tasks a-s

Task/Permutations/Haskell/permutations-1.hs

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+import Data.List (permutations)
+
+main = mapM_ print (permutations [1,2,3])

Task/Permutations/Haskell/permutations-2.hs

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+import Data.List (delete)
+
+permutations :: Eq a -> [a] -> [[a]]
+permutations [] = [[]]
+permutations xs = [ x:ys | x <- xs, ys <- permutations (delete x xs)]

Task/Permutations/Haskell/permutations-3.hs

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+permutations :: [a] -> [[a]]
+permutations [] = [[]]
+permutations xs = [ y:zs | (y,ys) <- select xs, zs <- permutations ys]
+  where select []     = []
+        select (x:xs) = (x,xs) : [ (y,x:ys) | (y,ys) <- select xs ]

Task/Permutations/Haskell/permutations-4.hs

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+permutations :: [a] -> [[a]]
+permutations = foldr (concatMap . insertEverywhere) [[]]
+  where insertEverywhere :: a -> [a] -> [[a]]
+        insertEverywhere x [] = [[x]]
+        insertEverywhere x l@(y:ys) = (x:l) : map (y:) (insertEverywhere x ys)