(**) Determine the greatest common divisor of two positive integer numbers. Use Euclid's algorithm.
gcd' 0 y = y gcd' x y = gcd' (y `mod` x) x myGCD x y | x < 0 = myGCD (-x) y | y < 0 = myGCD x (-y) | y < x = gcd' y x | otherwise = gcd' x y
The Prelude includes a gcd function, so we have to choose another name for ours. The function gcd' is a straightforward implementation of Euler's algorithm, and myGCD is just a wrapper that makes sure the arguments are positive and in increasing order.
A more concise implementation is:
myGCD :: Integer -> Integer -> Integer myGCD a b | b == 0 = abs a | otherwise = myGCD b (a `mod` b)