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Date: Wed, 21 Jan 2015 14:58:47 +0000
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Subject: Re: [PHP-DEV] Fixing strange foreach behavior.
From: rowan.collins@gmail.com (Rowan Collins)

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Yasuo Ohgaki wrote on 21/01/2015 02:48:
>
>     I don't think that's an either/or situation: in most cases, it
>     would be up to the user to create that independent iterator,
>     because there's no general algorithm for cloning one that the
>     engine could use. (Think of an iterator proxying a database
>     cursor, for instance - it could be rewindable directly, but
>     cloning it would imply issuing a new query to create a new cursor
>     with separate state.) So the engine needs to issue an error
>     telling the user that they're reusing an object in an dangerous
>     way. An extended interface could be created later for those cases
>     that can handle the situation, although I doubt it would be that
>     common a use case.
>
>
> I think if users need nested database cursor based iterator or like, 
> they should create new iterator object by themselves.
> i.e. User should have __clone() that opens new cursor.

I agree, and that's why I think nesting should be disabled by default. 
It needs to be a run-time flag, as nesting could be arbitrarily indirect:

function iterator_dump($iter) {
     foreach ( $iter as $x ) {
         var_dump($x);
     }
}
foreach ( $some_iterator  as $foo ) {
     dump ( $foo );
     iterator_dump( $some_iterator ); // ERROR: Iterator cannot be used 
within nested foreach loops.
     iterator_dump( clone $some_iterator ); // OK, up to creator of 
iterator to implement __clone()
}

Perhaps foreach ( $some_iterator  as $foo ) { ... } should be translated 
something like this:

- if check_foreach_flag( $some_iterator ) == 1 then raise error
- set_foreach_flag( $some_iterator )
- if check_initialised( $some_iterator ) then $some_iterator->rewind()
- (if rewind fails, raise error)
--- perform loop until iterator exhausted or break statement encountered ---
- clear_foreach_flag( $some_iterator )

Where check_initialised() is some way of checking if the iterator has 
emitted values somewhere else, so needs rewinding. Without that, you 
couldn't use foreach on any iterator that couldn't be rewound, which 
would be pretty useless.

I've no idea how feasible such flags are in the current implementation, 
though.

Regards,
-- 
Rowan Collins
[IMSoP]

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