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added 'Relude.List.permutations' #397

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27 changes: 26 additions & 1 deletion src/Relude/List.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -20,16 +20,18 @@ module Relude.List
, (!!?)
, maybeAt
, partitionWith
, permutations
) where


import Relude.Base ((<))
import Relude.Bool (otherwise)
import Relude.Function (flip, (.))
import Relude.Function (flip, (.), id)
import Relude.List.NonEmpty
import Relude.List.Reexport
import Relude.Monad (Either, Maybe (..), partitionEithers)
import Relude.Numeric (Int, (-))
import Relude.Foldable (foldr)


-- $setup
Expand Down Expand Up @@ -108,6 +110,29 @@ partitionWith :: (a -> Either b c) -> [a] -> ([b], [c])
partitionWith f = partitionEithers . map f
{-# INLINE partitionWith #-}

{- | The 'permutations' function returns the list of all permutations of the argument.
Unlike its Prelude implementation, thiv version returns NonEmpty.

>>> permutations "abc"
"abc" :| ["bac","cba","bca","cab","acb"]

>>> permutations []
[] :| []

@since 1.1.0.0
-}
permutations :: [a] -> NonEmpty [a]
permutations xs0 = xs0 :| perms xs0 []
where
perms [] _ = []
perms (t:ts) is = foldr interleave (perms ts (t:is)) (permutations is)
where interleave xs r = let (_,zs) = interleave' id xs r in zs
interleave' _ [] r = (ts, r)
interleave' f (y:ys) r = let (us,zs) = interleave' (f . (y:)) ys r
in (y:us, f (t:y:us) : zs)
{-# INLINE permutations #-}


{- $reexport
Most of the "Data.List" types and function.

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2 changes: 1 addition & 1 deletion src/Relude/List/Reexport.hs
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Expand Up @@ -21,7 +21,7 @@ module Relude.List.Reexport

import Data.List (break, drop, dropWhile, filter, genericDrop, genericLength, genericReplicate,
genericSplitAt, genericTake, group, inits, intercalate, intersperse, isPrefixOf,
iterate, map, permutations, repeat, replicate, reverse, scanl, scanl', scanl1,
iterate, map, repeat, replicate, reverse, scanl, scanl', scanl1,
scanr, scanr1, sort, sortBy, sortOn, span, splitAt, subsequences, tails, take,
takeWhile, transpose, uncons, unfoldr, unzip, unzip3, zip, zip3, zipWith, (++))
import GHC.Exts (sortWith)
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