Making a Promise

[Anonymous Monk]

Hello Monks, I'm working on some asynchronous code and have started using the Promises module. I'm using collect to gather the results of several operations, but am not sure how to wrap a non-async function so that it returns a Promise object. The following example does not work:

    my $d = deferred->new;
    my $result = External::Parser->run( '--in', $file );
    $result
    ? $d->resolve( $result )
    : $d->reject();
    $d->promise;

[Corion]

In what sense does your snippet as shown not work?

Can you wrap the snippet in a small (20 lines) self-contained program that shows the problem?

Note that the call to External::Parser->run will always have happened before your function returns. This is different from the example in Promises because the http_get function is an asynchronous function which invokes the callback upon completion. If you want to move some arbitrary synchonous code to Promises, I would try to wrap a callback to that code with an object that implements a function ->result that actually calls the synchronous code and returns the result:

package Promise::Lazy;
use parent 'Promises::Promise';

sub new {
    my( $class, $usercallback ) = @_;
    my $self = $class::SUPER->new();
    $self->{ usercallback } = $usercallback;
    $self
};

sub result {
    my( $self ) = @_;
    my $result = $self->{ usercallback }->();
    if( $result ) {
        $self->resolve( $result )
    } else {
        $self->reject();
    };
};

... somewhere in your code ...

return Promise::Lazy->new(sub {
    External::Parser->run('--in', $file);
});

[Anonymous Monk]

The snippet wasn't working due to an unrelated documentation error -- the constructor for a Promises::Deferred object should have been deferred, not deferred->new. Doh.

I've implemented a wrapper for the synchronous code similar to your code, and now it's all working as anticipated. Thank you for the help!