NAME

    Future::IO - Future-returning IO methods

SYNOPSIS

       use Future::IO;
    
       my $delay = Future::IO->sleep( 5 );
       # $delay will become done in 5 seconds time
    
       my $input = Future::IO->sysread( \*STDIN, 4096 );
       # $input will yield some input from the STDIN IO handle

DESCRIPTION

    This package provides a few basic methods that behave similarly to the
    same-named core perl functions relating to IO operations, but yield
    their results asynchronously via Future instances.

    This is provided primarily as a decoupling mechanism, to allow modules
    to be written that perform IO in an asynchronous manner to depend
    directly on this, while allowing asynchronous event systems to provide
    an implementation of these operations.

 Default Implementation

    If the override_impl method is not invoked, a default implementation of
    these operations is provided. This implementation allows a single queue
    of sysread calls on a single filehandle only, combined with sleep
    calls. It is provided for the simple cases where modules only need one
    filehandle (most likely a single network socket or hardware device
    handle), allowing such modules to work without needing a better event
    system.

    If there are both sysread and sleep futures pending, the implementation
    will use select() to wait for either. This may be problematic on
    MSWin32, depending on what type of filehandle is involved.

    For cases where multiple filehandles are required, or for doing more
    involved IO operations, a real implementation based on an actual event
    loop should be provided.

 Unit Testing

    The replaceable implementation is also useful for writing unit test
    scripts. If the implementation is set to an instance of some sort of
    test fixture or mocking object, a unit test can check that the
    appropriate IO operations happen as part of the test.

METHODS

 sleep

       $f = Future::IO->sleep( $secs )

    Returns a Future that will become done a fixed delay from now, given in
    seconds. This value may be fractional.

 sysread

       $f = Future::IO->sysread( $fh, $length )
          $bytes = $f->get

    Returns a Future that will become done when at least one byte can be
    read from the given filehandle. It may return up to $length bytes. On
    EOF, the returned future will yield an empty list (or undef in scalar
    context). On any error (other than EAGAIN / EWOULDBLOCK which are
    ignored), the future fails with a suitable error message.

    Note specifically this may perform only a single sysread() call, and
    thus is not guaranteed to actually return the full length.

 override_impl

       Future::IO->override_impl( $impl )

    Sets a new implementation for Future::IO, replacing the minimal default
    internal implementation. This can either be a package name or an object
    instance reference, but must provide the methods named above.

    This method is intended to be called by event loops and other similar
    places, to provide a better integration. Another way, which doesn't
    involve directly depending on Future::IO or loading it, is to use the
    $IMPL variable; see below.

    Can only be called once, and only if the default implementation is not
    in use, therefore a module that wishes to override this ought to invoke
    it as soon as possible on program startup, before any of the main
    Future::IO methods may have been called.

THE $IMPL VARIABLE

    Since version 0.02.

    As an alternative to setting an implementation by using override_impl,
    a package variable is also available that allows modules such as event
    systems to opportunistically provide an implementation without needing
    to depend on the module, or loading it require. Simply directly set
    that package variable to the name of an implementing package or an
    object instance.

    Additionally, implementors may use a name within the Future::IO::Impl::
    namespace, suffixed by the name of their event system.

    For example, something like the following code arrangement is
    recommended.

       package Future::IO::Impl::BananaLoop;
    
       {
          no warnings 'once';
          ( $Future::IO::IMPL //= __PACKAGE__ ) eq __PACKAGE__ or
             warn "Unable to set Future::IO implementation to " . __PACKAGE__ .
                " as it is already $Future::IO::IMPL\n";
       }
    
       sub sleep
       {
          ...
       }
    
       sub sysread
       {
          ...
       }

AUTHOR

    Paul Evans <leonerd@leonerd.org.uk>

