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Date: Mon, 11 Mar 2019 13:16:19 +0100
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To: Larry Garfield <larry@garfieldtech.com>
Cc: PHP internals <internals@lists.php.net>
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Subject: Re: [PHP-DEV] RFC Draft: Comprehensions
From: nikita.ppv@gmail.com (Nikita Popov)

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On Sun, Mar 10, 2019 at 10:45 PM Larry Garfield <larry@garfieldtech.com>
wrote:

> Hello, peoples.  I know it's been discussed once or twice before on the
> list, many years ago, but not recently.  I therefore feel OK putting forth
> the following draft proposal for Comprehensions, aka "compact generators",
> in PHP:
>
> https://wiki.php.net/rfc/cohttps://externals.io/message/61021mprehensions
> <https://wiki.php.net/rfc/comprehensions>
>
> Sara Golemon has written a preliminary patch that is partially complete
> (see the RFC link for details, it's impressively simple), but unfortunately
> doesn't have the bandwidth to complete it at this time.  I am therefore
> looking for collaborators with more knowledge of internals than I (which is
> almost everyone) to help finish it up.
>
> The syntax proposed is also subject to revision if a terser but still
> lexer-friendly alternative can be proposed.
>
> At the moment I'm not calling this Proposed yet, as I don't feel
> comfortable doing so until someone else is on board to finish coding it.
> That said, if someone wants to weigh in on the concept for now (hopefully
> supportively) that's also fine.
>
> Anyone excited enough to help finish the job?
>
> (This is my first RFC collaboration, so if you're going to smack me for
> goofing something please be gentle about it.)
>

I've brought up a similar proposal a few years ago, probably around the
time generators were first introduced: https://externals.io/message/61021

I think my main point of feedback would be to stick closer to existing PHP
syntax, even if it costs us some brevity. I would prefer

    $gen = [foreach ($list as $x) if ($x % 2) yield $x * 2];

over

    $gen = [for $list as $x if $x % 2 yield $x * 2];

The latter is nice in languages that generally use "for" for this purpose
and generally allow omitting parentheses, but I don't think it's good to
introduce this kind of syntax inconsistency in one place.

Similarly, I found the ability to omit the "yield" expression somewhat
confusing. Especially once you get to nesting

   $gen = [for $a as $b for $b as $c];

the behavior becomes non-obvious. Also wondering how this part interacts
with keys, do you decide whether or not to yield the keys based on whether
they are part of the for expression?

    $gen = [for $list as $v]; // drops keys
    $gen = [for $list as $k => $v]; // keeps keys

Finally, Python makes a distinction between list comprehensions using []
and generator expressions using (). This proposal effectively corresponds
to generator expressions, but uses the [] syntax. I'm wondering if that
will cause confusion.

Regards,
Nikita

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