On 15/02/17 10:35 AM, Olaf Klinke wrote:
You should not have to write tests for functions you did not define.
Correct me if I'm wrong, but any property of max can be derived from the properties of <=. Unfortunately, this isn't quite true. Suppose we have two values x y such that x <= y && y <= x && x == y *BUT* x and y are distinguishable some other way. For example, suppose we are modelling rational numbers by pairs (n,d) *without* insisting that gcd(n,d) == 0. Then we have (n1,d1) == (n2,d2) = n1*d2 == n2*d1 compare (n1,d1) (n2,d2) = compare (n1*d2) (n2*d1) BUT (1,2) and (2,4), while ==, are none-the-less distinguishable. Now ask about max x y. If we have max a b = if a > b then a else b then max x y delivers y. But if we have max a b = if a < b then b else a then max x y delivers x, and these two results can be distinguished. I know and freely admit that if I want a version of max that satisfies stronger conditions than Ord can guarantee, it is up to me to write it. So you could say that the bug was that I *should* have written my own max, and didn't. But that's basically my point. (For an example of two values that can't be distinguished by compare or ==, consider -0.0 and +0.0. If I give those to max, what do I get back? No, that wasn't the test case I needed.)