NAME
    `Future' - represent an operation awaiting completion

SYNOPSIS
     my $future = Future->new;

     perform_some_operation(
        on_complete => sub {
           $future->done( @_ );
        }
     );

     $future->on_ready( sub {
        say "The operation is complete";
     } );

DESCRIPTION
    A `Future' object represents an operation that is currently in progress,
    or has recently completed. It can be used in a variety of ways to manage
    the flow of control, and data, through an asynchronous program.

    Some futures represent a single operation and are explicitly marked as
    ready by calling the `done' or `fail' methods. These are called "leaf"
    futures here, and are returned by the `new' constructor.

    Other futures represent a collection sub-tasks, and are implicitly
    marked as ready depending on the readiness of their component futures as
    required. These are called "dependent" futures here, and are returned by
    the various `wait_*' and `need_*' constructors.

    It is intended that library functions that perform asynchonous
    operations would use future objects to represent outstanding operations,
    and allow their calling programs to control or wait for these operations
    to complete. The implementation and the user of such an interface would
    typically make use of different methods on the class. The methods below
    are documented in two sections; those of interest to each side of the
    interface.

    See also Future::Utils which contains useful loop-constructing
    functions, to run a future-returning function repeatedly in a loop.

  SUBCLASSING
    This class easily supports being subclassed to provide extra behavior,
    such as giving the `get' method the ability to block and wait for
    completion. This may be useful to provide `Future' subclasses with event
    systems, or similar.

    Each method that returns a new future object will use the invocant to
    construct its return value. If the constructor needs to perform
    per-instance setup it can override the `new' method, and take context
    from the given instance.

     sub new
     {
        my $proto = shift;
        my $self = $proto->SUPER::new;

        if( ref $proto ) {
           # Prototype was an instance
        }
        else {
           # Prototype was a class
        }

        return $self;
     }

    If an instance provides a method called `await', this will be called by
    the `get' and `failure' methods if the instance is pending.

     $f->await

    In most cases this should allow future-returning modules to be used as
    if they were blocking call/return-style modules, by simply appending a
    `get' call to the function or method calls.

     my ( $results, $here ) = future_returning_function( @args )->get;

    The examples directory in the distribution contains some examples of how
    futures might be integrated with various event systems.

  MODULE DOCUMENTATION
    Modules that provide future-returning functions or methods may wish to
    adopt the following styles in some way, to document the eventual return
    values from these futures.

     func( ARGS, HERE... ) ==> ( RETURN, VALUES... )

     OBJ->method( ARGS, HERE... ) ==> ( RETURN, VALUES... )

    Code returning a future that yields no values on success can use empty
    parentheses.

     func( ... ) ==> ()

  DEBUGGING
    By the time a `Future' object is destroyed, it ought to have been
    completed or cancelled. By enabling debug tracing of objects, this fact
    can be checked. If a future object is destroyed without having been
    completed or cancelled, a warning message is printed.

    This feature is enabled by setting an environment variable called
    `PERL_FUTURE_DEBUG' to some true value.

     $ PERL_FUTURE_DEBUG=1 perl -MFuture -E 'my $f = Future->new'
     Future=HASH(0xaa61f8) was constructed at -e line 1 and was lost near -e line 0 before it was ready.

    Note that due to a limitation of perl's `caller' function within a
    `DESTROY' destructor method, the exact location of the leak cannot be
    accurately determined. Often the leak will occur due to falling out of
    scope by returning from a function; in this case the leak location may
    be reported as being the line following the line calling that function.

     $ PERL_FUTURE_DEBUG=1 perl -MFuture
     sub foo {
        my $f = Future->new;
     }

     foo();
     print "Finished\n";

     Future=HASH(0x14a2220) was constructed at - line 2 and was lost near - line 6 before it was ready.
     Finished

CONSTRUCTORS
  $future = Future->new
  $future = $orig->new
    Returns a new `Future' instance to represent a leaf future. It will be
    marked as ready by any of the `done', `fail', or `cancel' methods. It
    can be called either as a class method, or as an instance method. Called
    on an instance it will construct another in the same class, and is
    useful for subclassing.

    This constructor would primarily be used by implementations of
    asynchronous interfaces.

  $future = Future->wrap( @values )
    If given a single argument which is already a `Future' reference, this
    will be returned unmodified. Otherwise, returns a new `Future' instance
    that is already complete, and will yield the given values.

