tasks a-s

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

@@ -0,0 +1,18 @@
+insertEverywhere :: a -> [a] -> [[a]]
+insertEverywhere x [] = [[x]]
+insertEverywhere x l@(y:ys) = (x:l) : map (y:) (insertEverywhere x ys)
+
+s_perm :: [a] -> [[a]]
+s_perm = foldl aux [[]]
+  where aux items x = do (f, item) <- zip (cycle [reverse, id]) items
+                         f (insertEverywhere x item)
+
+s_permutations :: [a] -> [([a], Int)]
+s_permutations = flip zip (cycle [1, -1]) . s_perm
+
+main :: IO ()
+main = do
+  putStrLn "3 items:"
+  mapM_ print $ s_permutations [0..2]
+  putStrLn "4 items:"
+  mapM_ print $ s_permutations [0..3]