順列

順列をリストモナドでどう書くのだろう，と，ふと思いたち書いてみた．

remove _ [] = []

remove x (y:ys) = if x == y then ys else y : (remove x ys)
perm _ [] = [[]]

perm 1 l = do x <- l

              return [x]

perm n l = do x <- l

              xs <- perm (n - 1) (remove x l)

              return (x:xs)