[RFC] Deprecate and remove continue targeting switch

Hi!

This leaves us in a position where "continue" means different things in
PHP and in other syntactically similar languages, causing confusing for
programmers familiar with them. This is solved by simply prohibiting the
case that is ambiguous. This is not a loss for PHP developers (who use

I see your point, in this case I'd add a notice for continue in switch
that would say something along the lines "continue in switch is the same
as break, did you mean "continue 2;"?"

Stas Malyshev
smalyshev@gmail.com

-----Original Message-----
From: Nikita Popov [mailto:nikita.ppv@gmail.com]
Sent: Tuesday, June 26, 2018 9:28 PM
To: Stanislav Malyshev smalyshev@gmail.com
Cc: PHP internals internals@lists.php.net
Subject: Re: [PHP-DEV] [RFC] Deprecate and remove continue targeting switch

This RFC resolves the following issue:

a) In PHP "switch" is considered a looping structure, for this reason "break" and
"continue" both apply to "switch", as aliases. For PHP, these are reasonable
semantics, as PHP supports multi-level breaks. It would be very questionable if
"break N" and "continue N" could refer to different loop structures just because
there is a "switch" involved somewhere.

b) On the other hand, other languages, such as C and C-based languages, use a
different behavior, where "continue" refers to the enclosing loop. As these
languages don't have multi-level breaks, these semantics are also reasonable.

This leaves us in a position where "continue" means different things in PHP and
in other syntactically similar languages, causing confusing for programmers
familiar with them. This is solved by simply prohibiting the case that is
ambiguous. This is not a loss for PHP developers (who use "break" instead of
"continue" for switch statements anyway), but it is a gain for programmers from
other languages not familiar with this detail (as they get an error pointing them
to one of the correct alternatives).

All of this is not a big issue, it's just removing a papercut. IIRC this was originally
motivated by me seeing some bug reports about this behavior.

I think it would be helpful to gauge the levels of these bug reports to understand whether it's worth the trouble.

Ultimately - even after this change, C developers won't be able to use the C semantics they're used to, as (e.g. in the example in the RFC) - 'continue;' won't work as they'd expect - and they'd have to acquaint themselves with 'continue 2;'. It's true that if we 'force' them to acquaint themselves with multi-level loop control structures there's reduced 'continue;' erroneously. But unless this is really A Thing, as in, something pretty common - I think the mess involved with forcing people who have working code to try and (a) understand the change, and (b) understand their (potentially quite ancient) code flows may outweigh the benefits.

Zeev

a) In PHP "switch" is considered a looping structure, for this reason
"break" and "continue" both apply to "switch", as aliases. For PHP, these
are reasonable semantics, as PHP supports multi-level breaks. It would be
very questionable if "break N" and "continue N" could refer to different
loop structures just because there is a "switch" involved somewhere.

I think this point should be highlighted: this is not just an unfortunate
legacy decision, it would be really bad if "break 2;" sometimes referred
to a different loop from "continue 2;"

Other languages don't have that problem, because they don't have the
"continue N" syntax.

I'm +0.5 on Nikita's proposal to make this an error - it can't mean
anything other than "continue with this switch statement", but that doesn't
actually mean anything. The error is effectively saying "Can't 'continue' a
switch, because it's not really a loop."

Perhaps rather than deprecating and removing it, we should simply raise a
Warning on its use: "Use of 'continue' to terminate switch; did you mean
'break', or to continue a loop instead?" In other words, "we've done what
you asked, but we think it might not be what you meant".

Regards,

Rowan Collins
[IMSoP]

Hi!

Do you mean that this code would be valid in both PHP 7 and PHP 8, but
mean different things?

while($outer) {
    while($inner) {
        switch($foo) {
            case 1:
                continue 2;
        }
    }
}

I think this would be unacceptable, because WTF potential and subtle
code breakage that would result would be huge. If there are several
wrapped loops, it'd be almost impossible to detect that you're now
breaking a wrong loop. I think the only thing that could be changed is
"continue;" - without an argument - and the thing that should happen is
a notice that it may not do what you (might) think it's doing. If you
use something like "continue 2;" I would assume you know what you're
doing, this is a PHP-specific construct so it is reasonable to assume
you checked it does what you think it does.

