# Difference between revisions of "Data.List.Split"

Line 28: | Line 28: | ||

<haskell> |
<haskell> |
||

⚫ | |||

+ | {- |
||

⚫ | |||

{-# LANGUAGE ViewPatterns #-} |
{-# LANGUAGE ViewPatterns #-} |

## Revision as of 14:27, 17 December 2008

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.

This has been brought up many times on the mailing lists, and there's a summary of the split proposals on List function suggestions. Just for reference:

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?
- keep blank splits between consecutive delimiters, or merge multiple consecutive delimiters into one delimiter?

An important caveat: we should strive to keep things flexible yet SIMPLE. The more complicated things get, the closer this gets to just being a general parsing or regex library. So the right balance needs to be struck.

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

```
{-
-- NOTE: This is buggy code and should not be trusted
{-# 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 = fmap (q:) match ps qs -- BUGFIX: Previously did not fmap (q:)
match (_:_) (_:_) | otherwise = Nothing
chopWith delimiter (match delimiter -> Just tail) = return ([], tail) -- BUG: causes infinite production when it shouldn't
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] -> [[a]]
splitEq e = splitOn (==e)
-- | split at regular intervals
chunk :: Int -> [a] -> [[a]]
chunk _ [] = [[]]
chunk n xs = y1 : chunk n y2
where
(y1, y2) = splitAt n xs
-- another version (CPS) of chunk
chunk n list = case list of { [] -> [] ; (y:ys) -> ch' ys (n-1) (y:) } where
ch' [] _ k = k [] : []
ch' (y:ys) 0 k = k [] : ch' ys (n-1) (y:)
ch' (y:ys) (c+1) k = ch' ys c (k . (y:))
```

## A combinator approach?

Here are some initial thoughts on a combinator approach. The trick is to find nice implementations of the declarations below. Please add your own thoughts, other combinators, etc.

```
data Splitter a
split :: Splitter a -> [a] -> [[a]]
onElts :: [a] -> Splitter a -- split on any of these elements
onSublist :: [a] -> Splitter a -- split on this exact subsequence
whenElt :: (a -> Bool) -> Splitter a
keepingDelims :: Splitter a -> Splitter a
collapsingNulls :: Splitter a -> Splitter a
-- other basic combinators?
-- now you can write things like
--
-- split (collapsingNulls $ onElts " ,") "abc,def , gh"
--
-- which should evaluate to ["abc", "def", "gh"].
-- some convenience functions can be provided, such as...
splitOn = split . onElts
splitWhen = split . whenElt
```

## Splits of known lengths

I frequently require two types of splits, splitting into blocks of fixed length and splitting into lists of sizes of increasing powers of 2. My implementation was designed to be fold/builded as much as possible, so here goes:

```
splitEvery :: Int -> [e] -> [[e]]
splitEvery i l = map (take i) (build (splitter l)) where
splitter [] _ n = n
splitter l c n = l `c` splitter (drop i l) c n
For more general splits with foreknown lengths,
splitPlaces :: [Int] -> [e] -> [[e]]
splitPlaces ls xs = build (splitPlacer ls xs) where
splitPlacer [] _ _ n = n
splitPlacer _ [] _ n = n
splitPlacer (l:ls) xs c n = let (x1, x2) = splitAt l xs in x1 `c` splitPlacer ls x2 c n
splitPowersOf2 :: [e] -> [[e]]
splitPowersOf2 = splitPlaces (iterate (*2) 1)
```

To be sure, neither is a good consumer, but I don't think that's avoidable, given that drop isn't a good consumer either.

Here splitEvery is equivalent to "chunks" above, but it is a much better producer, I think. (It is also intended to be mapped on, given that the (map (take i)) makes every element of the list into a producer.

## Break on Nothing

```
import Data.Maybe
import Data.Either
import Data.List (find, isPrefixOf)
breaks :: [Maybe a] -> [[a]]
breaks xs = (if null cur then id else ((map fromJust cur):))
(if null rem then [] else breaks (tail rem))
where (cur, rem) = break isNothing xs
replaces :: (Eq a) => [([a], [b])] -> (a -> b) -> [a] -> [b]
replaces reps f = process
where process [] = []
process xs = case find ((`isPrefixOf` xs).fst) reps of
Nothing -> f (head xs) : process (tail xs)
Just (pat, rep) -> rep ++ (process $ drop (length pat) xs)
split :: (Eq a) => [([a], [Maybe a])] -> [a] -> [[a]]
split reps = breaks . replaces reps Just
onSeq, onSeqKeep :: (Eq a) => [a] -> ([a], [Maybe a])
onSeq xs = (xs, [Nothing])
onSeqKeep xs = (xs, Nothing : (map Just xs ++ [Nothing]))
onElt, onEltKeep :: (Eq a) => a -> ([a], [Maybe a])
onElt x = onSeq [x]
onEltKeep x = onSeqKeep [x]
insertAfter :: [Int] -> a -> [a] -> [a]
insertAfter [] _ xs = xs
insertAfter (i:_) _ [] | i > 0 = []
insertAfter (i:is) x xs = pre ++ [x] ++ insertAfter is x post
where (pre, post) = splitAt i xs
splitEvery :: Int -> [a] -> [[a]]
splitEvery i = splitPlaces (repeat i)
splitPlaces :: [Int] -> [a] -> [[a]]
splitPlaces is xs = breaks $ insertAfter is Nothing $ map Just xs
splitPowersOf2 = splitPlaces (iterate (*2) 1)
```

Implements all of the above ideas (except predicate matching). In order to split up an arithmetic expression, for example:

```
split (onElt ' ' : map onEltKeep "+-/*^()")
```