import Data.List (tails)

main :: IO ()
main = mapM_ print [ combi n "abcde" | n <- [-1..6] ]

-- | Non-repeat combination select @n@ from @xs@. 
combi n xs
    | n < 0 = []
    | n == 0 = [[]]
    | otherwise = [ y:zs | (y:ys) <- tails xs, zs <- combi (n - 1) ys ]