--
Stas Malyshev
smalyshev@gmail.com

Hi internals,

Another small deprecation for your consideration...

https://wiki.php.net/rfc/continue_on_switch_deprecation

Regards,
Nikita

IMO the proposed illustrations will lead to as much, if not more, WTF
from other language users as current behaviour. Why? Because the
programmer still has to account for the 'switch' in how continue
works. This will just cause a BC problem for older code without adding
anything for people new to PHP.

My suggestion, is to make the break (no pun intended) be clean.
Deprecate use of continue from usage to escape switch completely in
next PHP, then remove that functionality in PHP 8.

Do you mean that this code would be valid in both PHP 7 and PHP 8, but
mean different things?

How is that different to the illustrations provided? That code is valid
in PHP 7.0-7.3 but will have different meaning in 7.3.

while($outer) {
    while($inner) {
        switch($foo) {
            case 1:
                continue 2;
        }
    }
}

If so, I think that would be even more confusing.

Regards,

Ultimately, I was trying to express what Zeev said better:

Ultimately - even after this change, C developers won't be able to use the C semantics they're used to, as (e.g. in the example in the RFC) - 'continue;' won't work as they'd expect - and they'd have to acquaint themselves with 'continue 2;'. It's true that if we 'force' them to acquaint themselves with multi-level loop control structures there's reduced 'continue;' erroneously. But unless this is really A Thing, as in, something pretty common - I think the mess involved with forcing people who have working code to try and (a) understand the change, and (b) understand their (potentially quite ancient) code flows may outweigh the benefits.

On Wed, Jun 27, 2018 at 5:59 PM, Rowan Collins rowan.collins@gmail.com
wrote:

Hi internals,

Another small deprecation for your consideration...

https://wiki.php.net/rfc/continue_on_switch_deprecation

Regards,
Nikita

Could you clarify the PHP 8 changes:

"PHP 8 generates a compile error."

Does this mean any use of continue in a switch is a compile error, or
only continue without a number

Neither:

Continue can still be used inside switch statements, as long as it does
not target the switch.

The lines marked "Deprecated" in the examples are the ones which will be
compile errors.

Thank you Rowan. I have added a code example to
https://wiki.php.net/rfc/continue_on_switch_deprecation#unaffected_php_functionality
to further clarify what is deprecated and what continues to be allowed.

Nikita

Hi internals,

Another small deprecation for your consideration...

https://wiki.php.net/rfc/continue_on_switch_deprecation

Regards,
Nikita

Could you clarify the PHP 8 changes:

"PHP 8 generates a compile error."

Does this mean any use of continue in a switch is a compile error, or
only continue without a number

Neither:

Continue can still be used inside switch statements, as long as it does
not target the switch

The lines marked "Deprecated" in the examples are the ones which will be
compile errors.

This appears to contradict your 'neither'. continue as part of a
case will always be a compile error in 8? Apologies if my mention of
switch previously was ambiguous.

Will the PHP 8 version still require the programmer to count over the
switches, i.e. in the third illustration, would the continue 3 still be
needed for C-like behaviour?
If that is true, then I really have no use for this proposal as it
doesn't fix the problem, just moves it.

Den tor. 5. jul. 2018 kl. 00.15 skrev Yasuo Ohgaki yohgaki@ohgaki.net:

Since the issue is incompatibility between current "continue" and "break",
how about provide a tool replace "continue" to "break" where it is
applicable.
(Raise error for invalid "continue" usage also. This would be a bug most
likely.)

Then there wouldn't be compatibility issue anymore.

I like the idea of a tool for such, but I think its better to
implement it in some of the static analyzer projects like PHPStan and
the like to catch these.

Please note that the RFC has been superseded by
https://github.com/php/php-src/pull/3364.

--
Christoph M. Becker