64 lines
1.6 KiB
Haskell
64 lines
1.6 KiB
Haskell
module ListHelp
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( module Data.List
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, initOrNull
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, insertOver
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, swapIndices
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, (!?)
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, safeHead
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, safeUncons
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, errorHead
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, foldPairs
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, loopPairs
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) where
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import Data.List
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loopPairs :: [a] -> [(a,a)]
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loopPairs (a:as) = snd $ foldr f (Just a,[]) (a:as)
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where
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f x (Nothing,ps) = (Just x,ps)
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f x (Just y,ps) = (Just x,(x,y):ps)
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loopPairs [] = error "tried to make loop with empty list of elements"
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foldPairs :: [a] -> [(a,a)]
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foldPairs = snd . foldr f (Nothing, [])
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where
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f x (Nothing,ps) = (Just x,ps)
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f x (Just y,ps) = (Just x,(x,y):ps)
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initOrNull :: [a] -> [a]
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initOrNull [] = []
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initOrNull xs = init xs
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insertOver :: Int -> a -> [a] -> [a]
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insertOver i x xs = take i xs ++ x : drop (i+1) xs
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swapIndices :: Int -> Int -> [a] -> [a]
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swapIndices i j xs = insertOver i (xs !! j) . insertOver j (xs !! i) $ xs
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--copied from Data.List.Extra:
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-- | A total variant of the list index function `(!!)`.
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--
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-- > [2,3,4] !? 1 == Just 3
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-- > [2,3,4] !? (-1) == Nothing
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-- > [] !? 0 == Nothing
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(!?) :: [a] -> Int -> Maybe a
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xs !? n
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| n < 0 = Nothing
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-- Definition adapted from GHC.List
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| otherwise = foldr (\x r k -> case k of
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0 -> Just x
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_ -> r (k-1)) (const Nothing) xs n
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{-# INLINABLE (!?) #-}
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safeHead :: [a] -> Maybe a
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safeHead (x:_) = Just x
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safeHead _ = Nothing
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safeUncons :: [a] -> Maybe (a,[a])
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safeUncons (x:xs) = Just (x,xs)
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safeUncons _ = Nothing
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errorHead :: String -> [a] -> a
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errorHead _ (x:_) = x
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errorHead s [] = error s
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