Why can't I understand what coroutines are?

From	Juha Nieminen <nospam@thanks.invalid>
Newsgroups	comp.lang.c++
Subject	Why can't I understand what coroutines are?
Date	2019-08-04 09:37 +0000
Organization	Netfront http://www.netfront.net/
Message-ID	<qi6910$1eht$1@adenine.netfront.net> (permalink)

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C++20 will introduce support for coroutines. To this day, for reasons
that are completely mysterious even to me, I have *absolutely no idea*
what they are. I just don't get it.

I go to the Wikipedia page "Coroutine"... and it tells me nothing.

"Coroutines are computer program components that generalize subroutines
for non-preemptive multitasking, by allowing execution to be suspended
and resumed. Coroutines are well-suited for implementing familiar program
components such as cooperative tasks, exceptions, event loops, iterators,
infinite lists and pipes."

This tells me nothing. I don't get it.

"By contrast, coroutines can exit by calling other coroutines, which may
later return to the point where they were invoked in the original
coroutine; from the coroutine's point of view, it is not exiting but
calling another coroutine."

Can exit by calling other coroutines... And then the execution later
resumes from that calling point. This is not "exiting" but "yielding",
whatever that's supposed to mean.

Sounds like a normal function call to me, but what do I know? I can't
understand what this is saying. It's like it's speaking in legible
English, yet it somehow is complete gibberish at the same time. I don't
get it.

It then gives a pseudocode example of coroutines... that look exactly
like a function calling another (which then just returns to the
original caller).

==========================================
var q := new queue

coroutine produce
    loop
        while q is not full
            create some new items
            add the items to q
        yield to consume

coroutine consume
    loop
        while q is not empty
            remove some items from q
            use the items
        yield to produce
==========================================

This looks to me exactly like

==========================================
var q := new queue

coroutine produce
    loop
        while q is not full
            create some new items
            add the items to q
        call consume

coroutine consume
    while q is not empty
        remove some items from q
        use the items
    return
==========================================

But what do I know? I don't get it.

Ok, Wikipedia is just a generic article about coroutines. Maybe
https://en.cppreference.com/w/cpp/language/coroutines has a better
explanation. It's done from the perspective of C++ in particular,
after all.

"A coroutine is a function that can suspend execution to be resumed
later. Coroutines are stackless: they suspend execution by returning
to the caller. This allows for sequential code that executes
asynchronously (e.g. to handle non-blocking I/O without explicit
callbacks), and also supports algorithms on lazy-computed infinite
sequences and other uses."

A function that can suspend execution to be resumed later...
Yeah, I don't get it. Allows sequential code that executes
asynchronously... What? Supports algorithms on lazy-computed infinite
sequences... Ok, but what does this all mean? I don't get it.

The examples given are complete gibberish. I have *absolutely no
idea* what something like this is supposed to mean:

    co_await async_write(socket, buffer(data, n));

co_await? What?

It seems that everybody else is awaiting (hah!) and praising coroutines
like the second coming of Christ. Yet I just can't understand what they
are, or how exactly they are used or useful. All web pages I can find
are legible English that still somehow manages to be completely meaningless
gibberish that tells me nothing.

