Hi Nikita,
On 26 February 2020 11:47:14 GMT+00:00, Nikita Popov
<[email protected]> wrote:
>There is a relatively simple (at least conceptually) way to make generators
>rewindable: Remember the original arguments of the function, and basically
>"re-invoke" it on rewind().
This is an interesting idea.
There is a gotcha though, neatly demonstrated by your example:
>function map(callable $function, iterable $iterable): \Iterator {
> foreach ($iterable as $key => $value) {
> yield $key => $function($value);
> }
>}
If the $iterable passed in is anything other than an array,
re-invoking the function won't actually rewind it. That can be fixed
by explicitly rewinding at the start:
function map(callable $function, iterable $iterable): \Iterator {
reset($iterable);
foreach ($iterable as $key => $value) {
yield $key => $function($value);
}
}
But now we have a different problem: if we pass an iterator that
doesn't support rewinding, we'll get an error immediately.
In other cases, the side-effects of running the "constructor" itself
might be undesirable. For instance, it might re-run an SQL query,
rather than rewinding a cursor:
public function getResultsIterator($sql) {
$cursor = $this->runQuery($sql);
foreach ( $cursor->getNext() as $row ) {
yield $this->formatRow($row);
}
}
I think the fix would be to return a generator rather than yielding
directly, like this:
public function getResultsIterator($sql) {
$generator = function($cursor) {
$cursor->rewind();
foreach ( $cursor->getNext() as $row ) {
yield $this->formatRow($row);
}
};
$cursor = $this->runQuery($sql);
return $generator($cursor);
}
In general, it feels like it would be useful for generators that knew
it was going to happen, but a foot-gun for generators that weren't
expecting it, so I like Judah's suggestion of an opt-in mechanism of
some sort.
Regards,
--
Rowan Tommins
[IMSoP]
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