main :: IO ()
main = print $ combi 4 ['a'..'d']

-- | Non-repeat combination select @n@ from @xs@. 
combi n xs = combi' 0 n xs

-- | Non-repeat combinations recursing at @i@ (splitAt).
combi' :: Int -> Int -> [a] -> [[a]]
combi' i n xs = case splitAt i xs of
    ([], []) -> []
    ([], hs) -> combi' (i + 1) n hs
    (h:hs, ts) -> ( if n == 1 then [[h]]
                    else fmap (h :) (combi' 0 (n - 1) (hs <> ts))
                  ) -- keep h
                  <> combi' 0 n (hs <> ts) -- drop h