PROMISE.pl

use strict;
use warnings;
use feature 'say';
 
package Promise {
 
    sub state {
        return shift->{'state'};
    }
 
    sub fulfill {
        my ($self, $fulfillvalue) = @_;
        return
            unless $self->state eq 'PENDING';
        $self->{'fulfillvalue'} = $fulfillvalue;
        $self->{'state'} = 'FULFILLED';
        for my $onFulfilled (@{$self->{'fulfillqueue'}}) {
            $onFulfilled->($fulfillvalue);
        }
        return;
    }
 
    sub reject {
        my ($self, $rejectreason) = @_;
        return
            unless $self->state eq 'PENDING';
        $self->{'rejectreason'} = $rejectreason;
        $self->{'state'} = 'REJECTED';
        return;
    }
 
    sub then {
        my ($self, $onFulfilled, $onRejected) = @_;
        if ($self->state eq 'PENDING') {
            push @{$self->{'fulfillqueue'}}, $onFulfilled if defined $onFulfilled;
        #   push @rejectqueue,  $onRejected  if defined $onRejected;
            $self->{'rejrun'} = $onRejected;
            $self->{'fulrun'} = $onFulfilled;
            #$self->{'fulrun'}("foo");
        }
        elsif ($self->state eq 'FULFILLED') {
            $self->{'fulrun'}($self->{'fulfillvalue'});
        #   for my $onFulfilled (@fulfillqueue) {
        #       $onFulfilled->($fulfillvalue);
        #   }
        }
        else {
            $self->{'rejrun'}->($self->{'rejectreason'});
        #   for my $onRejected (@rejectqueue) {
        #   $onRejected->($rejectreason);
        #   }
        }
    }
 
    sub new {
        my $class = shift;
     
        my %opts = @_;
        $opts{'state'}  //= 'PENDING';
        $opts{'value'}  //= '';
        $opts{'fulfillqueue'} //= [];
        $opts{'rejectqueue'}  //= [];
     
        return bless \%opts, $class;
    }
}
 
use Test::More tests => 12;
 
{
    my $promise = Promise->new;
 
    my $value = 0;
    $promise->then(sub { $value = shift });
    ok !$value, "Hasn't been fulfilled yet";
 
    $promise->fulfill("OH HAI");
 
    is $promise->state, "FULFILLED", "Correct state";
    is $value, "OH HAI", "Code has been run";
}
 
{
    my $promise = Promise->new(state => "FULFILLED", value => "yay!");
    is $promise->state, "FULFILLED", "Already correct state";
 
    my $value = 0;
    $promise->then(sub { $value = shift });
 
    is $value, "yay!", "Code ran immediately";
}
 
{
    my $promise = Promise->new;
 
    my $value = 0;
    my $reason = 0;
    $promise->then(sub { $value = shift }, sub { $reason = shift });
    ok !$reason, "Hasn't been rejected yet";
 
    $promise->reject("OH NOES");
 
    is $promise->state, "REJECTED", "Correct state";
    ok !$value, "Fulfill code didn't run";
    is $reason, "OH NOES", "Reject code has been run";
}
 
{
    my $promise = Promise->new;
 
    my $value = 0;
    $promise->then(sub { $value++ });
    $promise->then(sub { $value++ });
    $promise->then(sub { $value++ });
    ok !$value, "Hasn't been fulfilled yet";
 
    $promise->fulfill("OH HAI");
 
    is $promise->state, "FULFILLED", "Correct state";
    is $value, 3, "Code has been run";
}