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Thread

Why can't I understand what coroutines are? Juha Nieminen <nospam@thanks.invalid> - 2019-08-04 09:37 +0000
  Re: Why can't I understand what coroutines are? "Chris M. Thomasson" <invalid_chris_thomasson_invalid@invalid.com> - 2019-08-04 02:58 -0700
    Re: Why can't I understand what coroutines are? Sam <sam@email-scan.com> - 2019-08-04 09:18 -0400
      Re: Why can't I understand what coroutines are? "Chris M. Thomasson" <invalid_chris_thomasson_invalid@invalid.com> - 2019-08-04 11:15 -0700
  Re: Why can't I understand what coroutines are? Louis Krupp <lkrupp@nospam.pssw.com.invalid> - 2019-08-04 04:58 -0600
    Re: Why can't I understand what coroutines are? Juha Nieminen <nospam@thanks.invalid> - 2019-08-05 08:31 +0000
      Re: Why can't I understand what coroutines are? Martijn van Buul <pino@dohd.org> - 2019-08-05 09:51 +0000
        Re: Why can't I understand what coroutines are? "Chris M. Thomasson" <invalid_chris_thomasson_invalid@invalid.com> - 2019-08-05 14:44 -0700
  Re: Why can't I understand what coroutines are? Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2019-08-04 14:08 +0100
    Re: Why can't I understand what coroutines are? Sam <sam@email-scan.com> - 2019-08-04 09:23 -0400
      Re: Why can't I understand what coroutines are? Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2019-08-04 14:36 +0100
        Re: Why can't I understand what coroutines are? Sam <sam@email-scan.com> - 2019-08-04 10:31 -0400
          Re: Why can't I understand what coroutines are? Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2019-08-04 16:42 +0100
            Re: Why can't I understand what coroutines are? Sam <sam@email-scan.com> - 2019-08-04 16:43 -0400
              Re: Why can't I understand what coroutines are? "Chris M. Thomasson" <invalid_chris_thomasson_invalid@invalid.com> - 2019-08-04 13:58 -0700
                Re: Why can't I understand what coroutines are? Sam <sam@email-scan.com> - 2019-08-04 17:36 -0400
                Re: Why can't I understand what coroutines are? Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2019-08-04 22:43 +0100
                Re: Why can't I understand what coroutines are? Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2019-08-04 22:52 +0100
                Re: Why can't I understand what coroutines are? Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2019-08-04 23:10 +0100
                Re: Why can't I understand what coroutines are? Sam <sam@email-scan.com> - 2019-08-04 21:32 -0400
                Re: Why can't I understand what coroutines are? Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2019-08-05 18:32 +0100
                Re: Why can't I understand what coroutines are? scott@slp53.sl.home (Scott Lurndal) - 2019-08-05 17:48 +0000
                Re: Why can't I understand what coroutines are? Sam <sam@email-scan.com> - 2019-08-05 20:14 -0400
                Re: Why can't I understand what coroutines are? leigh.v.johnston@googlemail.com - 2019-08-06 04:13 -0700
                Re: Why can't I understand what coroutines are? leigh.v.johnston@googlemail.com - 2019-08-06 04:23 -0700
                Re: Why can't I understand what coroutines are? Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2019-08-06 22:19 +0100
                Re: Why can't I understand what coroutines are? "Chris M. Thomasson" <invalid_chris_thomasson_invalid@invalid.com> - 2019-08-06 15:19 -0700
                Re: Why can't I understand what coroutines are? Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2019-08-07 00:01 +0100
                Re: Why can't I understand what coroutines are? scott@slp53.sl.home (Scott Lurndal) - 2019-08-06 23:52 +0000
                Re: Why can't I understand what coroutines are? Öö Tiib <ootiib@hot.ee> - 2019-08-06 16:57 -0700
                Re: Why can't I understand what coroutines are? scott@slp53.sl.home (Scott Lurndal) - 2019-08-07 14:41 +0000
                Re: Why can't I understand what coroutines are? Öö Tiib <ootiib@hot.ee> - 2019-08-08 07:10 -0700
                Re: Why can't I understand what coroutines are? scott@slp53.sl.home (Scott Lurndal) - 2019-08-06 23:49 +0000
                Re: Why can't I understand what coroutines are? Robert Wessel <robertwessel2@yahoo.com> - 2019-08-04 22:04 -0500
                Re: Why can't I understand what coroutines are? "Chris M. Thomasson" <invalid_chris_thomasson_invalid@invalid.com> - 2019-08-04 20:10 -0700
        Re: Why can't I understand what coroutines are? "Chris M. Thomasson" <invalid_chris_thomasson_invalid@invalid.com> - 2019-08-04 11:20 -0700
