Hi internals,
Generators currently do not support rewinding -- or rather, only support it
if the generator is at/before the first yield, in which case rewinding is a
no-op.
Generators make it real breeze to implement primitives like
function map(callable $function, iterable $iterable): \Iterator {
foreach ($iterable as $key => $value) {
yield $key => $function($value);
}
}
without having to do through the whole Iterator boilerplate. However, if
you do this, you end up with an iterator that is not rewindable. If you
want to make map() rewindable, you need to go back to a manual Iterator
implementation. As iterators in PHP are assumed to be rewindable by
default, this is somewhat annoying.
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().
I'm wondering what people think about adding this functionality. I think
the main argument against it is that not all generators may behave sensibly
if you re-run their code -- there's probably a reasonable expectation that
an iterator will return the same sequence of values are rewinding,
something which we cannot guarantee with generators, but also don't enforce
with normal iterators either.
Regards,
Nikita