  $future = Future->call( \&code, @args )
    A convenient wrapper for calling a `CODE' reference that is expected to
    return a future. In normal circumstances is equivalent to

     $future = $code->( @args )

    except that if the code throws an exception, it is wrapped in a new
    immediate fail future. If the return value from the code is not a
    blessed `Future' reference, an immediate fail future is returned instead
    to complain about this fact.

  $future = $f1->followed_by( \&code )
    Returns a new `Future' instance that allows a sequence of operations to
    be performed. Once `$f1' is ready, the code reference will be invoked
    and is passed one argument, being `$f1'. It should return a future,
    `$f2'. Once `$f2' indicates completion the combined future `$future'
    will then be marked as complete, with whatever result `$f2' gave.

     $f2 = $code->( $f1 )

    If `$future' is cancelled before `$f1' completes, then `$f1' will be
    cancelled. If it is cancelled after completion then `$f2' is cancelled
    instead.

    If the `$code' block dies entirely and throws an exception, this will be
    caught and set as the failure for the returned `$fseq'. The exception
    will not be propagated to the caller of the method that caused `$f1' to
    be ready.

    As it is always a mistake to call this method in void context and lose
    the reference to the returned future (because exception/error handling
    would be silently dropped), this method warns in void context.

  $future = $f1->and_then( \&code )
    A convenient shortcut to `followed_by', which invokes the supplied code
    reference only if the first future completes successfully. If it fails,
    then the returned future will fail with the same error and the code
    reference will not be invoked.

  $future = $f1->or_else( \&code )
    A convenient shortcut to `followed_by', which invokes the supplied code
    reference only if the first future fails. If it completes successfully,
    then the returned future will complete with the same result and the code
    reference will not be invoked.

  $future = $f1->then( \&done_code )
    Returns a new `Future' instance that allows a sequence of operations to
    be performed similar to `and_then', except that the code reference is
    passed the result of `$f1' rather than `$f1' itself.

    If `$f1' completes successfully, its result it passed into the
    `$done_code' function, which should return a new `Future' whose result
    will be used to set the result of the overall `$future'. If `$f1' fails
    this failure is used to set the result of `$future' directly.

     $f2 = $done_code->( @result )

    If `$future' is cancelled before `$f1' completes, then `$f1' will be
    cancelled. If it is cancelled after completion then `$f2' is cancelled
    instead.

    This is more convenient than `and_then' in the likely case that the code
    block does not need the initial future object itself, only the result.

  $future = $f1->else( \&fail_code )
    Returns a new `Future' instance that allows a sequence of operations to
    be performed similar to `or_else', except that the code reference is
    passed the failure of `$f1' rather than `$1' itself.

    If `$f1' fails, its failure is passed into the `$fail_code' function,
    which should return a new `Future' whose result will be used to set the
    result of the overall `$future'. If `$f1' completes successful this
    result is used to set the result of `$future' directly.

     $f2 = $fail_code->( $exception, @details )

    If `$future' is cancelled before `$f1' completes, then `$f1' will be
    cancelled. If it is cancelled after completion then `$f2' is cancelled
    instead.

    This is more convenient than `or_else' in the likely case that the code
    block does not need the initial future object itself, only the failure.

  $future = $f1->then( \&done_code, \&fail_code )
    The `then' method can also be passed the `$fail_code' block as well,
    giving a combination of `then' and `else' behaviour.

    This operation is designed to be compatible with the semantics of other
    future systems, such as Javascript's Q or Promises/A libraries.

  $future = $f1->transform( %args )
    Returns a new `Future' instance that wraps the one given as `$f1'. With
    no arguments this will be a trivial wrapper; `$future' will complete or
    fail when `$f1' does, and `$f1' will be cancelled when `$future' is.

    By passing the following named argmuents, the returned `$future' can be
    made to behave differently to `$f1':

    done => CODE
            Provides a function to use to modify the result of a successful
            completion. When `$f1' completes successfully, the result of its
            `get' method is passed into this function, and whatever it
            returns is passed to the `done' method of `$future'

    fail => CODE
            Provides a function to use to modify the result of a failure.
            When `$f1' fails, the result of its `failure' method is passed
            into this function, and whatever it returns is passed to the
            `fail' method of `$future'.

IMPLEMENTATION METHODS
    These methods would primarily be used by implementations of asynchronous
    interfaces.

