PROMISE.pl

use strict;
use warnings;
use feature 'say';

package Promise {

    my $STATE; # = 'PENDING';

    my $fulfillvalue;
    my $rejectreason;
    my @fulfillqueue;
    my @rejectqueue;
        my $rejrun;
    my $fulrun;

    sub state {  return $STATE; }

    sub fulfill {
                return unless $STATE eq 'PENDING';
                my $class = shift;
        $fulfillvalue = shift;
        $STATE = 'FULFILLED';
    }

    sub reject {
                return unless $STATE eq 'PENDING';
                my $class = shift;
                $rejectreason = shift;
        $STATE = 'REJECTED';
    }

    sub then {
        if ($STATE eq 'PENDING') {
                        my $class = shift;
            my $onFulfilled = shift // undef;
            my $onRejected = shift // undef;
#            push @fulfillqueue, $onFulfilled if defined $onFulfilled;
#            push @rejectqueue,  $onRejected  if defined $onRejected;
            $rejrun = $onRejected;
            $fulrun = $onFulfilled;
            #$fulrun->("foo");
        }
        elsif ($STATE eq 'FULFILLED') {
            $fulrun->($fulfillvalue);
#            for my $onFulfilled (@fulfillqueue) {
#                $onFulfilled->($fulfillvalue);
#            }
        }
        else {
            $rejrun->($rejectreason);
#            for my $onRejected (@rejectqueue) {
#                $onRejected->($rejectreason);
#            }
        }
    }

    sub new {
        my $class = shift;
        my $self = {};
        bless $self, $class;

        my %opts = @_;
                $STATE = $opts{'state'} // 'PENDING';
        $fulfillvalue = $opts{'value'} // '';
        $rejectreason = $opts{'reason'} // '';

        return $self;
    }
}

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";
}