On 2020-12-30 Gianni Ceccarelli <dak...@thenautilus.net> wrote:
> Also, my understanding of ``whenever`` is that it's adding a hook into
> the event loop, and only leaving the surrounding ``react`` (or
> ``supply``) will remove that hook (people who understand this better
> than I do: please correct me!). If that's true, adding a hook many
> times on the same condition looks wrong to me…
My understanding seems to be mostly correct::
use v6.d;
my Channel $outer .=new;
my Supplier $inner .=new;
my $inner-s = $inner.Supply;
my Promise $done-with-outer .=new;
my $receiver = start {
react {
whenever $outer -> $outer-value {
say "received outer $outer-value";
whenever $inner-s -> $inner-value {
say "received inner $inner-value for outer $outer-value";
done if $inner-value >= 3 && $outer-value >= 3;
}
$done-with-outer.keep() if $outer-value >= 3;
}
}
}
$outer.send($_) for ^4;
await $done-with-outer;
$inner.emit($_) for ^4;
await $receiver;
prints::
received outer 0
received outer 1
received outer 2
received outer 3
received inner 0 for outer 0
received inner 0 for outer 1
received inner 0 for outer 2
received inner 0 for outer 3
received inner 1 for outer 0
received inner 1 for outer 1
received inner 1 for outer 2
received inner 1 for outer 3
received inner 2 for outer 0
received inner 2 for outer 1
received inner 2 for outer 2
received inner 2 for outer 3
received inner 3 for outer 0
received inner 3 for outer 1
received inner 3 for outer 2
received inner 3 for outer 3
so the inner ``whenever`` really sets up a separate tap every time
it's executed.
If I change ``$inner`` to be a Channel, we only get 4 "received inner"
lines, as expected, because values in a channel are received exactly
once.
(If I change ``$outer`` to a Supplier/Supply pair… the program hangs
without printing anything, because there's a race between the main
process and the ``start``ed one, so all the ``$outer.emit`` happen
before the ``react / whenever`` can tap, and ``$done-with-outer``
never gets kept. Using ``Supplier::Preserving`` fixes that)
--
Dakkar - <Mobilis in mobile>
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