[Haskell] big integer in haskell

Hello all, I am a new learner, and i am trying to write a function to calculate factorial of a natural number: (I use ghc 6.4, window xp) fac :: Int -> Int fac n | n == 0 = 1 | n > 0 = fac (n-1) * n | otherwise = 0 this works when n < 14 and result is 1278945280, when n > 14 the answer is not right n = 15 result is 2004310016. I know it is maybe because of overflow. So i calclate 2^32, 2^64, 2^128 the result correct and is much bigger as 14!,Why? 2^n use some big integer or my implementation of n! is not correct, I'm really confused. Someone can help me? Thanks Ting Wang