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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: RFC: Prototype for a user threading library in Ada
Date: Thu, 23 Jun 2016 17:12:04 -0500
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Date: 2016-06-23T17:12:04-05:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
news:nkg78j$1keu$1@gioia.aioe.org...
> On 23/06/2016 03:42, Randy Brukardt wrote:
>
>> I'd like to understand better the motivations for these features, so if 
>> you
>> (or anyone else) wants to try to explain them to me, feel free.
>
> The motivation is a two-liner. Let you have some consumer of data:
>
>    procedure Write (Buffer : String; Last : out Integer);
>
> It may take less than the whole string when called, but will take more 
> data later. So, the parameter Last. Now you want to write a program in a 
> *normal* way:
>
>    Write ("This");
>    Write ("That");
>
> That's it.

That wasn't my question; I'm wondering the motivation for these features in 
terms of parallelization. In today's world, it's impossible to consider any 
feature in a purely sequential manner. The expressiveness gain (if any) is 
secondary. And the OP was talking about parallelism, not generators.

The generator proposal as expressed in AI12-0197-1 is just too expensive to 
consider for a purely sequential feature. For Janus/Ada on Windows, we'd 
either have to throw away 1/3 of the back-end of the compiler (and generally 
use slower instructions in some cases, impacting all code), or implement 
these *exactly* as we do tasks (with a TCB, context switching, and so on). 
[Not to mention the extensive changes needed to the front-end.] For 
something to be worth that sort of effort, it has to benefit a large 
percentage of programs. This (as a purely sequential feature) doesn't do 
that.

                                                Randy.