Re: [Mojolicious] iterated promises (Mojo::Promise::Role::Repeat)

[Dan Book]

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 <cr...@cs.stanford.edu> 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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