  $future->done( @result )
    Marks that the leaf future is now ready, and provides a list of values
    as a result. (The empty list is allowed, and still indicates the future
    as ready). Cannot be called on a dependent future.

    Returns the `$future' to allow easy chaining to create an immediate
    future by

     return Future->new->done( ... )

  $code = $future->done_cb
    Returns a `CODE' reference that, when invoked, calls the `done' method.
    This makes it simple to pass as a callback function to other code.

    The same effect can be achieved using curry:

     $code = $future->curry::done;

  $future->fail( $exception, @details )
    Marks that the leaf future has failed, and provides an exception value.
    This exception will be thrown by the `get' method if called.

    The exception must evaluate as a true value; false exceptions are not
    allowed. Further details may be provided that will be returned by the
    `failure' method in list context. These details will not be part of the
    exception string raised by `get'.

    Returns the `$future' to allow easy chaining to create an immediate
    failed future by

     return Future->new->fail( ... )

  $code = $future->fail_cb
    Returns a `CODE' reference that, when invoked, calls the `fail' method.
    This makes it simple to pass as a callback function to other code.

    The same effect can be achieved using curry:

     $code = $future->curry::fail;

  $future->die( $message, @details )
    A convenient wrapper around `fail'. If the exception is a non-reference
    that does not end in a linefeed, its value will be extended by the file
    and line number of the caller, similar to the logic that `die' uses.

    Returns the `$future'.

  $future->on_cancel( $code )
    If the future is not yet ready, adds a callback to be invoked if the
    future is cancelled by the `cancel' method. If the future is already
    ready, throws an exception.

    If the future is cancelled, the callbacks will be invoked in the reverse
    order to that in which they were registered.

     $on_cancel->( $future )

  $future->on_cancel( $f )
    If passed another `Future' instance, the passed instance will be
    cancelled when the original future is cancelled. This method does
    nothing if the future is already complete.

  $cancelled = $future->is_cancelled
    Returns true if the future has been cancelled by `cancel'.

USER METHODS
    These methods would primarily be used by users of asynchronous
    interfaces, on objects returned by such an interface.

  $ready = $future->is_ready
    Returns true on a leaf future if a result has been provided to the
    `done' method, failed using the `fail' method, or cancelled using the
    `cancel' method.

    Returns true on a dependent future if it is ready to yield a result,
    depending on its component futures.

  $future->on_ready( $code )
    If the future is not yet ready, adds a callback to be invoked when the
    future is ready. If the future is already ready, invokes it immediately.

    In either case, the callback will be passed the future object itself.
    The invoked code can then obtain the list of results by calling the
    `get' method.

     $on_ready->( $future )

    Returns the `$future'.

  $future->on_ready( $f )
    If passed another `Future' instance, the passed instance will have its
    `done', `fail' or `cancel' methods invoked when the original future
    completes successfully, fails, or is cancelled respectively.

  @result = $future->get
  $result = $future->get
    If the future is ready and completed successfully, returns the list of
    results that had earlier been given to the `done' method on a leaf
    future, or the list of component futures it was waiting for on a
    dependent future. In scalar context it returns just the first result
    value.

    If the future is ready but failed, this method raises as an exception
    the failure string or object that was given to the `fail' method.

    If the future was cancelled an exception is thrown.

    If it is not yet ready and is not of a subclass that provides an `await'
    method an exception is thrown. If it is subclassed to provide an `await'
    method then this is used to wait for the future to be ready, before
    returning the result or propagating its failure exception.

  $future->on_done( $code )
    If the future is not yet ready, adds a callback to be invoked when the
    future is ready, if it completes successfully. If the future completed
    successfully, invokes it immediately. If it failed or was cancelled, it
    is not invoked at all.

    The callback will be passed the result passed to the `done' method.

     $on_done->( @result )

    Returns the `$future'.

  $future->on_done( $f )
    If passed another `Future' instance, the passed instance will have its
    `done' method invoked when the original future completes successfully.

  $exception = $future->failure
  $exception, @details = $future->failure
    Returns the exception passed to the `fail' method, `undef' if the future
    completed successfully via the `done' method, or raises an exception if
    called on a future that is not yet ready.

    If called in list context, will additionally yield a list of the details
    provided to the `fail' method.

    Because the exception value must be true, this can be used in a simple
    `if' statement:

     if( my $exception = $future->failure ) {
        ...
     }
     else {
        my @result = $future->get;
        ...
     }

  $future->on_fail( $code )
    If the future is not yet ready, adds a callback to be invoked when the
    future is ready, if it fails. If the future has already failed, invokes
    it immediately. If it completed successfully or was cancelled, it is not
    invoked at all.

