stack overflow in tail recursive function

Hi all,

I have this tail recursive factorial function:

factorial :: Integer -> Integer
factorial 0 = 1
factorial n = fat' n 1 where
    fat' 1 fat = fat
    fat' n fat = fat' (n-1) (n*fat)

Whenever I run it with a number of 20000 or more I get a stack
overflow error. It doesn't seem a problem with the large resulting
number because, if so, the message should be something like "Garbage
collection fails to reclaim sufficient space". Other functions seem to
able to handle a larger number of recursive calls.

So, what is the problem with this particular function?

-- 
Bruno Schneider
http://www.dcc.ufla.br/~bruno/