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Date: Mon, 1 Dec 2025 15:38:27 +0000
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Subject: Re: [PHP-DEV] [RFC] Context Managers
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From: imsop.php@rwec.co.uk ("Rowan Tommins [IMSoP]")

On 01/12/2025 14:26, Tim Düsterhus wrote:
> You are wrong here. Manually interacting with an Iterator is not just 
> safe, it is also necessary for some use cases. The simplest example 
> would be iterating two Iterators in lock-step (i.e. doing a `zip()` 
> operation / array_combine()). 


It is certainly *possible* to use the Iterator methods safely without a 
foreach() construct or built-in aggregate; but it's also easy to miss 
the implications, and cause very confusing behaviour. For instance, you 
could call next() in two different pieces of code, and each would miss 
half the items; or you could call rewind() on an iterator that was 
already in use elsewhere.

The same is true of the ContextManager interface: you could certainly 
call enterContext() and exitContext() manually with no problems at all. 
It might even be necessary, for much the same reasons as iterators: you 
might want to write code that aggregates two context managers in a 
specific arrangement.


As I understand it, your concern is that someone will call 
enterContext() without exitContext(). You could do exactly the same 
thing if you manually handle an Iterator.

Consider this method:

function getDataIterator(): Generator {
     $this->acquireLock();
     while ( $this->hasMoreResults() ) {
         yield $this->getNextResult();
     }
     $this->releaseLock();
}

If the user calls $it = $foo->getDataIterator(), they can call 
$it->current() and get a result, but never call $it->next(). They could 
even use it in a foreach() loop but `break` out before consuming all the 
results. In such cases, the lock would never be released, unless there's 
an extra safety check in the destructor.


I think the currently proposed names actually make that *less* likely 
for a ContextManager: if you enter something, you probably want to exit 
it later.

We could reinforce that further by also prefixing the method names with 
"__", but we don't have any precedent for that, and misuse would still 
be possible.


-- 
Rowan Tommins
[IMSoP]