate x is idempotent. unintercalate x . intercalate x 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 x does not occur in any elements of the input list. Working out why is left as an exercise for the reader.)

Split into words, with word boundaries indicated by the given predicate. Satisfies words === wordsBy isSpace; equivalent to split . dropBlanks . dropDelims . whenElt. For example:

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

Split into lines, with line boundaries indicated by the given predicate. Satisfies lines === linesBy (=='\n'); equivalent to split . dropFinalBlank . dropDelims . whenElt. For example:

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

splitEvery n splits a list into length-n pieces. The last piece will be shorter if n does not evenly divide the length of the list. If n <= 0, splitEvery n l returns an infinite list of empty lists.

Note that splitEvery n [] is [], not [[]]. This is intentional, and is consistent with a recursive definition of splitEvery; it satisfies the property that

splitEvery n xs ++ splitEvery n ys == splitEvery n (xs ++ ys)

whenever n evenly divides the length of xs.

A common synonym for splitEvery.

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]]

The behavior of splitPlaces ls xs when sum ls /= length xs can be inferred from the above examples and the fact that splitPlaces is total.

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. 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],[]]

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)