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RFC: First Class Callables in constant expressions

php.internals

Tim Düsterhus

1 year ago

Hi

Volker and I would like to start discussion on our RFC to support "First
Class Callables in constant expressions".

Please find the following resources for your reference:

- RFC: https://wiki.php.net/rfc/fcc_in_const_expr
- Implementation: https://github.com/php/php-src/pull/17213

- The related "Support Closures in constant expressions" RFC:
https://wiki.php.net/rfc/closures_in_const_expr
- The "Support Closures in constant expressions" discussion:
https://externals.io/message/125872

Best regards
Tim Düsterhus

Larry Garfield

1 year ago

On Wed, Jan 22, 2025, at 8:35 AM, Tim Düsterhus wrote:

> Hi
>
> Volker and I would like to start discussion on our RFC to support "First
> Class Callables in constant expressions".
>
> Please find the following resources for your reference:
>
> - RFC: https://wiki.php.net/rfc/fcc_in_const_expr
> - Implementation: https://github.com/php/php-src/pull/17213
>
> - The related "Support Closures in constant expressions" RFC:
> https://wiki.php.net/rfc/closures_in_const_expr
> - The "Support Closures in constant expressions" discussion:
> https://externals.io/message/125872
>
> Best regards
> Tim Düsterhus

I don't know how to interpret the radio silence from the list, but this seems like an obvious and good follow-on to the previous RFC and has my support.

--Larry Garfield

Arvids Godjuks

1 year ago

On Fri, 31 Jan 2025 at 01:59, Larry Garfield <larry@garfieldtech.com> wrote:

> On Wed, Jan 22, 2025, at 8:35 AM, Tim Düsterhus wrote:
> > Hi
> >
> > Volker and I would like to start discussion on our RFC to support "First
> > Class Callables in constant expressions".
> >
> > Please find the following resources for your reference:
> >
> > - RFC: https://wiki.php.net/rfc/fcc_in_const_expr
> > - Implementation: https://github.com/php/php-src/pull/17213
> >
> > - The related "Support Closures in constant expressions" RFC:
> > https://wiki.php.net/rfc/closures_in_const_expr
> > - The "Support Closures in constant expressions" discussion:
> > https://externals.io/message/125872
> >
> > Best regards
> > Tim Düsterhus
>
> I don't know how to interpret the radio silence from the list, but this
> seems like an obvious and good follow-on to the previous RFC and has my
> support.
>
> --Larry Garfield
>

Ship it.
Fin.
;)

--

Arvīds Godjuks
+371 26 851 664
arvids.godjuks@gmail.com
Telegram: @psihius https://t.me/psihius

Tim Düsterhus

1 year ago

Hi

Am 2025-01-31 00:58, schrieb Larry Garfield:

> I don't know how to interpret the radio silence from the list, but this
> seems like an obvious and good follow-on to the previous RFC and has my
> support.

We also interpreted the lack of responses as “this is an obvious
follow-up and I don't have any questions or concerns”, especially since
FCC support also came up in the discussion of the Closure RFC and was
split into a separate RFC primarily to give each of these features the
proper attention with regard to implementation and edge cases. In
hindsight this was the correct decision, since the implementation for
FCC was much more complicated and edge-casey than initially expected,
whereas Closures were reasonably straight forward.

Best regards
Tim Düsterhus

Larry Garfield

1 year ago

On Fri, Jan 31, 2025, at 5:03 AM, Tim Düsterhus wrote:

> Hi
>
> Am 2025-01-31 00:58, schrieb Larry Garfield:
>> I don't know how to interpret the radio silence from the list, but this
>> seems like an obvious and good follow-on to the previous RFC and has my
>> support.
>
> We also interpreted the lack of responses as “this is an obvious
> follow-up and I don't have any questions or concerns”, especially since
> FCC support also came up in the discussion of the Closure RFC and was
> split into a separate RFC primarily to give each of these features the
> proper attention with regard to implementation and edge cases. In
> hindsight this was the correct decision, since the implementation for
> FCC was much more complicated and edge-casey than initially expected,
> whereas Closures were reasonably straight forward.
>
> Best regards
> Tim Düsterhus

Purely out of curiosity and for educational value, what was so complicated about it?  I would have expected it to be straightforward.

