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axionbuster/Zipper.hs

Created August 16, 2025 06:56
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Easy nonrecursive list zipper
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Zipper.hs
{-# LANGUAGE ParallelListComp #-}
module Data.Zipper (zipper) where
import Data.List
import qualified Data.List.NonEmpty as NE
-- helper
dropLast :: [a] -> [a]
dropLast = maybe [] fst . unsnoc
zipper :: [a] -> [([a], a, [a])]
zipper as =
[ (dropLast bs, a, drop 1 cs)
| bs <- NE.toList <$> inits1 as
| cs <- NE.toList <$> tails1 as
| a <- as
]
{- ex: zipper [1, 2, 4] = [([],1,[2,4]),([1],2,[4]),([1,2],4,[])] -}
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