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Date: Wed, 13 Aug 2014 13:52:30 +0100
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Subject: Re: [PHP-DEV] [RFC] Disallow multiple default blocks in a single
 switch statement
From: rowan.collins@gmail.com (Rowan Collins)

Ferenc Kovacs wrote (on 13/08/2014):
> not sure what do you mean here, multiple default cases can be reached the
> same way as any other duplicated case branch(as shown in my snippet in the
> mail you replied to).

If you're talking about http://3v4l.org/eZdPU then those duplicates are 
definitely being silently ignored. Remember that the switch statement is 
just a glorified goto, so execution starts at the first matching case 
and then carries on until you hit a "break" or the end of the switch block.

In that example, it finds the first "case 'foo':", does a goto, and at 
that point all the other labels are completely irrelevant; the 4 echo 
statements are executed in order as though the other labels didn't exist.

If you feed it something other than 'foo', then the behaviour being 
discussed kicks in: for whatever reason, the *last* default label is 
selected as the target of the goto, so only the last echo is executed: 
http://3v4l.org/TfYiQ

Interestingly, HHVM apparently behaves the same on that example, 
although others mentioned it selecting the first rather than last 
default label.

> not sure about the multiple defaults, but I'm fairly sure that there are
> code out there which uses switch blocks as state-machines, and changes the
> switch variable in the case blocks, and have duplicated case blocks for
> executing code after the switch variable reached it's final form.

Multiple cases do not make any difference here; the label to jump to is 
selected only once, so if you want to change the state and jump to a 
different label, you have to wrap the switch in a loop, and evaluate it 
again. If you specify multiple labels matching the same state, the 
second one can never be reached. http://3v4l.org/i3746

-- 
Rowan Collins
[IMSoP]