path: root/README

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 of sub-tasks, and are implicitly
    marked as ready depending on the readiness of their component futures as
    required. These are called "convergent" futures here as they converge
    control and data-flow back into one place. These are the ones returned
    by the various `wait_*' and `need_*' constructors.

    It is intended that library functions that perform asynchronous
    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.

    It should be noted however, that this module does not in any way provide
    an actual mechanism for performing this asynchronous activity; it merely
    provides a way to create objects that can be used for control and data
    flow around those operations. It allows such code to be written in a
    neater, forward-reading manner, and simplifies many common patterns that
    are often involved in such situations.

    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.

     $ 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

    A warning is also printed in debug mode if a `Future' object is
    destroyed that completed with a failure, but the object believes that
    failure has not been reported anywhere.

     $ PERL_FUTURE_DEBUG=1 perl -Mblib -MFuture -E 'my $f = Future->fail("Oops")'
     Future=HASH(0xac98f8) was constructed at -e line 1 and was lost near -e line 0 with an unreported failure of: Oops

    Such a failure is considered reported if the `get' or `failure' methods
    are called on it, or it had at least one `on_ready' or `on_fail'
    callback, or its failure is propagated to another `Future' instance (by
    a sequencing or converging method).

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->done( @values )
  $future = Future->fail( $exception, @details )
    Shortcut wrappers around creating a new `Future' then immediately
    marking it as done or failed.

  $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.

    This will ensure that an incoming argument is definitely a `Future', and
    may be useful in such cases as adapting synchronous code to fit
    asynchronous libraries driven by `Future'.

  $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.

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 convergent future.

    If the future is already cancelled, this request is ignored. If the
    future is already complete with a result or a failure, an exception is
    thrown.

  Future->done( @result )
    May also be called as a class method, where it will construct a new
    Future and immediately mark it as done.

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

     return Future->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.

    As the same effect can be achieved using curry, this method is
    deprecated now and may be removed in a later version.

     $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'.

    If the future is already cancelled, this request is ignored. If the
    future is already complete with a result or a failure, an exception is
    thrown.

  Future->fail( $exception, @details )
    May also be called as a class method, where it will construct a new
    Future and immediately mark it as failed.

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

     return Future->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.

    As the same effect can be achieved using curry, this method is
    deprecated now and may be removed in a later version.

     $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 convergent 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.

  $done = $future->is_done
    Returns true on a future if it is ready and completed successfully.
    Returns false if it is still pending, failed, or was cancelled.

  @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
    convergent 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.

  @values = Future->unwrap( @values )
    If given a single argument which is a `Future' reference, this method
    will call `get' on it and return the result. Otherwise, it returns the
    list of values directly in list context, or the first value in scalar.
    Since it involves an implicit `await', this method can only be used on
    immediate futures or subclasses that implement `await'.

    This will ensure that an outgoing argument is definitely not a `Future',
    and may be useful in such cases as adapting synchronous code to fit
    asynchronous libraries that return `Future' instances.

  $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.

  $failed = $future->is_failed
    Returns true on a future if it is ready and it failed. Returns false if
    it is still pending, completed successfully, or was cancelled.

  $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 convergent 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.

    As the same effect can be achieved using curry, this method is
    deprecated now and may be removed in a later version.

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

SEQUENCING METHODS
    The following methods all return a new future to represent the
    combination of its invocant followed by another action given by a code
    reference. The combined activity waits for the first future to be ready,
    then may invoke the code depending on the success or failure of the
    first, or may run it regardless. The returned sequence future represents
    the entire combination of activity.

    In some cases the code should return a future; in some it should return
    an immediate result. If a future is returned, the combined future will
    then wait for the result of this second one. If the combinined future is
    cancelled, it will cancel either the first future or the second,
    depending whether the first had completed. If the code block throws an
    exception instead of returning a value, the sequence future will fail
    with that exception as its message and no further values.

    As it is always a mistake to call these sequencing methods 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 = $f1->then( \&done_code )
    Returns a new sequencing `Future' that runs the code if the first
    succeeds. Once `$f1' succeeds the code reference will be invoked and is
    passed the list of results. It should return a future, `$f2'. Once `$f2'
    completes the sequence future will then be marked as complete with
    whatever result `$f2' gave. If `$f1' fails then the sequence future will
    immediately fail with the same failure and the code will not be invoked.

