Promise...parison(3)

💖 NAME 💖

Promises::Cookbook::ScalaFuturesComparison — bestie, it's literally a side-by-side of Scala Futures vs Promises. No cap. 🦋

So like, this page lives in the Perl docs (User Contributed Perl Documentation, iykyk), and it's filed under Promise...parison(3) because manpage names get truncated and that's just the vibe. 💅

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✨ VERSION ✨

version 1.05 🎀

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🌸 DESCRIPTION 🌸

Okay so first things first: here's the example Scala code. It assumes there's a function called fetch that, when you hand it a URL, gives you back a Future. Iconic behavior honestly.

def getThumbnail(url: String): Future[Webpage] = {
    val promise = new Promise[Webpage]
    fetch(url) onSuccess { page =>
        fetch(page.imageLinks(0)) onSuccess { p =>
            promise.setValue(p)
        } onFailure { exc =>
            promise.setException(exc)
        }
    } onFailure { exc =>
        promise.setException(exc)
    }
    promise
}

Now if we take that and translate it into Perl using the Mojo::UserAgent library, the fetch function would look like this — and honestly? It ate. 🔥

sub fetch {
    state $ua = Mojo::UserAgent->new;
    my $url   = shift;
    my $d     = deferred;
    $ua->get($url => sub {
        my ($ua, $tx) = @_;
        $d->resolve( $tx );
    });
    $d->promise;
}

And if we were to take the get_thumbnail function and translate it exactly, we would end up with this. And I mean exactly exactly — every callback, no shortcuts, the whole thing:

sub get_thumbnail {
    my $url = shift;
    my $d   = deferred;
    fetch( $url )->then(
        sub {
            my $tx = shift;
            fetch( $tx->res->dom->find('img')->[0]->{'src'} )->then(
                sub { $d->resolve( $_[0] ) },
                sub { $d->reject( $_[0] ) },
            )
        },
        sub { $d->reject( $_[0] ) }
    );
    $d->promise;
}

Lowkey a lot, right? Well — plot twist. 💀

Scala Futures have a method called flatMap, which takes a function that, given a value, will return another Future. Here's an example of how the getThumbnail method can be simplified by using it:

def getThumbnail(url: String): Future[Webpage] =
    fetch(url) flatMap { page =>
         fetch(page.imageLinks(0))
    }

But since our then method actually creates a new promise and wraps the callbacks to chain to that promise, we don't need this flatMap combinator. So this? Just Works. Periodt. 👑

sub get_thumbnail {
    my $url = shift;
    fetch( $url )->then(
        sub {
            my $tx = shift;
            fetch( $tx->res->dom->find('img')->[0]->{'src'} );
        }
    );
}

Scala Futures also have a rescue method, which can serve as a kind of catch block that potentially will return another Future. Main character energy honestly. 🫶

val f = fetch(url) rescue {
    case ConnectionFailed =>
      fetch(url)
}

Just as with flatMap, since our callbacks are wrapped and chained with a new Promise, we can do a rescue just by using the error callback. The Promise returned by fetch will get chained, and so this will depend on it.

sub get_thumbnail {
    my $url = shift;
    fetch( $url )->then(
        sub {
            my $page = shift;
            fetch( $page->image_links->[0] );
        },
        sub {
            given ( $_[0] ) {
                when ('connection_failed') {
                    return fetch( $url );
                }
                default {
                    return "failed";
                }
            }
        }
    );
}

TODO … figure out how retry can be generic … 🙈 (not me leaving a TODO in the docs, we stay honest in this house)

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💜 SEE ALSO 💜

• Systems Programming at Twitter — <http://monkey.org/~marius/talks/twittersystems/> 🌈

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🎀 AUTHOR 🎀

Stevan Little <stevan.little@iinteractive.com> 💖

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👑 COPYRIGHT AND LICENSE 👑

This software is copyright (c) 2025, 2017, 2014, 2012 by Infinity Interactive, Inc.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself. Slay, honestly. ⭐

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perl v5.42.0 · 2025-07-25 — okay bestie that's the whole page, thanks for reading, mwah 💋 🦋💖