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Date: Wed, 11 Mar 2020 10:50:22 +0100
Message-ID: <CAF+90c8BeG9GwQOVypcNe3p5Yp9LefsWHfH5Wrjq1o6HZvrZVQ@mail.gmail.com>
To: PHP internals <internals@lists.php.net>
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Subject: Making all Traversables an Iterator or IteratorAggregate
From: nikita.ppv@gmail.com (Nikita Popov)

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Hi internals,

Userland classes that implement Traversable must do so either through
Iterator or IteratorAggregate. The same requirement does not exist for
internal classes: They can implement the internal get_iterator mechanism,
without exposing either the Iterator or IteratorAggregate APIs. This makes
them usable in get_iterator(), but incompatible with any Iterator based
APIs.

A lot of internal classes do this, because exposing the userland APIs is
simply a lot of work. I would like to add a general mechanism to make this
simpler: https://github.com/php/php-src/pull/5216 adds a generic
"InternalIterator" class, that essentially converts the internal
get_iterator interface into a proper Iterator. Internal classes then only
need to a) implement the IteratorAggregate interface and b) add a
getIterator() method with an implementation looking like this:

// WeakMap::getIterator(): Iterator
ZEND_METHOD(WeakMap, getIterator)
{
    if (zend_parse_parameters_none() == FAILURE) {
        return;
    }
    zend_create_internal_iterator_zval(return_value, ZEND_THIS);
}

This allows internal classes to trivially implement IteratorAggregate, and
as such allows us to enforce that all Traversables implement Iterator or
IteratorAggregate.

Regards,
Nikita

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