Re: [Haskell-beginners] Powerset function

From base-4.6.0.1 [1] the filterM is implemented like this:

filterM :: (Monad m) => (a -> m Bool) -> [a] -> m [a] filterM _ [] = return [] filterM p (x:xs) = do flg <- p x ys <- filterM p xs return (if flg then x:ys else ys) So filterM (\x -> [True, False]) [1,2] is filterM (\x -> [True, False]) (1:[2]) do flg <- (\x -> [True, False]) 1 ys <- filterM (\x -> [True, False]) [2] return (if flg then x:ys else ys) do flg <- [True, False] ys <- [[2], []] return (if flg then 1:ys else ys) You get the idea. You'll end up with [[1,2],[1],[2],[]] Hope this helps, Alexey Shmalko [1] https://hackage.haskell.org/package/base-4.6.0.1/docs/src/Control-Monad.html