=begin pod

=TITLE class Promise

=SUBTITLE Status/result of an asynchronous computation

    my enum PromiseStatus (:Planned(0), :Kept(1), :Broken(2));
    class Promise { ... }

A I<Promise> is used to handle the result of a computation that might not have
finished. It allows the user to execute code once the computation is done
(with the C<then> method), execution after a time delay (with C<in>),
combining promises, and waiting for results.

    my $p = Promise.start({ sleep 2; 42});
    $p.then({ say .result });   # will print 42 once the block finished
    say $p.status;              # Planned
    $p.result;                  # waits for the computation to finish
    say $p.status;              # Kept

There are two typical scenarios for using promises. The first is to use a
factory method (C<start>, C<in>, C<anyof>, C<allof>) on the type object; those
will make sure that the promise is automatically kept or broken for you, and
you can't call C<break> or C<keep> on these promises yourself.

The second is to create your promises yourself with C<Promise.new>. If you
want to ensure that only your code can keep or break the promise, you can use
the C<vow> method to get a unique handle, and call C<keep> or C<break> on it:

=begin code
    sub async-get-with-promise($user-agent, $url) {
        my $p = Promise.new;
        my $v = $p.vow;

        # do an asynchronous call on a fictive user agent,
        # and return the promise:
        $user-agent.async-get($url,
                on-error => -> $error {
                    $v.break($error);
                },
                on-success => -> $response {
                    $v.keep($response);
                }
        );
        return $p;
    }
=end code

=head1 Methods

=head2 method start

    sub start(&code);
    method start(Promise:U: &code, :$scheduler = $*SCHEDULER) returns Promise:D

Creates a new Promise that runs the given code object. The promise will be
kept when the code terminates normally, or broken if it throws an exception.
The return value or exception can be inspected with the C<result> method.

The scheduler that handles this promise can be passed as a named argument.

    # these two are equivalent:
    my $p1 = Promise.start({ do something here });
    my $p2 = start { do something here };

=head2 method in

    method in(Promise:U: $seconds, :$scheduler = $*SCHEDULER) returns Promise:D

Creates a new Promise that will be kept in C<$seconds> seconds, or later.

    my $p = Promise.in(5).then({ say "5 seconds later" });
    # do other stuff here

    await $p;   # wait here until the 5 seconds are over

=head2 method at

    method at(Promise:U: $time, :$scheduler = $*SCHEDULER) returns Promise:D

Creates a new Promise that will be kept at the given time, or later.

    my $p = Promise.at(now + 5).then({ say "5 seconds later" });
    # do other stuff here

    await $p;   # wait here until the 5 seconds are over

=head2 method allof

    method allof(Promise:U: *@promises) returns Promise:D

Returns a new promise that will be kept when all the promises passed as
arguments are kept, and that will be broken as soon as any of the argument
promises is broken. You may need to run the following code several times
before none of the promises get broken with C<die> and we get to
the C<say> on the last line, instead of bailing out on C<await>.

    my @promises;
    for 1..5 -> $t {
        push @promises, start {
            sleep $t;
            die "OH NOEZ" if rand < 0.2;
        };
    }
    my $all-successful = Promise.allof(@promises);
    await $all-successful;
    say "Promises kept so we get to live another day!";

=head2 method anyof

    method anyof(Promise:U: *@promises) returns Promise:D

Returns a new promise that will be kept as soon as any of the promises passed as
arguments is kept, and will be broken when all of the argument promises are
broken.

You can use this to wait at most a number of seconds for a promise:

    my $timeout = 5;
    await Promise.anyof(
        Promise.in($timeout),
        start {
            # do a potentially long-running calculation here
        },
    );

=head2 method then

    method then(Promise:D: &code)

Schedules a piece of code to be run after the invocant has been kept or
broken, and returns a new promise for this computation. In other words,
creates a chained promise.

    my $timer = Promise.in(2);
    my $after = $timer.then({ say "2 seconds are over!"; 'result' });
    say $after.result;  # 2 seconds are over
                        # result

=head2 method keep

    multi method keep(Promise:D:);
    multi method keep(Promise:D: \result);

Keeps a promise, optionally setting the result. If no result is passed, the
result will be C<True>.

Throws an exception  of type X::Promise::Vowed if a vow has already been
taken. See method C<vow> for more information.

    my $p = Promise.new;

    if Bool.pick {
        $p.keep;
    }
    else {
         $p.break;
    }

=head2 method break

    multi method break(Promise:D:);
    multi method break(Promise:D: \result);

Breaks a promise, optionally setting the result. If no result is passed, the
result will be C<False>.

Throws an exception  of type X::Promise::Vowed if a vow has already been
taken. See method C<vow> for more information.

    my $p = Promise.new;

    $p.break('sorry');
    say $p.status;          # Broken
    say $p.cause;           # sorry

=head2 method result

    method result(Promise:D)

Waits for the promise to be kept or broken. If it is kept, returns the result;
otherwise throws the result as an exception.

=head2 method cause

    method cause(Promise:D)

If the promise was broken, returns the result (or exception). Otherwise, throws
an exception of type C<X::Promise::CauseOnlyValidOnBroken>.

=head2 method Bool

    multi method Bool(Promise:D:)

Returns C<True> for a kept or broken promise, and C<False> for one in state
C<Planned>.

=head2 method status

    method status(Promise:D) returns PromiseStatus

Returns the current state of the promise: C<Kept>, C<Broken> or C<Planned>.

=head2 method scheduler

    method scheduler(Promise:D:)

Returns the scheduler that manages the promise.

=head2 method vow

    my class Vow {
        has Promise $.promise;
        method keep() { ... }
        method break() { ... }
    }
    method vow(Promise:D:) returns Vow:D

Returns an object that holds the sole authority over keeping or breaking a
promise. Calling C<keep> or C<break> on a promise that has vow taken throws an
exception of type C<X::Promise::Vowed>.

    my $p   = Promise.new;
    my $vow = $p.vow;
    $vow.keep;
    say $p.status;          # Kept

=head2 sub await

    multi sub await(Promise:D)
    multi sub await(*@)

Waits until one or more promises are I<all> fulfilled, and then returns their
values. Also works on L<channels|/type/Channel>. Any broken promises will
rethrow their exceptions. 

=end pod
