On Mon, Sep 17, 2012 at 11:24 AM, Giovanni Piero Deretta <gpderetta@gmail.com> wrote:
About the design:
I want to comment on this proposed design but not argue if it should replace what is currently under review.
I like the idea that has been proposed during the review that the result of a coroutine should be retrivable at any time via a get method. I dislike the proposed optional<T> result for operator(). If the previous delayed result functionality is implemented, operator() should just return *this, so that the subsequent get() call can be chained or the result tested for non empty. This change has the potential of deeply affecting the interface, so I would really like to see it subject to a mini review before acceptance.
In this case, get() or operator* (like in your sketch impl) should return T&, not T. Otherwise you get a surprise move: T x = *c; // OK, value has been moved into x. T y = *c; // y contains an empty value
It has been suggested that coroutine::self be removed an have a thread specific object. I strongly disagree with this option. First of all it will make it too easy to break type safety:
typedef this_coroutine<int(int)> this_coro;
typedef this_coroutine<void()> that_coro;
coroutine<int(int)>([]{ that_coro::yield(); // oops compiles fine, UB at runtime. });
This is a good point. I now think I'd prefer type safety here so I agree that using a thread specific object is bad.
Instead I suggest doing away with a separate self type and instead making the first parameter to the coroutine functor a full fledged coroutine, with simmetric signature:
coroutine<int(void)> my_coroutine([](coroutine<void(int)> caller) { caller(10); });
int i = my_coroutine();
I like this.
This means that the parameter no longer represent the coroutine itself, but, more logically, the caller coroutine. This makes very obvious the analogy between coroutines and channels. It also blurs the division between symmetric and asymmetric coroutines. Bonus points if you had a function that infers the coroutine singnature from the coroutine functor operator() (I like to call this function callcc for obvious reasons [ http://en.wikipedia.org/wiki/Call-with-current-continuation]):
auto my_coroutine = callcc([](coroutine<void(int)> caller) { ... } );
I agree that there're similarities with call/cc but I'm not sure it's exactly the same. call/cc returns the "yielded" value, not a coroutine. It only returns a coroutine if the "yield" was bundled with another call to call/cc. So to avoid the confusion, a different name might be best.
I do not like the separation between coroutines and generators. Instead the generator class should be dropped. The reason that I had generator as a separate class in the original coroutine SoC is because I wanted to give it iterator semantics (this also required internally using a reference counted pointer to make the iterator copiable, which I disliked). Instead I propose to make every coroutine<T()> fulfill the the input range concept and every coroutine<void(T)> fulfill the output range concept (this is a functionality not covered by generators).
While I like this suggestion, I worry that having one template specialization model one concept and another specialization model another concept (Input and Output iterators) might be too confusing.
The last suggestion works very well if coupled with the suggestion of making 'self' a full coroutine:
std::vector<int> a = { ... }; std::vector<int> b = { ... };
std::cout << "The result of 'merge(a,b) is:\n"; for(auto x : callcc([&](coroutine<void(int)> c) { std::merge(a.begin(), a.end(), b.begin(), b.end(), c.begin()); }) { std::cout << x << "\n"; }
This does look compelling. Also, the sketch implementation's continuation class is copyable. Conceptually, continuation/coroutine encompasses a resource (context) and should be movable only (like other resources: e.g. file handle wrappers). It would also be great if the coroutine function could return void instead of making the user return (moved) coroutine. I understand why it's needed from the implementation point of view but I wonder if there's a way around it. Regards, Eugene