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
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