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Reply-To: Levi Morrison <levi.morrison@datadoghq.com>
Date: Sat, 30 Jan 2021 20:49:23 -0700
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To: tyson andre <tysonandre775@hotmail.com>
Cc: "internals@lists.php.net" <internals@lists.php.net>
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Subject: Re: [PHP-DEV] Proposal: Adding a RewindableKeyValueIterator type
 allowing mixed/repeated keys
From: internals@lists.php.net ("Levi Morrison via internals")

On Sat, Jan 30, 2021 at 5:37 PM tyson andre <tysonandre775@hotmail.com> wrote:
>
> Hi internals,
>
> Currently, there don't seem to be any internal classes that can be used to store a copy of the keys and values of an arbitrary Traversable.
>
> - This would help in eagerly evaluating the result of a generator in a memory efficient way that could be exactly stored and reused
>   e.g. `return $this->cachedResults ??= new \RewindableKeyValueIterator($this->innerResultsGenerator());`
>   https://externals.io/message/108767#108797
> - This would be useful to exactly represent the keys of sequences with repeated keys (e.g. `yield 'first'; yield 'second';` implicitly uses the key `0` twice.)
> - This would be convenient to have to differentiate between 1, '1', and true.
> - This would be useful if php were to add internal global functions that act on iterables and return Traversables with potentially repeated keys based on those iterables,
>    e.g. map(), filter(), take(), flip(), etc
> - If PHP were to add more iterable methods, being able to save an immutable copy of a traversable's result would be useful for end users.
> - If PHP were to add a Map (ordered hash map with null/any scalar/arrays/objects as keys) class type in the future,
>   and that implemented IteratorAggregate, the return type of getIterator() would need something like RewindableKeyValueIterator.
> - The lack of a relevant datatype is among the reasons why classes such as SplObjectStorage are still an Iterator instead of an IteratorAggregate. (and backwards compatibility)
>
> ```
> final class KeyValueSequenceIterator implements Iterator {
>     // loop over all values in $values and store a copy, converting
>     // references in top-level array values to non-references
>     public function __construct(iterable $values) {...}
>     public static function fromKeyValuePairs(iterable $entries): self {...} // fromKeyValuePairs([[$key1, $value1]])
>     public function rewind(): void {...}
>     public function next(): void {...}
>     public function current(): mixed {...}
>     public function key(): mixed {...}
>     public function valid(): bool {...}
>     // and __debugInfo, __clone(), etc.
> }
> ```
>
> Thanks,
> - Tyson
> --
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>

The names `RewindableKeyValueIterator` and `KeyValueSequenceIterator`
are just long-form descriptions of the Iterator API. I don't think we
need such long names. The name should focus on what it brings.

What it brings is a caching iterator around another iterator, that
includes re-windability. The SPL provides a `CachingIterator`, but I
assume it is inadequate somehow (it is basically SPL tradition). Can
you specifically discuss the shortcomings of `CachingIterator` and how
you will address them? To that end, have you implemented a
proof-of-concept for the proposed iterator?