Data commit

Task/Permutations-by-swapping/Haskell/permutations-by-swapping.hs

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+sPermutations :: [a] -> [([a], Int)]
+sPermutations = flip zip (cycle [-1, 1]) . foldr aux [[]]
+  where
+    aux x items = do
+      (f, item) <- zip (repeat id) items
+      f (insertEv x item)
+    insertEv x [] = [[x]]
+    insertEv x l@(y:ys) = (x : l) : ((y :) <$> insertEv x ys)
+
+main :: IO ()
+main = do
+  putStrLn "3 items:"
+  mapM_ print $ sPermutations [1 .. 3]
+  putStrLn "\n4 items:"
+  mapM_ print $ sPermutations [1 .. 4]