Promise

use v6;
use Test;

class Promise {
    enum STATE <PENDING FULFILLED REJECTED>;

    has STATE $.state = STATE::PENDING;
    has %!do-actions;
    has $!reject-reason;
    has $!fulfill-value;

    method fulfill($val!) {
        if $!state != STATE::FULFILLED {
            $!state = STATE::FULFILLED;
            $!fulfill-value = $val;
            for %!do-actions<fulfill>[] -> $f {
                $f($!fulfill-value);
            }
        }
    }
    method reject($reason!) {
        if $!state != STATE::REJECTED {
            $!state = STATE::REJECTED;
            $!reject-reason = $reason;
            for %!do-actions<reject>[] -> $r {
                $r[0]($!reject-reason);
            }
        }
    }

    method then(&on-fulfill, &on-reject?) {
        if $!state == STATE::PENDING {
            %!do-actions<fulfill>.push: { &on-fulfill } if &on-fulfill;
            %!do-actions<reject>.push: { &on-reject } if &on-reject;
        } elsif $!state == STATE::REJECTED {
            %!do-actions<reject>.map: { &on-reject($!reject-reason) };
        } else {
            %!do-actions<fulfill>.map: { &on-fulfill($!fulfill-value) };
        }
        return self;
    }
}

{
    my $promise = Promise.new;

    my $value = 0;
    $promise.then(sub ($arg) { $value = $arg });
    ok !$value, "Hasn't been fulfilled yet";

    $promise.fulfill("OH HAI");

    is $promise.state, Promise::STATE::FULFILLED, "Correct state";
    is $value, "OH HAI", "Code has been run";
}

{
    my $promise = Promise.new(state => Promise::STATE::FULFILLED,
    value => "yay!");
    is $promise.state, Promise::STATE::FULFILLED, "Already correct state";

    my $value = 0;
    $promise.then(sub ($arg) { $value = $arg });

    is $value, "yay!", "Code ran immediately";
}

{
    my $promise = Promise.new;

    my $value = 0;
    my $reason = 0;
    $promise.then(sub ($arg) { $value  = $arg },
    sub ($arg) { $reason = $arg });
    ok !$reason, "Hasn't been rejected yet";

    $promise.reject("OH NOES");

    is $promise.state, Promise::STATE::REJECTED, "Correct state";
    ok !$value, "Fulfill code didn't run";
    is $reason, "OH NOES", "Reject code has been run";
}

done;