Combinatorics.

Combinatorics.hs

module Combinatorics (powerset, combsOf, perms) where

powerset :: [a] -> [[a]]
powerset (x:xs) = let xs' = powerset xs in xs' ++ map (x:) xs'
powerset []     = [[]]

combsOf :: Int -> [a] -> [[a]]
combsOf n _ | n < 1 = [[]]
combsOf n (x:xs)    = combsOf n xs ++ map (x:) (combsOf (n - 1) xs)
combsOf _ _         = []

perms :: [a] -> [[a]]
perms [] = []
perms xs = foldr ((=<<) . ins) [[]] xs where
    ins y (x:xs) = (y:x:xs) : map (x:) (ins y xs)
    ins y _      = [[y]]