Future::IO - Future-returning IO methods


   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


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.



   $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.


   $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.


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

Since version 0.03.

Returns a Future that will become done when exactly the given number of bytes have been read from the given filehandle. It returns exactly $length bytes. On EOF, the returned future will yield an empty list (or undef in scalar context), even if fewer bytes have already been obtained. These bytes will be lost. On any error (other than EAGAIN / EWOULDBLOCK which are ignored), the future fails with a suitable error message.

This may make more than one syssread() call.


   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.


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


Paul Evans <>