[Mojolicious] iterated promises (Mojo::Promise::Role::Repeat)
So I've uploaded Mojo::Promise::Role::Repeat to CPAN.
This adds a single method to Mojo::Promise
$done_p = $promise->repeat(sub{...})
which is essentially equivalent to
$done_p = $promise->then(sub{...})->then(sub{...})->then(sub{...})->then(sub
{...}) #... forever
The promise returned represents the end of the chain off at infinity,
meaning
- if any instance of the handler sub{...} dies or returns a rejected
promise
$done_p is rejected with the same @reason(s).
- if any instance of sub {...} calls $_->(@values) -- $_ is bound to an
escape function that does not return --
$done_p is resolved with @values (and execution of everything else in
the loop is abandoned).
and these are the only ways out of the loop. If a handler run returns
normally or returns a resolved promise, the return values become the
arguments for the next iteration (as one would expect for consecutive
then()s).
The function bound to $_ also works from inside of nested handlers/loops
(though you'll want to stash it in a lexical if you're going to do that
since $_ will probably be set to something different by the time the nested
handler runs). Meaning you can do stuff like this
$ua->get_p(...)->repeat(sub{
my $break = $_;
...
$ua->get_p(...)->repeat(sub {
...
$ua->get_p(...)->then(sub {
...
$break->('bye') if (condition);
...
}
...
}
...
})
Anyway it's up. Enjoy.
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Re: [Mojolicious] iterated promises (Mojo::Promise::Role::Repeat)
You might look at the existing art of
https://metacpan.org/pod/Future::Utils#REPEATING-A-BLOCK-OF-CODE which can
be used with Mojo::Promise via futurify/promisify, and seems a lot clearer
to me.
-Dan
On Tue, Oct 1, 2019 at 9:04 AM Roger Crew wrote:
> something I have found vaguely useful that I just uploaded to CPAN for
> people to play with
> Mojo::Promise::Role::Repeat,
>
> adds a *repeat()* method to the promise class/objects.
>
> The essential idea is that
>
> $promise->repeat(sub{...})
>
>
> is equivalent to
>
> $promise->then(sub{...})->then(sub{...})->then(sub{...})->then(sub{...}) #
> forever
>
>
> except that repeat() also returns a "final" promise, which is
>
>- rejected, if any iteration dies with a value or returns a promise
>that gets rejected
>- resolved with @values, if any iteration calls $_->(@values)
>
> and if any run of the handler sub{...} returns normally or with a promise
> that gets resolved, those values get fed to the next run,
>
> $_ being bound, at the start of each run, to an escape function that
> doesn't return -- the moral equivalent of a break statement -- *but *you
> can also use this function to break out of nested loops/handlers as well
> (though you'll need to stash it in a lexical if you're going to do that).
>
> The pattern that seems to be coming up a lot for me is
>
> $ua->get_p($first_url)->repeat(sub {
> # ... parse page ...
> $_->() if (victory);
> # ...
> $ua->get_p($next_url);
> })
>
>
> (I may eventually want to petition for some form of this to be added to
> Mojo::Promise, but one thing at a time...)
>
> --
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> "Mojolicious" group.
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>
> .
>
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[Mojolicious] iterated promises (Mojo::Promise::Role::Repeat)
something I have found vaguely useful that I just uploaded to CPAN for
people to play with
Mojo::Promise::Role::Repeat,
adds a *repeat()* method to the promise class/objects.
The essential idea is that
$promise->repeat(sub{...})
is equivalent to
$promise->then(sub{...})->then(sub{...})->then(sub{...})->then(sub{...}) #
forever
except that repeat() also returns a "final" promise, which is
- rejected, if any iteration dies with a value or returns a promise that
gets rejected
- resolved with @values, if any iteration calls $_->(@values)
and if any run of the handler sub{...} returns normally or with a promise
that gets resolved, those values get fed to the next run,
$_ being bound, at the start of each run, to an escape function that
doesn't return -- the moral equivalent of a break statement -- *but *you
can also use this function to break out of nested loops/handlers as well
(though you'll need to stash it in a lexical if you're going to do that).
The pattern that seems to be coming up a lot for me is
$ua->get_p($first_url)->repeat(sub {
# ... parse page ...
$_->() if (victory);
# ...
$ua->get_p($next_url);
})
(I may eventually want to petition for some form of this to be added to
Mojo::Promise, but one thing at a time...)
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