     $f2 = $done_code->( @result )

  $future = $f1->else( \&fail_code )
    Returns a new sequencing `Future' that runs the code if the first fails.
    Once `$f1' fails the code reference will be invoked and is passed the
    failure and details. It should return a future, `$f2'. Once `$f2'
    completes the sequence future will then be marked as complete with
    whatever result `$f2' gave. If `$f1' succeeds then the sequence future
    will immediately succeed with the same result and the code will not be
    invoked.

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

  $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 = $f1->transform( %args )
    Returns a new sequencing `Future' that wraps the one given as `$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 arguments, 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'.

  $future = $f1->then_with_f( \&code )
    Returns a new sequencing `Future' that runs the code if the first
    succeeds. Identical to `then', except that the code reference will be
    passed both the original future, `$f1', and its result.

     $f2 = $code->( $f1, @result )

    This is useful for conditional execution cases where the code block may
    just return the same result of the original future. In this case it is
    more efficient to return the original future itself.

  $future = $f->then_done( @result )
  $future = $f->then_fail( $exception, @details )
    Convenient shortcuts to returning an immediate future from a `then'
    block, when the result is already known.

  $future = $f1->else_with_f( \&code )
    Returns a new sequencing `Future' that runs the code if the first fails.
    Identical to `else', except that the code reference will be passed both
    the original future, `$f1', and its exception and details.

     $f2 = $code->( $f1, $exception, @details )

    This is useful for conditional execution cases where the code block may
    just return the same result of the original future. In this case it is
    more efficient to return the original future itself.

  $future = $f->else_done( @result )
  $future = $f->else_fail( $exception, @details )
    Convenient shortcuts to returning an immediate future from a `else'
    block, when the result is already known.

  $future = $f1->followed_by( \&code )
    Returns a new sequencing `Future' that runs the code regardless of
    success or failure. Once `$f1' is ready the code reference will be
    invoked and is passed one argument, `$f1'. It should return a future,
    `$f2'. Once `$f2' completes the sequence future will then be marked as
    complete with whatever result `$f2' gave.

     $f2 = $code->( $f1 )

  $future = $f1->without_cancel
    Returns a new sequencing `Future' that will complete with the success or
    failure of the original future, but if cancelled, will not cancel the
    original. This may be useful if the original future represents an
    operation that is being shared among multiple sequences; cancelling one
    should not prevent the others from running too.

CONVERGENT 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 derived class component future will be used
    as the prototype for constructing the return value, so it respects
    subclassing correctly, or failing that a plain `Future'.

  $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, failure or cancellation. 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. Any
    component futures that are cancelled are ignored, apart from the final
    component left; at which point the result will be a 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. Any component futures that are
    cancelled will cause an immediate failure of the result.

    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. Any component futures
    that are cancelled are ignored, apart from the final component left; at
    which point the result will be a failure.

    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 CONVERGENT FUTURES
    The following methods apply to convergent (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.

TRACING METHODS
  $future = $future->set_label( $label )
  $label = $future->label
    Chaining mutator and accessor for the label of the `Future'. This should
    be a plain string value, whose value will be stored by the future
    instance for use in debugging messages or other tooling, or similar
    purposes.

  [ $sec, $usec ] = $future->btime
  [ $sec, $usec ] = $future->rtime
    Accessors that return the tracing timestamps from the instance. These
    give the time the instance was contructed ("birth" time, `btime') and
    the time the result was determined (the "ready" time, `rtime'). Each
    result is returned as a two-element ARRAY ref, containing the epoch time
    in seconds and microseconds, as given by `Time::HiRes::gettimeofday'.

    In order for these times to be captured, they have to be enabled by
    setting `$Future::TIMES' to a true value. This is initialised true at
    the time the module is loaded if either `PERL_FUTURE_DEBUG' or
    `PERL_FUTURE_TIMES' are set in the environment.

  $sec = $future->elapsed
    If both tracing timestamps are defined, returns the number of seconds of
    elapsed time between them as a floating-point number. If not, returns
    `undef'.

  $cb = $future->wrap_cb( $operation_name, $cb )
    *Since version 0.31.*

    *Note: This method is experimental and may be changed or removed in a
    later version.*

    This method is invoked internally by various methods that are about to
    save a callback CODE reference supplied by the user, to be invoked
    later. The default implementation simply returns the callback agument
    as-is; the method is provided to allow users to provide extra behaviour.
    This can be done by applying a method modifier of the `around' kind, so
    in effect add a chain of wrappers. Each wrapper can then perform its own
    wrapping logic of the callback. `$operation_name' is a string giving the
    reason for which the callback is being saved; currently one of
    `on_ready', `on_done', `on_fail' or `sequence'; the latter being used
    for all the sequence-returning methods.

