On Mon, Nov 6, 2023 at 4:58 AM Frederick Virchanza Gotham via Std-Proposals
<std-proposals_at_[hidden]> wrote:

> I reply in series below to Ville, Giuseppe, Jonathan, Marcin and Brian
>
>
> Ville Voutilainen <ville.voutilainen_at_[hidden]> wrote:
> >
> > Yeah well, perhaps the solution to that problem isn't a facility
> > spelled "not continue". Because for
> > !continue, that's a valid alternative spelling.
>
>
> I've no idea what you mean here. "!continue" is presently a compiler
> error with g++:
>
> <source>: In function 'int main()':
> <source>:10:36: error: expected primary-expression before
> 'continue'
> 10 | for ( int i = 0; i < 5; ++i ) !continue;
> | ^~~~~~~~
> Compiler returned: 1
>
>
> Giuseppe D'Angelo wrote:
> >
> > In Perl this is called `redo`, which is a much much much better name
> > than `!continue`.
>
>
> Yeah fair enough we could have "__redo" or "_Redo" instead of
> "!continue". Personally I like "!continue" but the exact syntax isn't
> important right now.
>
>
> Jonathan Wakely wrote:
> >
> > Yeah, obviously it means "don't continue" i.e. stop looping. But we have
> break for that.
>
>
> I'm trying to learn German at the moment, and I still pause for a
> second before saying "You must not do that", because I don't want to
> mistakenly say "You don't need to do that". First I remind myself that
> "Du musst das nicht machen" imposes no restrictions, and so then by
> deductive reasoning I know that I've to pick out a different verb to
> say "You must not".
>
> Similarly with C++, if everyone knows that 'break' breaks out of a
> loop, then everybody's gonna know that "!continue" must do something
> else. Nobody who's been programming in C++ for more than a week is
> going to think that "!continue" breaks out of a loop.
>
> Here's another few possible syntaxes:
> --continue;
> continue(skip);
>
>
> Marcin Jaczewski wrote:
> >
> > There is idiom like this with `erase`:
> >
> > ```
> > if (condition)
> > {
> > p = v.erase(p);
> > }
> > else
> > {
> > ++p;
> > }
> > ```
> >
> > but do not know if this is good code practice as it is very easy to
> > make mistakes.
>
>
> This is exactly what I'm getting at here -- having a simple language
> feature that allows us to write simple code. When we have to do
> gymnastics -- and yes I'll admit it's fun sometimes especially when
> programming microcontrollers -- those gymnastics can have bugs in
> them, and just make the code more complex.
>
> By the way at the start of this thread, Brian said that he jumped back
> to the start of the loop when he wanted to skip the post-iterative
> step. Brian, if you consider the following loop:
>
> for ( unsigned i = 0u; i < 67u; ++i )
> {
> if ( !Erase() ) !continue;
> }
>
> which behaves the same as:
>
> for ( unsigned i = 0u; i < 67u; ++i )
> {
> if ( !Erase() ) { --i; continue; }
> }
>
> then how would you use goto here? something like:
>
> for ( unsigned i = 0u; i < 67u; ++i )
> {
> start_of_loop:
> if ( !Erase() ) goto start_of_loop;
> }
>
> That of course would skip the pre-iterative condition check, and so
> we'd have to write:
>
> for ( unsigned i = 0u; ; ++i )
> {
> start_of_loop:
> if ( !(i < 67u) ) break;
> if ( !Erase() ) goto start_of_loop;
> }
>

I think the situations where you need to not only skip the incrementation
statement but *also* re-check the condition *despite not having incremented*
are very rare.


>
> But that isn't a very nice loop. Sure it works, but it would be much
> nicer to have it as:
>
> for ( unsigned i = 0u; i < 67u; ++i )
> {
> if ( !Erase() ) !continue;
> }
>
> or maybe:
>
> for ( unsigned i = 0u; i < 67u; ++i )
> {
> if ( !Erase() ) --continue;
> }
>
> or maybe:
>
> for ( unsigned i = 0u; i < 67u; ++i )
> {
> if ( !Erase() ) continue(skip);
> }
> --
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>

-- 
*Brian Bi*