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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Generators/coroutines in future Ada?
Date: Thu, 13 Jul 2017 10:18:20 +0200
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Xref: news.eternal-september.org comp.lang.ada:47396
Date: 2017-07-13T10:18:20+02:00
List-Id: <comp.lang.ada>

On 12/07/2017 22:49, G.B. wrote:

> Assume that the language has special generator syntax, e.g.:
> 
>     while X of Stack with 0 < X and X < 10 loop
> 
> This is to mean that generating a value happens once per iteration,
> binding the result to the X before *of* and then evaluating the
> condition. Stack being a Stateful, it has the prim ops needed.

OK, but this is not generator syntax. It is the problem of using some 
computed value twice. Yes we need a syntax for that. Far more important 
than the loop-generator case are cases like:

"Generator-new":

    X := new S'Class;
    declare
       Object : S'Class renames S'Class (X.all);
    begin
       ...
    end;

"Generator-return":

    like above, create a more specific instance returning it as a more 
generic class-wide.

"generator-test":

    if X in T'Class then
       declare
          Object : T'Class renames T'Class (X);
       begin
          ...
       end;

> If X, still a generator, were instead to be repeatedly evaluated,
> as in
> 
>     while 0 < X and X < 10 loop
> 
> then it isn't clear how many values will be generated if,
> under the hood, the expression entails evaluating
> 
>      0 <Stack.Pop and Stack.Pop < 10.

"Generator-test" above.

> Note that this argument emphasizes the use of a generator,
> not the construct that furnishes it.

I see why co-routines are useful when dressed as tasks. I see great use 
in syntax constructs which might reduce above cases into single liners. 
I see no use in stateful functions, only damage.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de