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

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</haskell> |
</haskell> |
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+ | == Splits of known lengths == |
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+ | |||

+ | 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: |
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+ | |||

+ | <haskell> |
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+ | splitEvery :: Int -> [e] -> [[e]] |
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+ | splitEvery i l = map (take i) (build (splitter l)) where |
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+ | splitter [] _ n = n |
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+ | splitter l c n = l `c` splitter (drop i l) c n |
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+ | |||

+ | For more general splits with foreknown lengths, |
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+ | splitPlaces :: [Int] -> [e] -> [[e]] |
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+ | splitPlaces ls xs = build (splitPlacer ls xs) where |
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+ | splitPlacer [] _ _ n = n |
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+ | splitPlacer _ [] _ n = n |
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+ | splitPlacer (l:ls) xs c n = let (x1, x2) = splitAt l xs in x1 `c` splitPlacer ls x2 c n |
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+ | |||

+ | splitPowersOf2 :: [e] -> [[e]] |
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+ | splitPowersOf2 = splitPlaces (iterate (*2) 1) |
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+ | </haskell> |
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+ | 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. |

## Revision as of 22:19, 13 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.

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?

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.

```
{-# 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] -> [[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
```

## 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.