Data.List.Split

A theoretical module which contains implementations/combinators for implementing every possible method of list-splitting known to man. This way no one has to argue about what the correct interface for split is, we can just have them all.

Some possible ways to split a list, to get your creative juices flowing:

• what to split on?
  • single-element separator
  • sublist separator
  • use a list of possible separators instead of just one
  • use a predicate on elements or sublists instead of giving explicit separators
  • use approximate matching?
  • chunks of fixed length
• how to split?
  • discard the separators
  • keep the separators with the preceding or following splits
  • keep the separators as their own separate pieces of the result list
  • what to do with separators at the beginning/end? create a blank split before/after, or not?

Add your implementations below! Once we converge on something good we can upload it to hackage.

{-# LANGUAGE ViewPatterns #-}

import Data.List (unfoldr)


-- intercalate :: [a] -> [[a]] -> [a]
-- intercalate x [a,b,c,x,y,z] = [a,x,b,x,c,x,x,y,x,z,x]

-- unintercalate :: [a] -> [a] -> [[a]]
-- unintercalate x [a,x,b,x,c,x,x,y,x,z,x] = [a,b,c,[],y,z]

-- unintercalate is the "inverse" of intercalate

match [] string = Just string
match (_:_) [] = Nothing
match (p:ps) (q:qs) | p == q = match ps qs
match (_:_)  (_:_)  | otherwise = Nothing

chopWith delimiter (match delimiter -> Just tail) = return ([], tail)
chopWith delimiter (c:cs) = chopWith delimiter cs >>= \(head, tail) ->
                              return (c:head, tail)
chopWith delimiter [] = Nothing
-- note: chopWith could be make 'more efficient' i.e. remove the >>=\-> bit
--       by adding an accumulator


unintercalate delimiter = unfoldr (chopWith delimiter)

-- > unintercalate "x" "axbxcxxyxzx"
-- ["a","b","c","","y","z"]

splitOn :: (a -> Bool) -> [a] -> [[a]]
splitOn _ [] = []
splitOn f l@(x:xs)
  | f x = splitOn f xs
  | otherwise = let (h,t) = break f l in h:(splitOn f t)

-- take the element who make predict true as delimiter
-- > splitOn even [1,3,5,6,7,3,3,2,1,1,1]
-- [[1,3,5],[7,3,3],[1,1,1]]

-- | like String split, except for any element that obeys Eq
splitEq :: Eq a -> [a] -> [[a]]
splitEq _ [] = []
splitEq e l@(x:xs)
  | e == x = splitOn e xs
  | otherwise = let (h,t) = break f l in h:(splitEq e t)


-- | split at regular intervals
splitEquidistant :: Int -> [a] -> [[a]]
splitEquidistant _ [] = []
splitEquidistant n xs = y1 : split n y2
  where
    (y1, y2) = splitAt n xs