    The callback will be passed the exception and details passed to the
    `fail' method.

     $on_fail->( $exception, @details )

    Returns the `$future'.

  $future->on_fail( $f )
    If passed another `Future' instance, the passed instance will have its
    `fail' method invoked when the original future fails.

    To invoke a `done' method on a future when another one fails, use a CODE
    reference:

     $future->on_fail( sub { $f->done( @_ ) } );

  $future->cancel
    Requests that the future be cancelled, immediately marking it as ready.
    This will invoke all of the code blocks registered by `on_cancel', in
    the reverse order. When called on a dependent future, all its component
    futures are also cancelled. It is not an error to attempt to cancel a
    future that is already complete or cancelled; it simply has no effect.

    Returns the `$future'.

  $code = $future->cancel_cb
    Returns a `CODE' reference that, when invoked, calls the `cancel'
    method. This makes it simple to pass as a callback function to other
    code.

    The same effect can be achieved using curry:

     $code = $future->curry::cancel;

DEPENDENT FUTURES
    The following constructors all take a list of component futures, and
    return a new future whose readiness somehow depends on the readiness of
    those components. The first non-immediate component future will be used
    as the prototype for constructing the return value, so it respects
    subclassing correctly.

  $future = Future->wait_all( @subfutures )
    Returns a new `Future' instance that will indicate it is ready once all
    of the sub future objects given to it indicate that they are ready,
    either by success or failure. Its result will a list of its component
    futures.

    When given an empty list this constructor returns a new immediately-done
    future.

    This constructor would primarily be used by users of asynchronous
    interfaces.

  $future = Future->wait_any( @subfutures )
    Returns a new `Future' instance that will indicate it is ready once any
    of the sub future objects given to it indicate that they are ready,
    either by success or failure. Any remaining component futures that are
    not yet ready will be cancelled. Its result will be the result of the
    first component future that was ready; either success or failure.

    When given an empty list this constructor returns an immediately-failed
    future.

    This constructor would primarily be used by users of asynchronous
    interfaces.

  $future = Future->needs_all( @subfutures )
    Returns a new `Future' instance that will indicate it is ready once all
    of the sub future objects given to it indicate that they have completed
    successfully, or when any of them indicates that they have failed. If
    any sub future fails, then this will fail immediately, and the remaining
    subs not yet ready will be cancelled.

    If successful, its result will be a concatenated list of the results of
    all its component futures, in corresponding order. If it fails, its
    failure will be that of the first component future that failed. To
    access each component future's results individually, use `done_futures'.

    When given an empty list this constructor returns a new immediately-done
    future.

    This constructor would primarily be used by users of asynchronous
    interfaces.

  $future = Future->needs_any( @subfutures )
    Returns a new `Future' instance that will indicate it is ready once any
    of the sub future objects given to it indicate that they have completed
    successfully, or when all of them indicate that they have failed. If any
    sub future succeeds, then this will succeed immediately, and the
    remaining subs not yet ready will be cancelled.

    If successful, its result will be that of the first component future
    that succeeded. If it fails, its failure will be that of the last
    component future to fail. To access the other failures, use
    `failed_futures'.

    Normally when this future completes successfully, only one of its
    component futures will be done. If it is constructed with multiple that
    are already done however, then all of these will be returned from
    `done_futures'. Users should be careful to still check all the results
    from `done_futures' in that case.

    When given an empty list this constructor returns an immediately-failed
    future.

    This constructor would primarily be used by users of asynchronous
    interfaces.

METHODS ON DEPENDENT FUTURES
    The following methods apply to dependent (i.e. non-leaf) futures, to
    access the component futures stored by it.

  @f = $future->pending_futures
  @f = $future->ready_futures
  @f = $future->done_futures
  @f = $future->failed_futures
  @f = $future->cancelled_futures
    Return a list of all the pending, ready, done, failed, or cancelled
    component futures. In scalar context, each will yield the number of such
    component futures.

EXAMPLES
    The following examples all demonstrate possible uses of a `Future'
    object to provide a fictional asynchronous API.

    For more examples, comparing the use of `Future' with regular
    call/return style Perl code, see also Future::Phrasebook.