          Re: Why can't I understand what coroutines are? Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2019-08-04 20:30 +0100
            Re: Why can't I understand what coroutines are? "Chris M. Thomasson" <invalid_chris_thomasson_invalid@invalid.com> - 2019-08-04 14:03 -0700
            Re: Why can't I understand what coroutines are? Paavo Helde <myfirstname@osa.pri.ee> - 2019-08-05 09:36 +0300
              Re: Why can't I understand what coroutines are? "Chris M. Thomasson" <invalid_chris_thomasson_invalid@invalid.com> - 2019-08-05 14:34 -0700
        Re: Why can't I understand what coroutines are? scott@slp53.sl.home (Scott Lurndal) - 2019-08-04 19:46 +0000
      Re: Why can't I understand what coroutines are? Juha Nieminen <nospam@thanks.invalid> - 2019-08-05 08:37 +0000
        Re: Why can't I understand what coroutines are? David Brown <david.brown@hesbynett.no> - 2019-08-05 12:06 +0200
          Re: Why can't I understand what coroutines are? Juha Nieminen <nospam@thanks.invalid> - 2019-08-05 12:20 +0000
            Re: Why can't I understand what coroutines are? David Brown <david.brown@hesbynett.no> - 2019-08-06 15:06 +0200
              Re: Why can't I understand what coroutines are? David Brown <david.brown@hesbynett.no> - 2019-08-06 15:18 +0200
          Re: Why can't I understand what coroutines are? Bonita Montero <Bonita.Montero@gmail.com> - 2019-08-06 13:58 +0200
  Re: Why can't I understand what coroutines are? Sam <sam@email-scan.com> - 2019-08-04 09:17 -0400
  Re: Why can't I understand what coroutines are? Robert Wessel <robertwessel2@yahoo.com> - 2019-08-04 10:12 -0500
    Re: Why can't I understand what coroutines are? Juha Nieminen <nospam@thanks.invalid> - 2019-08-05 08:42 +0000
      Re: Why can't I understand what coroutines are? Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2019-08-05 13:35 +0100
      Re: Why can't I understand what coroutines are? Bart <bc@freeuk.com> - 2019-08-05 20:19 +0100
      Re: Why can't I understand what coroutines are? Christian Gollwitzer <auriocus@gmx.de> - 2019-08-05 22:26 +0200
        Re: Why can't I understand what coroutines are? "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com> - 2019-08-05 23:09 +0200
          Re: Why can't I understand what coroutines are? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-08-09 19:49 -0700
            Re: Why can't I understand what coroutines are? "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com> - 2019-08-10 05:13 +0200
          Re: Why can't I understand what coroutines are? Christian Gollwitzer <auriocus@gmx.de> - 2019-08-10 08:23 +0200
            Re: Why can't I understand what coroutines are? "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com> - 2019-08-10 10:43 +0200
              Re: Why can't I understand what coroutines are? Christian Gollwitzer <auriocus@gmx.de> - 2019-08-11 22:19 +0200
                Re: Why can't I understand what coroutines are? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-08-14 08:25 -0700
        Re: Why can't I understand what coroutines are? "Chris M. Thomasson" <invalid_chris_thomasson_invalid@invalid.com> - 2019-08-10 00:19 -0700
        Re: Why can't I understand what coroutines are? Juha Nieminen <nospam@thanks.invalid> - 2019-08-11 21:16 +0000
      Re: Why can't I understand what coroutines are? "Chris M. Thomasson" <invalid_chris_thomasson_invalid@invalid.com> - 2019-08-05 23:43 -0700
      Re: Why can't I understand what coroutines are? Bonita Montero <Bonita.Montero@gmail.com> - 2019-08-10 10:50 +0200
  Re: Why can't I understand what coroutines are? Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2019-08-04 17:57 +0100
  Re: Why can't I understand what coroutines are? "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com> - 2019-08-05 01:11 +0200
    Re: Why can't I understand what coroutines are? Juha Nieminen <nospam@thanks.invalid> - 2019-08-05 08:50 +0000
      Re: Why can't I understand what coroutines are? "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com> - 2019-08-05 16:59 +0200
    Re: Why can't I understand what coroutines are? Manfred <noname@add.invalid> - 2019-08-06 16:24 +0200
    Re: Why can't I understand what coroutines are? "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com> - 2019-08-07 00:52 +0200
      Re: Why can't I understand what coroutines are? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-08-09 18:11 -0700
  Re: Why can't I understand what coroutines are? Martijn van Buul <pino@dohd.org> - 2019-08-05 07:46 +0000

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