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Date: Thu, 6 Feb 2020 12:35:01 +0100
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To: tyson andre <tysonandre775@hotmail.com>
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Subject: Re: [PHP-DEV] Planning an RFC to allow calls to global functions in
 constant expressions
From: nikita.ppv@gmail.com (Nikita Popov)

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On Sun, Feb 2, 2020 at 12:00 AM tyson andre <tysonandre775@hotmail.com>
wrote:

>
> Hi internals,
>
> I have a working implementation for calling global functions in constant
> expressions at https://github.com/php/php-src/pull/5139
> (see PR description for more details, see tests for how edge cases get
> resolved)
>
> If there was interest, and no implementation or process blockers I was
> unaware of, I was planning to set up an RFC with votes along these lines:
>
> 1. (Primary) Yes/no for supporting calls to global functions, with or
> without the below limitation (requires 2/3 majority)
> 2. (Secondary vote requiring 2/3 majority to allow calls to all global
> functions) Support calls to any user-defined or internal global functions,
> not just unambiguously resolved calls to the deterministic, side effect
> free ones that are guaranteed to be installed in core extensions.
>
> Alternately, have a first vote on allowing a small subset of global
> functions calls, then if that passes, start a second vote on allowing all
> global functions.
>
> For example, allow `\count()`, `\strlen()`, `\array_merge()`, and
> `\in_array()`,
> but don't allow functions such as
> `\strtolower()` (different in Turkish locale),
> \sprintf() (Depends on locale and  `ini_get('precision')`, but so does
> `(string)EXPR`),
> `json_encode()` (The `json` extension can be disabled),
> or calls which aren't unambiguously resolved with `\` or `use function`.
> I'll assemble the list when writing the RFC.
>
> https://github.com/phan/phan/blob/2.4.8/src/Phan/Plugin/Internal/UseReturnValuePlugin.php#L190
> has a list of candidate functions (non-deterministic ones such as rand()
> will be excluded).
>
> I mentioned this earlier in https://externals.io/message/108238#108238
> The use case I have in mind is different from what was mentioned there,
> and this is *just* adding supports for function calls by name, not any
> other expressions (variables, properties, methods, etc)
>
> Motivation for the voting structure: There are many opinions on what a
> constant *should* be used for, e.g. one opinion is
> https://externals.io/message/108238#107992
>
>
> > Off the top of my head, I can think of three types of things that you
> might call "constants":
> >
> > 1. A value that will never change, and you just want to give a name to.
> For instance, MINUTES_IN_HOUR=60
> > 2. A value set at compile-time, to hard-code a particular condition. For
> instance, DEBUG=false
> > 3. A value which is set at run-time, but happens not to change during
> the course of the application, so should not be over-written.
> >
> > PHP's constants are designed primarily to be type 1 - they are literal
> > values, hard-coded in the source. Unlike C, there is no standard
> > pre-processor for PHP, so type 2 is rarer, but it's possible to
> > implement by manipulating the source file before deployment, or even
> > when running the autoloader.
>

Hey Tyson,

As you've already realized, the main problem here is the behavior for
functions that have side-effects or state. Currently we mostly get away
with the illusion that it doesn't matter when exactly constexpr
initializers are evaluated. Apart from the error location and (admittedly
quite a few) other details, you ostensibly can't distinguish whether the
expression is going to evaluated on declaration, on first access or on each
access.

I'm not a big fan of whitelisting functions, especially as this runs into
the name resolution issue (you suggest that essentially the use of fully
qualified/imported names will be forced -- unlike everywhere else in PHP),
and because I've seen this "mark all the functions const/constexpr" race
play out one too many times in other languages, which have a much stronger
motivation for it than we do.

If we want to expand the allowed operations inside constexpr initializers,
I think this needs to start by considering precise evaluation semantics in
the places where it matters. Those are primarily initializers for function
parameters and non-static properties, because both of these have a
reasonable expectation of evaluation at each use. I think the best way to
approach those two may well be to relax the constexpr restrictions on them
entirely (allowing say "public Foo $prop = new Foo()", something people
want anyway) and make sure that things are evaluated on each access (unless
they can be pre-evaluated without affecting behavior, of course).

For the remaining cases (constant, class constant and static property
initializers) we can probably get away with the current semantics, which is
evaluation on first access, with a very broad definition of what "access"
means.

Regards,
Nikita

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