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Subject: Re: [boost] [Review.Coroutine] Vicente's review
From: Oliver Kowalke (oliver.kowalke_at_[hidden])
Date: 2012-09-13 11:38:55

Am 13.09.2012 16:42, schrieb Vicente Botet:
>> generator< int > gen( f);
>> if ( optional< int > val = gen() ) {...}
>> if the generator function simple returns (no yield() called) then gen()
>> returns a none (== invalid/unset optional).
> I don't think optional should be used. The user must know if it can call the
> generator to get the next value or not.
That was my previous design. The user has to check the generator like:

if ( gen) before asking the next value

if ( gen) {
    int x = gen();

This design required pre-fetching and storing the return value via a
context jump into the generator routine.
Only with this mechanism I was able to know if the next call to
generator<>::operator() will return a value.
(requires that the first pre-fetch is done in the ctor of generator<>

The difference to the actual design

if ( optional< int > val = gen() ) {

is that I don't need to pre-fetch and not to store the optional<> as
parameter (optional is only created inside generator<>::operator()).

> My point was independent of the generator interface. I don't think the
> asymmetry is beneficial, but maybe I'm missing something important.
>> In the case of coroutines coroutine<R>::operator() should still return R
>> (not optional<R>).
>> void f( self_t &) {}
>> coroutine< string( int, int) > coro( f);
>> string s = coro( 1, 2);
>> Which value should coroutine< string >::operator() return if the
>> coroutine-function is required to return void and the user simply returns
>> without calling yield()?
> A coroutine should yield a value once it is called/resumed. The library can
> check that a value has been yield when the function returns, isn't it?

If the coroutine function does not call self_t::yield() at all no return
value can be fetched and returned to the caller.
That means in the corrected example above:

void f( self_t &, int x, int y)
coroutine < string( int, int) > coro( f);
string s = coro( 1, 2); // f did not return a value, what value should
s have after this?

's' would contain garbage - therefore I think the user must be forced to
return an value at least at the final 'return' statement.

>> My impression is that this asymmetry between coroutines and generators
>> should be kept.
> I would like to be convinced. For the time been I'm not.
I hope with my argument above you are convinced now ;)
>>> * The access to the actual coroutine parameters is asymmetric, letting
>>> access to old actual parameters that can point to objects that have
>>> already
>>> been destroyed.
>>> Maybe the self_t object could take care of this (or this_coroutine). We
>>> can use a get<> function to give access to the actual parameters.
>>> int f20(coro_t::self_t & self)
>>> {
>>> self.yield(2*self.get<0>());
>>> }
>>> int f20()
>>> {
>>> typedef this_coroutine&lt;int(int)&gt; this_coro;
>>> this_coro::yield(2*this_coro::get<0>());
>>> }
>> interesting idea, I find it better than the version using bind because you
>> don't can't forget to 'bind'. you are always required to use
>> self_t::get<>().

Would not prevent user from doing something like this:

in f20( coro_t::self_t & s, int x, int y)
    s.yield( s.get< 0 > * x + y);

coroutine< int(int) > coro( bind( f20, _1, _2, 2, 3) );
int x = coro( 5); // x == 13

? don't know which interface would be small and hardly to misuse ?


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