    This method is intentionally invoked only for CODE references that are
    being saved on a pending `Future' instance to be invoked at some later
    point. It does not run for callbacks to be invoked on an
    already-complete instance. This is for performance reasons, where the
    intended behaviour is that the wrapper can provide some amount of
    context save and restore, to return the operating environment for the
    callback back to what it was at the time it was saved.

    For example, the following wrapper saves the value of a package variable
    at the time the callback was saved, and restores that value at
    invocation time later on. This could be useful for preserving context
    during logging in a Future-based program.

     our $LOGGING_CTX;

     no warnings 'redefine';

     my $orig = Future->can( "wrap_cb" );
     *Future::wrap_cb = sub {
        my $cb = $orig->( @_ );

        my $saved_logging_ctx = $LOGGING_CTX;

        return sub {
           local $LOGGING_CTX = $saved_logging_ctx;
           $cb->( @_ );
        };
     };

    At this point, any code deferred into a `Future' by any of its callbacks
    will observe the `$LOGGING_CTX' variable as having the value it held at
    the time the callback was saved, even if it is invoked later on when
    that value is different.

    Remember when writing such a wrapper, that it still needs to invoke the
    previous version of the method, so that it plays nicely in combination
    with others (see the `$orig->( @_ )' part).

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.
    This can also be used as a class method.

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

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

     my $f = Future->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->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.

KNOWN ISSUES
  Cancellation of Non-Final Sequence Futures
    The behaviour of future cancellation still has some unanswered questions
    regarding how to handle the situation where a future is cancelled that
    has a sequence future constructed from it.

    In particular, it is unclear in each of the following examples what the
    behaviour of `$f2' should be, were `$f1' to be cancelled:

     $f2 = $f1->then( sub { ... } ); # plus related ->then_with_f, ...

     $f2 = $f1->else( sub { ... } ); # plus related ->else_with_f, ...

     $f2 = $f1->followed_by( sub { ... } );

    In the `then'-style case it is likely that this situation should be
    treated as if `$f1' had failed, perhaps with some special message. The
    `else'-style case is more complex, because it may be that the entire
    operation should still fail, or it may be that the cancellation of `$f1'
    should again be treated simply as a special kind of failure, and the
    `else' logic run as normal.

    To be specific; in each case it is unclear what happens if the first
    future is cancelled, while the second one is still waiting on it. The
    semantics for "normal" top-down cancellation of `$f2' and how it affects
    `$f1' are already clear and defined.

  Cancellation of Divergent Flow
    A further complication of cancellation comes from the case where a given
    future is reused multiple times for multiple sequences or convergent
    trees.

    In particular, it is in clear in each of the following examples what the
    behaviour of `$f2' should be, were `$f1' to be cancelled:

     my $f_initial = Future->new; ...
     my $f1 = $f_initial->then( ... );
     my $f2 = $f_initial->then( ... );

     my $f1 = Future->needs_all( $f_initial );
     my $f2 = Future->needs_all( $f_initial );

    The point of cancellation propagation is to trace backwards through
    stages of some larger sequence of operations that now no longer need to
    happen, because the final result is no longer required. But in each of
    these cases, just because `$f1' has been cancelled, the initial future
    `$f_initial' is still required because there is another future (`$f2')
    that will still require its result.

    Initially it would appear that some kind of reference-counting mechanism
    could solve this question, though that itself is further complicated by
    the `on_ready' handler and its variants.

    It may simply be that a comprehensive useful set of cancellation
    semantics can't be universally provided to cover all cases; and that
    some use-cases at least would require the application logic to give
    extra information to its `Future' objects on how they should wire up the
    cancel propagation logic.

    Both of these cancellation issues are still under active design
    consideration; see the discussion on RT96685 for more information
    (https://rt.cpan.org/Ticket/Display.html?id=96685).

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

    *   "Futures advent calendar 2013"

        http://leonerds-code.blogspot.co.uk/2013/12/futures-advent-day-1.htm
        l

TODO
    *   Consider the ability to pass the constructor an `await' CODEref,
        instead of needing to use a subclass. This might simplify
        async/etc.. implementations, and allows the reuse of the idea of
        subclassing to extend the abilities of `Future' itself - for example
        to allow a kind of Future that can report incremental progress.

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