Hello everyone. Recently I've run into a situation where a monadic fold seems like an interesting option. However, I'm not particularly sure how to choose which fold - left or right - to use. For non-monadic folds the rule of thumb is 'avoid foldl, use foldr for infinite things and foldl' for performance'. What are the guidelines for the monadic ones? The fact that 'foldlM' is implemented (in base-4.12.0-0) using 'foldr' and 'foldrM' uses 'foldl' (also, non-strict foldl, which is worrying) adds somewhat to my confusion. I've played a bit with folding Eithers, and foldlM seems to work somewhat intuitively (breaks on the first Left, as is usual for Either), and also accepts infinite lists (which makes sense, given it's a foldr): import Data.Foldable main :: IO () main = do print $ sumNonNegative $ [1, 2, -4] ++ [5..] sumNonNegative :: [Int] -> Either String Int sumNonNegative = foldlM foo 0 foo :: Int -> Int -> Either String Int foo acc x | x >= 0 = Right $ x + acc | otherwise = Left $ "Negative number: " ++ show x This prints out "Negative number: -4". Odd things begin when I edit the above to use foldrM. It doesn't terminate on an infinite list, which is expected from a disguised foldl. But given several negative numbers in the input, it prints out the last one, seemingly turning the usual behaviour of Either on its head. It seems its foldl heritage results in applying side-effects (in case of Either, erroring out with a Left) in reverse order. Do I get this right and is this the general behaviour of foldrM for any monad? -- Michail.