You are aware aren't you, that your factorial function is recursive?

That is when you call fac on your really big number, it calls fac on
ones less than your really big number which calls fac on one less
than that ....... and so on down to when n == 0.

Every single one of those recursive calls requires stack space and
your stack is simply not big enough to hold it all.

The other problem you are going to have is that even if you make
that tail recursive, you are trying to calculate the factorial
of a very, very large number and i doubt your computer memory is
large enough to hold the result.

Erik

Your function is not tail-recursive. Try

fac :: Integer -> Integer
fac n = product [1..n]

and make sure it is strict (e.g. with GHC, compile with -O, no idea
how to do that in WinHugs).

BTW, don't expect an answer for "435435443543532343543254324325435".
With Stirling approximation

logfac b n = n * logBase b (n / exp(1)) + 0.5 * logBase b (2 * pi * n)

So

Prelude> logfac 10 435435443543532343543254324325435
1.402303704118515e34

means the result has 10^34 decimal digits, or in base 2

Prelude> logfac 2 435435443543532343543254324325435
4.658352072275911e34

that's 4*10^34 bits, or

Prelude> 4*10^34 / 8 / 1024^3
4.656612873077393e24

about 4000000000000000000000000 GB of memory, if I didn't make a
stupid mistake somwhere. I doubt you can buy this much memory
somewhere, much less find a CPU to address all of it :-)

Above some point, the results of the factorial function will get large, so
you'll run out of memory. Nothing you can do about that.

- Dirk