{-# LANGUAGE GADTs, Rank2Types #-} ----------------------------------------------------------------------------- -- | -- Module : Data.List.Split.Internal -- Copyright : (c) Brent Yorgey 2008 -- License : BSD-style (see LICENSE) -- Maintainer : byorgey@gmail.com -- Stability : experimental -- Portability : unportable (GADTs, Rank2Types) -- -- Implementation module for "Data.List.Split", a combinator library -- for splitting lists. See the "Data.List.Split" documentation for -- more description and examples. -- ----------------------------------------------------------------------------- module Data.List.Split.Internals where import Data.List (genericSplitAt) -- * Types and utilities -- | A splitting strategy. data Splitter a = Splitter { delimiter :: Delimiter a -- ^ What delimiter to split on , delimPolicy :: DelimPolicy -- ^ What to do with delimiters (drop -- from output, keep as separate -- elements in output, or merge with -- previous or following chunks) , condensePolicy :: CondensePolicy -- ^ What to do with multiple -- consecutive delimiters , initBlankPolicy :: EndPolicy -- ^ Drop an initial blank? , finalBlankPolicy :: EndPolicy -- ^ Drop a final blank? } -- | 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. defaultSplitter :: Splitter a defaultSplitter = Splitter { delimiter = DelimEltPred (const False) , delimPolicy = Keep , condensePolicy = KeepBlankFields , initBlankPolicy = KeepBlank , finalBlankPolicy = KeepBlank } -- | A delimiter can either be a predicate on elements, or a list of -- elements to be matched as a subsequence. data Delimiter a where DelimEltPred :: (a -> Bool) -> Delimiter a DelimSublist :: Eq a => [a] -> Delimiter 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. matchDelim :: Delimiter a -> [a] -> Maybe ([a],[a]) matchDelim (DelimEltPred p) (x:xs) | p x = Just ([x],xs) | otherwise = Nothing matchDelim (DelimEltPred _) [] = Nothing matchDelim (DelimSublist []) xs = Just ([],xs) matchDelim (DelimSublist _) [] = Nothing matchDelim (DelimSublist (d:ds)) (x:xs) | d == x = matchDelim (DelimSublist ds) xs >>= \(h,t) -> Just (d:h,t) | otherwise = Nothing -- | What to do with delimiters? data DelimPolicy = Drop -- ^ Drop delimiters from the output. | Keep -- ^ Keep delimiters as separate chunks -- of the output. | KeepLeft -- ^ Keep delimiters in the output, -- prepending them to the following -- chunk. | KeepRight -- ^ Keep delimiters in the output, -- appending them to the previous chunk. deriving (Eq, Show) -- | What to do with multiple consecutive delimiters? data CondensePolicy = Condense -- ^ Condense into a single delimiter. | KeepBlankFields -- ^ Insert blank chunks -- between consecutive -- delimiters. deriving (Eq, Show) -- | What to do with a blank chunk at either end of the list -- (i.e. when the list begins or ends with a delimiter). data EndPolicy = DropBlank | KeepBlank deriving (Eq, Show) -- | Tag chunks as delimiters or text. data Chunk a = Delim [a] | Text [a] deriving (Show, Eq) -- | Internal representation of a split list that tracks which pieces -- are delimiters and which aren't. type SplitList a = [Chunk a] -- | Untag a 'Chunk'. fromElem :: Chunk a -> [a] fromElem (Text as) = as fromElem (Delim as) = as -- | Test whether a 'Chunk' is a delimiter. isDelim :: Chunk a -> Bool isDelim (Delim _) = True isDelim _ = False -- | Test whether a 'Chunk' is text. isText :: Chunk a -> Bool isText (Text _) = True isText _ = False -- | Standard build function. build :: (forall b. (a -> b -> b) -> b -> b) -> [a] build g = g (:) [] -- * Implementation -- | 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@. splitInternal :: Delimiter a -> [a] -> SplitList a splitInternal _ [] = [] splitInternal d xxs | null xs = toSplitList match | otherwise = Text xs : toSplitList match where (xs,match) = breakDelim d xxs toSplitList Nothing = [] toSplitList (Just ([],r:rs)) = Delim [] : Text [r] : splitInternal d rs toSplitList (Just (delim,rest)) = Delim delim : splitInternal d rest breakDelim :: Delimiter a -> [a] -> ([a],Maybe ([a],[a])) breakDelim d@(DelimEltPred p) (x:xs) | p x = ([],Just ([x],xs)) | otherwise = let (ys,match) = breakDelim d xs in (x:ys,match) breakDelim (DelimEltPred _) [] = ([],Nothing) breakDelim (DelimSublist []) xs = ([],Just ([],xs)) breakDelim (DelimSublist _) [] = ([],Nothing) breakDelim (DelimSublist ds) xxs@(x:xs) = case matchSublist