> I can tell you conclusively what it does: it takes a monadic function that returns a Bool after performing some "action" as well as a list of values; it then

> I can tell you conclusively what it does: it takes a monadic function that returns a Bool after performing some "action" as well as a list of values; it then performs this action on each element and look at whether the result is True or False; when True, keep it otherwise discard it. The function below matches the signature of filterM for List monad, but does not do anything remotely similar to what you described. foo :: (Int -> [Bool]) -> [Int] -> [[Int]] foo f xs = case bool of [True] -> [] _ -> [xs] where bool = f $ length xs This is a contrived example, but I think a large part of your guess on what filterM does comes from the word "filter" in its name. This isn't any different from other mainstream programing language, so it is not a ding on Haskell.