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Date: Fri, 18 Oct 2019 09:54:15 +0100
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To: PHP Internals <internals@lists.php.net>
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Subject: Re: [PHP-DEV] Adding explicit intent for SWITCH/CASE fall through?
From: rowan.collins@gmail.com (Rowan Tommins)

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Hi Bishop,

I'm not the proposer, but would like to answer your questions anyway.


On Fri, 18 Oct 2019 at 04:14, Bishop Bettini <bishop@php.net> wrote:

> Do you envision this token accepting an optional argument, as break does,
> to fall-through multiple nesting levels? Eg:
>
> <?php
> $x = $y = $z = 1;
> switch ($x) {
> case 1:
>     switch ($y) {
>     case 1:
>         switch ($z) {
>         case 1: fallthrough 3;
>         case 2: echo 'z2';
>         }
>     }
>     break;
> case 2: echo 'x2'; break;
> }
> ?>
>


I think we should keep it simple - we can always add more features later if
a real need reveals itself.

I note that the Go version linked to explicitly forbids the use of
"fallthrough" in a conditional, and a multi-level fallthrough is
effectively the same thing: the control flow in your example is something
like if ( $y==1 && $z == 1 ) { fallthrough; } else { break; } I would have
thought that by the time you've got nested switch statements with
conditional fallthrough on multiple levels, the code is going to be very
hard to reason about, so unless there's a key use case, I'd leave the
complexity out of the language.

There might be use cases for single-level conditional fallthrough, but I'd
lean towards defining it as Go has: as a marker at the end of the case
block only. By far the most common use of it I've seen is like Mike's:

case x:
   additional pre-processing
   fallthrough
case y:
   shared processing
   break




> What do you envision the result of indicating fall-through to no subsequent
> case? Eg:
>
> <?php
> switch ($x) {
> case 1: fallthrough;
> }
>


I think it would be perfectly fine for this to do nothing. Currently you
can write both

switch ($x) {
case 1:
}

and

switch ($x) {
case 1: break;
}



In general, I really like this idea; I've always wished switch worked this
way.

Regards,
-- 
Rowan Tommins
[IMSoP]

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