Haskell permutation generation algorithm with list comprehension.

perm.hs

import Data.List

perm :: (Eq a) => [a] -> Int -> [[a]]
perm _ 0 = [[]]
perm xs r | length xs < r = [[]]
          | otherwise = [x:ys | x <- xs, ys <- perm (delete x xs) (r - 1)]

Your Haskell implementation is brilliant!!! It is similar to the nPn Haskell version at https://stackoverflow.com/questions/49280797/permutation-algorithm-in-haskell
namely,

import Data.List ((\\),delete)

perms [] = [[]]
perms p = [x:xs | x <- p, xs <- perms (delete x p) ]

main = print $ perms [1,2,3]

However, yours is nPr or P(n,r), i.e., "n choose r", which is more general. I have been working on such an algorithm for a while. Do you have any suggestions regarding how you arrived at your solution/implementation?

Thank you.

Richard E. Adams
Email: readams256@gmail.com

Hi, @readamd256,

To be honest, I forgot how I came up with this code snippets. But I think this gist is just a step of generalisation of your code.

Thank you!