Newsgroups: php.internals
Path: news.php.net
Xref: news.php.net php.internals:108768
Date: Wed, 26 Feb 2020 07:41:03 -0700
To: Nikita Popov <nikita.ppv@gmail.com>
Cc: PHP internals <internals@lists.php.net>
Message-ID: <1582728063.10268.2@judahwright.me>
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 <CAF+90c8AVchjJ6VNtY=4Qg3aWmmK2REVEK6r+rQtKB_-3X905w@mail.gmail.com>
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Subject: Re: [PHP-DEV] Support rewinding of generators
From: hello@judahwright.me (Judah Wright)

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Perhaps an easy userland implementation could be type-hinting a new 
generator type, to indicate that the generator should be rewindable by 
simply re-calling the function?

// Safe to rewind
function fooRange(int $from, int $to): RewindableGenerator {
  for ($i = $from; $i <= $to; $i++) {
    yield $i;
  }
}

That should be safe to re-call again, but my concern is with generators 
that modify some external state that should not be called twice. In 
these cases having a fatal error is a handy feature to prevent 
re-iterating over things that might cause issues.

~Judah

On Wed, Feb 26, 2020 at 12:47 pm, Nikita Popov <nikita.ppv@gmail.com> 
wrote:
> Hi internals,
> 
> Generators currently do not support rewinding -- or rather, only 
> support it
> if the generator is at/before the first yield, in which case 
> rewinding is a
> no-op.
> 
> Generators make it real breeze to implement primitives like
> 
> function map(callable $function, iterable $iterable): \Iterator {
>     foreach ($iterable as $key => $value) {
>         yield $key => $function($value);
>     }
> }
> 
> without having to do through the whole Iterator boilerplate. However, 
> if
> you do this, you end up with an iterator that is not rewindable. If 
> you
> want to make map() rewindable, you need to go back to a manual 
> Iterator
> implementation. As iterators in PHP are assumed to be rewindable by
> default, this is somewhat annoying.
> 
> There is a relatively simple (at least conceptually) way to make 
> generators
> rewindable: Remember the original arguments of the function, and 
> basically
> "re-invoke" it on rewind().
> 
> I'm wondering what people think about adding this functionality. I 
> think
> the main argument against it is that not all generators may behave 
> sensibly
> if you re-run their code -- there's probably a reasonable expectation 
> that
> an iterator will return the same sequence of values are rewinding,
> something which we cannot guarantee with generators, but also don't 
> enforce
> with normal iterators either.
> 
> Regards,
> Nikita


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