Proposal: Make gcd total

[Daniel Fischer]

I would like to propose the elimination of the special error case

gcd 0 0 = error "Prelude.gcd: gcd 0 0 is undefined"

to replace it with

gcd 0 0 = 0

(which would be an automatic consequence of removing the above line).

Rationale:

1. It makes gcd a total function.
2. It makes gcd associative.
3. It makes folding gcd over a list safe.
4. It conforms to common mathematical practice:

In a commutative ring, a greatest common divisor of two elements a, b is
a common divisor g of a and b such that every common divisor d of a and b
also divides g (if R is a commutative ring, a \in R, d \in R is a divisor
of a iff there exists c \in R with a = d*c [leaving out noncommutative
rings for simplicity]).
Since every ring element divides 0, the above definition entails
gcd 0 0 = 0.

Further, in a principal ideal ring, the greatest common divisors of two
elements a and b are the generators of the ideal (a,b), again that
characterisation of greatest common divisors says gcd 0 0 = 0:
(0,0) = {0} = (0).

Pros: see above.

Cons: ?
I'm not aware of any, the change shouldn't break existing code, since that
would have to check for the special case anyway.

Daniel