On Tue, Sep 28, 2010 at 12:44 PM, joel falcou <joel.falcou@lri.fr> wrote:
On 28/09/10 17:10, Stewart, Robert wrote:
'...one can write a sum_int_impl using this Concept...'
"Yes and"
'...is it too hacky?'
"yes"
Guess I embarassed myself then *blushes* Why is it hacky ? The fact that second(int) return 0 ? Then can't we say that second(int) is ill-formed and have HasFirstComponent and HasSecondComponent and thus making the std::pair sum require AND relation ?
(or am I still guessing it wrong ?)
Actually, I don't think it's a bad hack. Formally, summation in the integers is a _binary_ relation; i.e. a relation from the cartesian product Z^2 into Z. I would write Eric's functions using something like... int sum_int(int x, int y = 0) { return x + y; } int sum_int(pair<int,int> p) { return sum(first(p), second(p)); } To "conceptually" describe the argument(s) of sum_int, you need an abstraction to represent members of Z^2. But in this case I'm not sure there's a satisfying way to do that, because an ordered pair from Z^2 can be encoded in C++ as a single function parameter (a pair) or by two function parameters... To me this seems related to a persistent kink in mathematical notation. Let f : Z^2 -> Z. But we write f(x, y) for <x,y> in Z^2, instead of the more formal f(<x,y>). Also, for those who haven't read it, I would recommend chapter 2 of Jeremy Siek's book The Boost Graph Library as an introduction to concepts. It's a great piece of expository writing and helped me a lot. Daniel Walker