Re: [Haskell-cafe] Permutations

Daniel Fischer wrote:

Needs an Ord constraint:

inserts :: [a] -> [a] -> [[a]] inserts [] ys = [ys] inserts xs [] = [xs] inserts xs@(x:xt) ys@(y:yt) = [x:zs | zs <- inserts xt ys] ++ [y:zs | zs <- inserts xs yt]

Heh, I came up with basically the same thing. I'd call this function 'merges' - it returns all possibilities for merging two given lists.

uniquePermutations :: Ord a => [a] -> [[a]] uniquePermutations = foldr (concatMap . inserts) [[]] . group . sort

Which can also be written as uniquePermutations = foldM merges [] . group . sort Bertram