kcombinations

Main.hs

{-# LANGUAGE MultiWayIf #-}
import Data.List
import Control.Monad
import Control.Monad.Trans.Writer

kcombinations n arr = snd . runWriter $ combos [] 1 n arr

combos :: (Eq a) => [a] -> Int -> Int -> [a] -> Writer [[a]] ()
combos acc step n array = forM_ (zip array [1..]) $ \(val, i) -> if
  | val `elem` acc -> return ()
  | n == step    -> tell [acc ++ [val]]
  | otherwise    -> combos (acc ++ [val]) (step + 1) n (drop i array)

main = print $ kcombinations 2 "abc"