The following code, when compiled with GHC 6.6.1 --make -O gives a stack overflow when I enter 1000000 as a command line argument: (please don't look at the efficiency of the code, it can of course be improved a lot both in time performance and numeric precision...) import System leibnizPI :: Integer -> Double leibnizPI n = sum (map leibnizTerm [0..n]) where leibnizTerm n = let i = fromIntegral n in 4 * (((-1) ** i) / (2*i+1)) main = do args <- getArgs let n = read (head args) print (leibnizPI n) However, if I replace main = print (leibnizPI 1000000) is does not stack overflow. Now, if I leave the original main, but replace sum in leibnizPI by mysum xs = aux 0 xs where aux s (x:xs) = aux (s+x) xs aux s [] = s then I don't get a stack overflow. However, I do get a stack overflow when I compile it without -O, in all cases. This puzzles me. I don't see any non-tail calls in my code... I guess it has to do with strictness? http://www.haskell.org/haskellwiki/Performance/Strictness Why isn't it possible to annotate strictness on the type signature in Haskell as in Clean? Is this on the TODO list? Many thanks, Peter