[Haskell-cafe] testing for exceptions

Hello, I have made this exercise which can be found at the Craft of Functional Programming book. -- exercise 32 -- Suppose we have to raise 2 to the power n. If n is even, 2*m say, then -- 2n = 22*m = (2m)2 -- If n is odd, 2*m+l say, then -- 2n = 22*m+l = (2n)2*2 -- Give a recursive function to compute 2n which uses these insights. f2 :: Integer -> Integer f2 n | n < 0 = error "This will only run for positive numbers" | n == 0 = 1 | even n = f2 ( n `div` 2) ^ 2 | odd n = (f2 ( n `div` 2) ^ 2) * 2 Now I have to make Hunit tests for it, But is there a way I can test if the error message is shown when a negative number is being used ? Roelof