Hi! Inspred from the discussion http://www.haskell.org/pipermail/beginners/2010-February/003396.html , I just try to understand the seq function a bit better. When I compare these two functions: firstSum :: Num a => a -> [a] -> a firstSum s [] = s firstSum s (x:xs) = seq help (firstSum help xs) where help = x + s secondSum :: Num a => [a] -> a secondSum [] = 0 secondSum (x:xs) = seq help (x + help) where help = secondSum xs What should be the difference? In my opinion both functions do not return a complete unevaluated thunk (secondSum returns a thunk of the form (THUNK + b) where THUNK contains a single numeral value). But it seems to me that the first function computes the output somehow linear in the sense that it does just a set of substitutions while the second functions has to create a tree to handle all the recursive calls of seq (sorry, my terminology is for sure a bit poor). So I would say the first function delivers a better performance. (In the discussion I mentioned, the second function was not discussed in this form with the seq implementation). Am I right with my thoughts? Thanks, fweth