ds xxs of Nothing -> let (ys,match) = breakDelim (DelimSublist ds) xs in (x:ys,match) Just rest -> ([],Just (ds,rest)) matchSublist :: Eq a => [a] -> [a] -> Maybe [a] matchSublist [] xs = Just xs matchSublist _ [] = Nothing matchSublist (d:ds) (x:xs) = if d==x then matchSublist ds xs else Nothing -- | 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'. postProcess :: Splitter a -> SplitList a -> SplitList a postProcess s = dropFinal (finalBlankPolicy s) . dropInitial (initBlankPolicy s) . doMerge (delimPolicy s) . doDrop (delimPolicy s) . insertBlanks . doCondense (condensePolicy s) -- | Drop delimiters if the 'DelimPolicy' is 'Drop'. doDrop :: DelimPolicy -> SplitList a -> SplitList a doDrop Drop l = [ c | c@(Text _) <- l ] doDrop _ l = l -- | Condense multiple consecutive delimiters into one if the -- 'CondensePolicy' is 'Condense'. doCondense :: CondensePolicy -> SplitList a -> SplitList a doCondense KeepBlankFields ls = ls doCondense Condense ls = condense' ls where condense' [] = [] condense' (c@(Text _) : l) = c : condense' l condense' l = (Delim $ concatMap fromElem ds) : condense' rest where (ds,rest) = span isDelim l -- | Insert blank chunks between any remaining consecutive delimiters, -- and at the beginning or end if the first or last element is a -- delimiter. insertBlanks :: SplitList a -> SplitList a insertBlanks [] = [Text []] insertBlanks (d@(Delim _) : l) = Text [] : insertBlanks' (d:l) insertBlanks l = insertBlanks' l -- | Insert blank chunks between consecutive delimiters. insertBlanks' :: SplitList a -> SplitList a insertBlanks' [] = [] insertBlanks' (d1@(Delim _) : d2@(Delim _) : l) = d1 : Text [] : insertBlanks' (d2:l) insertBlanks' [d@(Delim _)] = [d, Text []] insertBlanks' (c : l) = c : insertBlanks' l -- | Merge delimiters into adjacent chunks according to the 'DelimPolicy'. doMerge :: DelimPolicy -> SplitList a -> SplitList a doMerge KeepLeft = mergeLeft doMerge KeepRight = mergeRight doMerge _ = id -- | 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). mergeLeft :: SplitList a -> SplitList a mergeLeft [] = [] mergeLeft ((Delim d) : (Text c) : l) = Text (d++c) : mergeLeft l mergeLeft (c : l) = c : mergeLeft l -- | Merge delimiters with adjacent chunks to the left. mergeRight :: SplitList a -> SplitList a mergeRight [] = [] mergeRight ((Text c) : (Delim d) : l) = Text (c++d) : mergeRight l mergeRight (c : l) = c : mergeRight l -- | Drop an initial blank chunk according to the given 'EndPolicy'. dropInitial :: EndPolicy -> SplitList a -> SplitList a dropInitial DropBlank (Text [] : l) = l dropInitial _ l = l -- | Drop a final blank chunk according to the given 'EndPolicy'. dropFinal :: EndPolicy -> SplitList a -> SplitList a dropFinal _ [] = [] dropFinal DropBlank l = dropFinal' l where dropFinal' [] = [] dropFinal' [Text []] = [] dropFinal' (x:xs) = x:dropFinal' xs dropFinal _ l = l -- * Combinators -- | Split a list according to the given splitting strategy. This is -- how to \"run\" a 'Splitter' that has been built using the other -- combinators. split :: Splitter a -> [a] -> [[a]] split s = map fromElem . postProcess s . splitInternal (delimiter s) -- ** Basic strategies -- -- $ All these basic strategies have the same parameters as the -- 'defaultSplitter' except for the delimiters. -- | 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"] oneOf :: Eq a => [a] -> Splitter a oneOf elts = defaultSplitter { delimiter = DelimEltPred (`elem` elts) } -- | 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' (:[])@. onSublist :: Eq a => [a] -> Splitter a onSublist lst = defaultSplitter { delimiter = DelimSublist lst } -- | 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]] whenElt :: (a -> Bool) -> Splitter a whenElt p = defaultSplitter { delimiter = DelimEltPred p } -- ** Strategy transformers -- | 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"] dropDelims :: Splitter a -> Splitter a dropDelims s = s { delimPolicy = Drop } -- | Keep delimiters in the output by prepending them to adjacent -- chunks. For example: -- -- > split (keepDelimsL $ oneOf "xyz") "aazbxyzcxd" == ["aa","zb","x","y","zc","xd"] keepDelimsL :: Splitter a -> Splitter a keepDelimsL s = s { delimPolicy = KeepLeft } -- | Keep delimiters in the output by appending them to adjacent -- chunks. For example: -- -- > split (keepDelimsR $ oneOf "xyz") "aazbxyzcxd" == ["aaz","bx","y","z","cx","d"] keepDelimsR :: Splitter a -> Splitter a keepDelimsR s = s { delimPolicy = KeepRight } -- | 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"] condense :: Splitter a -> Splitter a condense s = s { condensePolicy = Condense } -- | 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"] dropInitBlank :: Splitter a -> Splitter a dropInitBlank s = s { initBlankPolicy = DropBlank } -- | 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",":"] dropFinalBlank :: Splitter a -> Splitter a dropFinalBlank s = s { finalBlankPolicy = DropBlank } -- ** Derived combinators -- | 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"] dropBlanks :: Splitter a -> Splitter a dropBlanks = dropInitBlank . dropFinalBlank . condense -- | 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"] startsWith :: Eq a => [a] -> Splitter a startsWith = dropInitBlank . keepDelimsL . onSublist -- | 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"] startsWithOneOf :: Eq a => [a] -> Splitter a startsWithOneOf = dropInitBlank . keepDelimsL . oneOf -- | 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"] endsWith :: Eq a => [a] -> Splitter a endsWith = dropFinalBlank . keepDelimsR . onSublist -- | 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?"] endsWithOneOf :: Eq a => [a] -> Splitter a endsWithOneOf = dropFinalBlank . keepDelimsR . oneOf -- ** 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 even -- though it is not the default. -- | Split on any of the given elements. Equivalent to @'split' -- . 'dropDelims' . 'oneOf'@. For example: -- -- > splitOneOf ";.," "foo,bar;baz.glurk" == ["foo","bar","baz","glurk"] splitOneOf :: Eq a => [a] -> [a] -> [[a]] splitOneOf = split . dropDelims . oneOf -- | Split on the given sublist. Equivalent to @'split' -- . 'dropDelims' . 'onSublist'@. For example: -- -- > splitOn ".." "a..b...c....d.." == ["a","b",".c","","d",""] splitOn :: Eq a => [a] -> [a] -> [[a]] splitOn = split . dropDelims . onSublist -- | 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]] splitWhen :: (a -> Bool) -> [a] -> [[a]] splitWhen = split . dropDelims . whenElt -- | A synonym for 'splitOn'. sepBy :: Eq a => [a] -> [a] -> [[a]] sepBy = splitOn -- | A synonym for 'splitOneOf'. sepByOneOf :: Eq a => [a] -> [a] -> [[a]] sepByOneOf = 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\"@. endBy :: Eq a => [a] -> [a] -> [[a]] endBy = split . dropFinalBlank . dropDelims . onSublist -- | Split into chunks terminated by one of the given elements. -- Equivalent to @'split' . 'dropFinalBlank' . 'dropDelims' . 'oneOf'@. endByOneOf :: Eq a => [a] -> [a] -> [[a]] endByOneOf = 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.) unintercalate :: Eq a => [a] -> [a] -> [[a]] unintercalate = sepBy -- | 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"] wordsBy :: (a -> Bool) -> [a] -> [[a]] wordsBy = split . dropBlanks . dropDelims . whenElt -- | 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",""] linesBy :: (a -> Bool) -> [a] -> [[a]] linesBy = split . dropFinalBlank . dropDelims . whenElt -- * Other splitting methods -- | @'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@. splitEvery :: Int -> [e] -> [[e]] splitEvery i ls = map (take i) (build (splitter ls)) where splitter :: [e] -> ([e] -> a -> a) -> a -> a splitter [] _ n = n splitter l c n = l `c` splitter (drop i l) c n -- | A common synonym for 'splitEvery'. chunk :: Int -> [e] -> [[e]] chunk = 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. splitPlaces :: Integral a => [a] -> [e] -> [[e]] splitPlaces is ys = build (splitPlacer is ys) where splitPlacer :: forall i b t. Integral i => [i] -> [b] -> ([b] -> t -> t) -> t -> t splitPlacer [] _ _ n = n splitPlacer _ [] _ n = n splitPlacer (l:ls) xs c n = let (x1, x2) = genericSplitAt l xs in x1 `c` splitPlacer ls x2 c n -- | 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],[]] splitPlacesBlanks :: Integral a => [a] -> [e] -> [[e]] splitPlacesBlanks is ys = build (splitPlacer is ys) where splitPlacer :: forall i b t. Integral i => [i] -> [b] -> ([b] -> t -> t) -> t -> t splitPlacer [] _ _ n = n splitPlacer (l:ls) xs c n = let (x1, x2) = genericSplitAt l xs in x1 `c` splitPlacer ls x2 c n -- | 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) chop :: ([a] -> (b, [a])) -> [a] -> [b] chop _ [] = [] chop f as = b : chop f as' where (b, as') = f as