PHP: rfc:iterator_chaining

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====== PHP RFC: Iterator chaining ======
  * Version: 0.9
  * Date: 2021-03-20
  * Author: Max Semenik, maxsem.wiki@gmail.com
  * Status: Draft
  * First Published at: http://wiki.php.net/rfc/iterator_chaining

===== Introduction =====
SPL provides a set of iterators that allow feeding one into another to create powerful data pipelines, however doing that is not particularly convenient:

<code php>
$iterator = new RegexIterator($myIterator, '/some pattern/');
$iterator = new CallbackFilterIterator($iterator, fn(...) {...});
$iterator = new LimitIterator($iterator, 0, 123);
$iterator = new InfiniteIterator($iterator);
</code>

Phew, that's a lot of typing! Not easy to comprehend, either. Alternatively, a functional approach could be used:
<code php>
$iterator = new InfiniteIterator(new LimitIterator(new CallbackFilterIterator(new RegexIterator($myIterator, '/some pattern/'), fn(...) {...}), 0, 123));
</code>

Note how the order of writing is gorgeously opposite to the order of data flow.

Can we make it better?

===== Proposal =====
I like how this is implemented Rust. Translated to PHP, that would look like:
<code php>
$iterator = $myIterator->regex('/some pattern/')
                       ->callbackFilter(fn(...) {...})
                       ->skip(123)
                       ->infinite();
</code>

Wouldn't that be way more readable and easy to write? I'm proposing to implement it the following way:

**Create a trait** that extends iterators and allows to feed them into other iterators:
<code php>
trait IteratorChain {
    public function skip(int $count): LimitIterator {}
    public function limit(int $count): LimitIterator {}
    public function skipAndLimit(int $numToSkip, int $limit): LimitIterator {}
    public function regex(string $regex, int $mode = RegexIterator::MATCH, int $flags = 0 , int $preg_flags = 0): RegexIterator {}
    public function callbackFilter(callable $callback): CallbackIterator {}
    public function noRewind(): NoRewindIterator {}
    public function cached(int $flags = CachingIterator::CALL_TOSTRING): CachingIterator {}
    public function multiple(int $flags = MultipleIterator::MIT_NEED_ALL|MultipleIterator::MIT_KEYS_NUMERIC, string|int|null $info = null): MultipleIterator {}
    public function infinite(): InfiniteIterator {}
}
</code>

**Use this trait** in all SPL iterators, where appropriate. Full list:
  * ''ArrayIterator''
  * ''DirectoryIterator''
  * ''EmptyIterator'' - for uniformness
  * ''IteratorIterator'' and thus all its subclasses
  * ''MultipleIterator''
  * ''RecursiveArrayIterator''

===== Backward Incompatible Changes =====
None.

===== Proposed PHP Version(s) =====
8.1.

===== RFC Impact =====
Should not impact existing functionality (other than making some of it easier to use).

===== Open Issues =====
Need to decide on precise details of interface before going into voting.

===== Future Scope =====
This section details areas where the feature might be improved in future, but that are not currently proposed in this RFC.

===== Proposed Voting Choices =====
Accept this RFC (yes/no)? - 2/3 votes required

===== Patches and Tests =====
Links to any external patches and tests go here.

If there is no patch, make it clear who will create a patch, or whether a volunteer to help with implementation is needed.

Make it clear if the patch is intended to be the final patch, or is just a prototype.

For changes affecting the core language, you should also provide a patch for the language specification.

===== Implementation =====
After the project is implemented, this section should contain 
  - the version(s) it was merged into
  - a link to the git commit(s)
  - a link to the PHP manual entry for the feature
  - a link to the language specification section (if any)

===== References =====
Links to external references, discussions or RFCs

===== Rejected Features =====
Keep this updated with features that were discussed on the mail lists.
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