Module representing rational numbers
Not too bad.


You do not export Rational' from the module so no outside code can make one directly
numer and denom will conflict with a similar function name in any other code which imports your module. Perhaps rational'Numer and rational'Denom?
See http://www.haskell.org/tutorial/numbers.html for a good intro to numbers in Haskell. You missed the Fractional class which defines div, recip, and fromRational.
Why are rationalSub and rationalDiv not defined like add and mult? Be consistent.
In rationalSignum your comment says the reader would expect a 0 or 1 Int but the return needs to be of the Rational' type and then you actually return 0 or 1. Be consistent and return Rational' 0 1 and Rational' 1 1
rationalFromInteger could be rationalFromInteger = (flip Rational') 1
I would define reduce like this

reduce (Rational' n 0) = divByZero n
reduce (Rational' n d) = Rational' (n `div` common) (d `div` common)
  where common = (gcd n d) * (signum d)