Defines a function nCombination which generates and prints out all n-combinations as a list. Uses Data.IntSet.

IntSet Combinations.hs

import Control.Monad (unless)
import Data.IntSet (IntSet)
import qualified Data.IntSet as IntSet

-- Generate and print out all combinations up to size n
nCombination :: Int -> IO ()
nCombination n = do
                   print $ IntSet.toList IntSet.empty
                   combination' n IntSet.empty $ makeBaseCase n

-- create the base case for the combination algorithm
makeBaseCase :: Int -> IntSet.IntSet
makeBaseCase 0 = IntSet.empty
makeBaseCase n = IntSet.insert n $ makeBaseCase (n-1)

{- prints out all n combinations
   use it like this:
   combination 8 IntSet.empty $ makeBaseCase 8
 -}
combination' :: Int -> IntSet.IntSet -> IntSet.IntSet -> IO ()
combination' n xs bs = do
                         let nextxs = nextIntSet n xs
                         print $ IntSet.toList nextxs
                         unless (nextxs == bs) $ combination' n nextxs bs

-- get the first element from the right that is not in the set
findN :: Int -> IntSet.IntSet -> Int
findN 0 _ = 0
findN n xs = if not (IntSet.member n xs)
               then n
               else findN (n-1) xs

-- removes all values from x+1 to n in the IntSet
removeUpToN :: Int -> Int -> IntSet.IntSet -> IntSet.IntSet
removeUpToN x n xs
            | x > n = xs
            | otherwise = removeUpToN (x+1) n $ IntSet.delete (x+1) xs

-- Generate the next IntSet
nextIntSet :: Int -> IntSet.IntSet -> IntSet.IntSet
nextIntSet n xs = removeUpToN x n $ IntSet.insert x xs
                    where
                      x = findN n xs