Newsgroups: php.internals
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To: Levi Morrison <levi.morrison@datadoghq.com>
CC: "internals@lists.php.net" <internals@lists.php.net>
Thread-Topic: [PHP-DEV] Proposal: Adding a RewindableKeyValueIterator type
 allowing mixed/repeated keys
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Date: Sun, 31 Jan 2021 15:43:48 +0000
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Subject: Re: [PHP-DEV] Proposal: Adding a RewindableKeyValueIterator type
 allowing mixed/repeated keys
From: tysonandre775@hotmail.com (tyson andre)

Hi Levi Morrison,=0A=
=0A=
> > 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.=0A=
> >=0A=
> > - This would help in eagerly evaluating the result of a generator in a =
memory efficient way that could be exactly stored and reused=0A=
> >=A0=A0 e.g. `return $this->cachedResults ??=3D new \RewindableKeyValueIt=
erator($this->innerResultsGenerator());`=0A=
> >=A0=A0 https://externals.io/message/108767#108797=0A=
> > - This would be useful to exactly represent the keys of sequences with =
repeated keys (e.g. `yield 'first'; yield 'second';` implicitly uses the ke=
y `0` twice.)=0A=
> > - This would be convenient to have to differentiate between 1, '1', and=
 true.=0A=
> > - This would be useful if php were to add internal global functions tha=
t act on iterables and return Traversables with potentially repeated keys b=
ased on those iterables,=0A=
> >=A0=A0=A0 e.g. map(), filter(), take(), flip(), etc=0A=
> > - If PHP were to add more iterable methods, being able to save an immut=
able copy of a traversable's result would be useful for end users.=0A=
> > - If PHP were to add a Map (ordered hash map with null/any scalar/array=
s/objects as keys) class type in the future,=0A=
> >=A0=A0 and that implemented IteratorAggregate, the return type of getIte=
rator() would need something like RewindableKeyValueIterator.=0A=
> > - 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)=0A=
> >=0A=
> > ```=0A=
> > final class KeyValueSequenceIterator implements Iterator {=0A=
> >=A0=A0=A0=A0 // loop over all values in $values and store a copy, conver=
ting=0A=
> >=A0=A0=A0=A0 // references in top-level array values to non-references=
=0A=
> >=A0=A0=A0=A0 public function __construct(iterable $values) {...}=0A=
> >=A0=A0=A0=A0 public static function fromKeyValuePairs(iterable $entries)=
: self {...} // fromKeyValuePairs([[$key1, $value1]])=0A=
> >=A0=A0=A0=A0 public function rewind(): void {...}=0A=
> >=A0=A0=A0=A0 public function next(): void {...}=0A=
> >=A0=A0=A0=A0 public function current(): mixed {...}=0A=
> >=A0=A0=A0=A0 public function key(): mixed {...}=0A=
> >=A0=A0=A0=A0 public function valid(): bool {...}=0A=
> >=A0=A0=A0=A0 // and __debugInfo, __clone(), etc.=0A=
> > }=0A=
> > ```=0A=
>=0A=
> The names `RewindableKeyValueIterator` and `KeyValueSequenceIterator`=0A=
> are just long-form descriptions of the Iterator API. I don't think we=0A=
> need such long names. The name should focus on what it brings.=0A=
>=0A=
> What it brings is a caching iterator around another iterator, that=0A=
> includes re-windability. The SPL provides a `CachingIterator`, but I=0A=
> assume it is inadequate somehow (it is basically SPL tradition). Can=0A=
> you specifically discuss the shortcomings of `CachingIterator` and how=0A=
> you will address them? To that end, have you implemented a=0A=
> proof-of-concept for the proposed iterator?=0A=
=0A=
It caches the results by coercing keys and inserting them into an array, wh=
ich is a different use case.=0A=
If you rewind a CachingIterator, it rewinds the iterator that it wraps,=0A=
which throws for Generators and other types of iterators.=0A=
=0A=
For RewindableKeyValueIterator, it needs to store keys that can't be used a=
s array keys.=0A=
=0A=
I was planning to implement a proof of concept if there wasn't widespread o=
pposition=0A=
and if nobody pointed out the functionality already existed.=0A=
=0A=
```=0A=
<?php=0A=
function dump_iterable(iterable $x) {=0A=
    foreach ($x as $key =3D> $value) {=0A=
        printf("Key: %s\nValue: %s\n", json_encode($key), json_encode($valu=
e));=0A=
    }=0A=
}=0A=
function yields_values(): Generator { yield 0 =3D> 'first'; yield '0' =3D> =
'second'; }=0A=
$c =3D new CachingIterator(yields_values(), CachingIterator::FULL_CACHE);=
=0A=
echo "First CachingIterator iteration\n";=0A=
dump_iterable($c);=0A=
var_export($c->getCache()); echo "\n"; // array(0 =3D> 'second') does not r=
epresent that data=0A=
echo "Second CachingIterator iteration\n";=0A=
// Fatal error: Uncaught Exception: Cannot rewind a generator that was alre=
ady run=0A=
dump_iterable($c);=0A=
```=0A=
=0A=
Thanks,=0A=
- Tyson=