Get all permutations of a list in Haskell

Question

I\'m trying to do this from scratch, without the use of a library outside the standard lib. Heres my code:

permutations :: [a] -> [[a]]
permutations (x:xs         


        

Answer 1

For a simple implementation without considering duplications in the input

permutations :: Eq a => [a] -> [[a]]
permutations [] = [[]]
permutations as = do a <- as
                     let l = delete a as
                     ls <- permutations l
                     return $ a : ls

Test:

λ> permutations [1,2,3]
[[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]]
λ> permutations "abc"
["abc","acb","bac","bca","cab","cba"]
λ> 

Algorithm Reference