--Larry Garfield

Tim Düsterhus

1 year ago

Hi

Am 2025-01-31 23:36, schrieb Larry Garfield:

> Purely out of curiosity and for educational value, what was so
> complicated about it?  I would have expected it to be straightforward.

FCC internally are effectively treated as function calls with a funny
parameter list. This means that all valid ways to perform a function
call are valid ways to create a FCC. And likewise do all constraints of
a function call also apply to creating a function call. FCCs in
const-expr need to replicate those semantics as applicable. For us this
means: A FCC in a parameter default value behaves, as if the assignment
was the first line in the function body. Likewise must a FCC in a
property default value behave as if it was assigned in the first line of
the constructor body.

And then amongst others you have the following (edge-)cases to take into
account:

- All kinds of expressions can be used as a function name (this RFC
excludes those for simplicity).
- A FCC can resolve to a dynamic function when __call() or
__callStatic() is implemented (this RFC excludes those for simplicity).
- Static methods can directly be called on a trait (this is deprecated,
but supported by the RFC).
- Calls to free-standing functions need to take the global namespace
fallback into account (which will be cached for each individual call,
which has funny effects when the function in the same namespace gets
defined after some of the calls already executed at least once).
- Visibility and names needs to be resolved correctly, including
references to `self` and `static`.

For regular closures (i.e. the first RFC) it was as simple as: Disallow
`use` and require `static` and you're done. They are much less
context-dependent.

Best regards
Tim Düsterhus

Rob Landers

1 year ago

On Wed, Jan 22, 2025, at 15:35, Tim Düsterhus wrote:

> Hi
>
> Volker and I would like to start discussion on our RFC to support "First
> Class Callables in constant expressions".
>
> Please find the following resources for your reference:
>
> - RFC: https://wiki.php.net/rfc/fcc_in_const_expr
> - Implementation: https://github.com/php/php-src/pull/17213
>
> - The related "Support Closures in constant expressions" RFC:
> https://wiki.php.net/rfc/closures_in_const_expr
> - The "Support Closures in constant expressions" discussion:
> https://externals.io/message/125872
>
> Best regards
> Tim Düsterhus
>

Hey Tim,

My only concern with the RFC is something a bit silly. If I understand correctly, this will be a constant expression:

fn(string $foo) => print $foo

But this will not:

(fn(string $foo) => print $foo)(...)

despite them both basically being the same thing.

It is an incredibly small inconsistency (imho), but other than that, ship it.

— Rob

Tim Düsterhus

1 year ago

Hi

Am 2025-02-02 15:35, schrieb Rob Landers:

> My only concern with the RFC is something a bit silly. If I understand
> correctly, this will be a constant expression:

You understood that correctly.

> It is an incredibly small inconsistency (imho), but other than that,
> ship it.

I wouldn't call it an inconsistency, it's natural that the allowed
expressions in a constant-expression context are limited. This
limitation is mentioned in the second bullet point of the “Constraints”
section: Only the "basic form" of first-class callables is supported,
mainly to keep the implementation simple, due to the limited usefulness
of the more complex function call variants when limited to
constant-expressions. As an "escape hatch", it's always possible to
wrote a proper closure that just forwards the calls to the desired
method.

It will also result in a useful error message that is consistent with
dynamic new-expressions within const-expr context:

Fatal error: Cannot use dynamic function name in constant expression
in test5.php on line 3

and for new-expressions:

$ cat test5.php
<?php

const Foo = new (Foo)();
$ sapi/cli/php test5.php

Fatal error: Cannot use dynamic class name in constant expression in
test5.php on line 3

Best regards
Tim Düsterhus

Volker Dusch

1 year ago

On Wed, Jan 22, 2025 at 3:35 PM Tim Düsterhus <tim@bastelstu.be> wrote:

> - RFC: https://wiki.php.net/rfc/fcc_in_const_expr
> - Implementation: https://github.com/php/php-src/pull/17213

With the two weeks discussion time elapsing later today and given the
feedback we received, we plan to open the vote tomorrow pending any
major concerns.

Kind Regards,
Volker

--
Volker Dusch
Head of Engineering
Tideways GmbH
Königswinterer Str. 116
53227 Bonn
https://tideways.io/imprint

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