Hi, I guess it's time to get acquainted with foldr and foldl. prelude>xs = [1..5] ++ undefined prelude> foldr const 0 xs 1 I assume it goes something like this: ( 1 `const`(2 `const`(3 `const`(4 `const`(5 `const`(undefined `const` 0)))))) ( 5 `const` undefined) (4 `const` 5) (3 `const` 4) ( 2 `const` 3) (1 `const`2) = 1 ======================================================================== What i don't get is the opposite: prelude> foldl const 0 xs error in my mind this should go like this: ((((((0`const`1)`const` 2) `const` 3 )`const` 4)`const` 5) `const` undefined) (0 `const`2) (0`const` 3) ( 0`const`4) (0`const` 5) (0 `const` undefined ) = 0 I have been told that the main difference between foldl and foldr is that foldl needs to evaluate the whole spline before it continues. And i guess that has something to do with it. What I don't understand is WHY foldl need to do this and foldr doesn't. thanks in advance! best,