  Providing Results
    By returning a new `Future' object each time the asynchronous function
    is called, it provides a placeholder for its eventual result, and a way
    to indicate when it is complete.

     sub foperation
     {
        my %args = @_;

        my $future = Future->new;

        do_something_async(
           foo => $args{foo},
           on_done => sub { $future->done( @_ ); },
        );

        return $future;
     }

    In most cases, the `done' method will simply be invoked with the entire
    result list as its arguments. In that case, it is simpler to use the
    `done_cb' wrapper method to create the `CODE' reference.

        my $future = Future->new;

        do_something_async(
           foo => $args{foo},
           on_done => $future->done_cb,
        );

    The caller may then use this future to wait for a result using the
    `on_ready' method, and obtain the result using `get'.

     my $f = foperation( foo => "something" );

     $f->on_ready( sub {
        my $f = shift;
        say "The operation returned: ", $f->get;
     } );

  Indicating Success or Failure
    Because the stored exception value of a failed future may not be false,
    the `failure' method can be used in a conditional statement to detect
    success or failure.

     my $f = foperation( foo => "something" );

     $f->on_ready( sub {
        my $f = shift;
        if( not my $e = $f->failure ) {
           say "The operation succeeded with: ", $f->get;
        }
        else {
           say "The operation failed with: ", $e;
        }
     } );

    By using `not' in the condition, the order of the `if' blocks can be
    arranged to put the successful case first, similar to a `try'/`catch'
    block.

    Because the `get' method re-raises the passed exception if the future
    failed, it can be used to control a `try'/`catch' block directly. (This
    is sometimes called *Exception Hoisting*).

     use Try::Tiny;

     $f->on_ready( sub {
        my $f = shift;
        try {
           say "The operation succeeded with: ", $f->get;
        }
        catch {
           say "The operation failed with: ", $_;
        };
     } );

    Even neater still may be the separate use of the `on_done' and `on_fail'
    methods.

     $f->on_done( sub {
        my @result = @_;
        say "The operation succeeded with: ", @result;
     } );
     $f->on_fail( sub {
        my ( $failure ) = @_;
        say "The operation failed with: $failure";
     } );

  Immediate Futures
    Because the `done' method returns the future object itself, it can be
    used to generate a `Future' that is immediately ready with a result.

     my $f = Future->new->done( $value );

    This is neater handled by the `wrap' class method, which encapsulates
    its arguments in a new immediate `Future', except if it is given a
    single argument that is already a `Future':

     my $f = Future->wrap( $value );

    Similarly, the `fail' and `die' methods can be used to generate a
    `Future' that is immediately failed.

     my $f = Future->new->die( "This is never going to work" );

    This could be considered similarly to a `die' call.

    An `eval{}' block can be used to turn a `Future'-returning function that
    might throw an exception, into a `Future' that would indicate this
    failure.

     my $f = eval { function() } || Future->new->fail( $@ );

    This is neater handled by the `call' class method, which wraps the call
    in an `eval{}' block and tests the result:

     my $f = Future->call( \&function );

  Sequencing
    The `then' method can be used to create simple chains of dependent
    tasks, each one executing and returning a `Future' when the previous
    operation succeeds.

     my $f = do_first()
                ->then( sub {
                   return do_second();
                })
                ->then( sub {
                   return do_third();
                });

    The result of the `$f' future itself will be the result of the future
    returned by the final function, if none of them failed. If any of them
    fails it will fail with the same failure. This can be considered similar
    to normal exception handling in synchronous code; the first time a
    function call throws an exception, the subsequent calls are not made.

  Merging Control Flow
    A `wait_all' future may be used to resynchronise control flow, while
    waiting for multiple concurrent operations to finish.

     my $f1 = foperation( foo => "something" );
     my $f2 = foperation( bar => "something else" );

     my $f = Future->wait_all( $f1, $f2 );

     $f->on_ready( sub {
        say "Operations are ready:";
        say "  foo: ", $f1->get;
        say "  bar: ", $f2->get;
     } );

    This provides an ability somewhat similar to `CPS::kpar()' or
    Async::MergePoint.

SEE ALSO
    *   curry - Create automatic curried method call closures for any class
        or object

    *   "The Past, The Present and The Future" - slides from a talk given at
        the London Perl Workshop, 2012.

        https://docs.google.com/presentation/d/1UkV5oLcTOOXBXPh8foyxko4PR28_
        zU_aVx6gBms7uoo/edit

AUTHOR
    Paul Evans <leonerd@leonerd.org.uk>