This is an example that shows how foldl and foldr work (from RWH p.93-94): foldl (+) 0 (1:2:3:[]) == foldl (+) (0 + 1) (2:3:[]) == foldl (+) ((0 + 1) + 2) (3:[]) == foldl (+) (((0 + 1) + 2) + 3) [] == (((0 + 1) + 2) + 3) foldr (+) 0 (1:2:3:[]) == 1 + foldr (+) 0 (2:3:[]) == 1 + (2 + foldr (+) 0 (3:[]) == 1 + (2 + (3 + foldr (+) 0 [])) == 1 + (2 + (3 + 0)) The book says on p.94: ----- The difference between foldl and foldr should be clear from looking at where the parentheses and the empty list elements show up. With foldl, the empty list element is on the left, and all the parentheses group to the left. With foldr, the zero value is on the right, and the parentheses group to the right. ---- Huh? With foldl, the only empty list element I see is on the right. Initially, it looked to me ike they did the same thing, and that the only difference was the way they called step. I think "step" is a horrible, non-descriptive name, so I'm going to use "accFunc" instead: foldl calls: accFunc acc x foldr calls: accFunc x acc So it looks like you can define a function using either one and get the same result. Here is a test: --I am going to use odd for pfunc and [1, 2, 3] for xs: myFilter1 pfunc xs = foldl accFunc [] xs where accFunc acc x | pfunc x = acc ++ [x] | otherwise = acc myFilter2 pfunc xs = foldr accFunc [] xs where accFunc x acc | pfunc x = acc ++ [x] | otherwise = acc *Main> myFilter1 odd [1, 2, 3] [1,3] *Main> myFilter2 odd [1, 2, 3] [3,1] Hmmm. So there is a difference. foldr appears to grab elements from the end of the list. Therefore, to get the same result from the function that uses foldr, I did this: myFilter3 pfunc xs = foldr accFunc [] xs where accFunc x acc | pfunc x = x : acc | otherwise = acc *Main> myFilter3 odd [1, 2, 3] [1,3] But then RWH explains that you would never use foldl in practice because it thunks the result, which for large lists can overwhelm the maximum memory alloted for a thunk. But it appears to me the same thunk problem would occur with foldr. So why is foldr used in practice but not foldl?