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

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

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

Phew, that's a lot of typing! Not easy to comprehend, either. Alternatively, a functional approach could be used:

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

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

Can we make it better?

I like how this is implemented Rust. Translated to PHP, that would look like:

$iterator = $myIterator->regex('/some pattern/')
                       ->callbackFilter(fn(...) {...})
                       ->skip(123)
                       ->infinite();

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:

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 {}
}

Use this trait in all SPL iterators, where appropriate. Full list:

• ArrayIterator
• DirectoryIterator
• EmptyIterator - for uniformness
• IteratorIterator and thus all its subclasses
• MultipleIterator
• RecursiveArrayIterator

None.

8.1.

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

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

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

Accept this RFC (yes/no)? - 2/3 votes required

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.

After the project is implemented, this section should contain

1. the version(s) it was merged into
2. a link to the git commit(s)
3. a link to the PHP manual entry for the feature
4. a link to the language specification section (if any)

Links to external references, discussions or RFCs

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