[OT] Stackless fibers/coroutines

[Nick Sabalausky via Digitalmars-d]

On 09/26/2016 02:42 AM, Jacob Carlborg wrote:
> On 2016-09-25 20:16, Nick Sabalausky wrote:
>> I don't see why not, but there would be one notable drawback: You could
>> only have one instance existing at a time (per thread).
>
> But how does it work in languages which don't pass in the state
> explicitly, like with "yield" in C#?
>
> Or async/await? I guess one doesn't call the function multiple times in
> the same way instead it's done automatically under the hood, passing in
> the state which might be stored in the "Task" that is returned?
>

I don't know about async/await, because all my C# work was with versions 
of C# that predated those. But as for C#'s yield functions, yes, that's 
my understanding: It's hidden magic automatically inserted by the compiler.

As a side note, that actually makes D's opApply a very interesting case, 
really:

Instead of doing like the other implementations and "yielding" by 
removing the top function from the callstack (ie "returning"), opApply 
does the oppposite: It yields by pushing whatever function it's yielding 
*to*, *onto* the callstack. So its state is stored on the stack, no need 
for magic. The downside is, that's exactly what makes it impossible for 
opApply to be used as a range (at least without introducing a true fiber 
to save/restore the callstack): popFront *must* return in order for the 
user's algorithm to call front and obtain the current element - because 
that's how the range interface works.

>
> The above code compiles, after removing the extra "return+case:".
>

Really? I may have to look into this more then.

Hmm, well for starters, according to 
<http://dlang.org/spec/statement.html#GotoStatement>:
"It is illegal for a GotoStatement to be used to skip initializations."

Although if that's true, I don't know why that example compiled. Maybe 
because "foo" wasn't used from "case 2:" onward? Maybe it failed to 
notice the initialization since it was in an if condition? Or maybe the 
compiler forgot to implement the same "can't skip initialization" check 
for switch/case?