++ed by:
Author image Paul Evans


Future::Buffer - a string buffer that uses Futures


   use Future::Buffer;

   use Future::AsyncAwait;
   use Future::IO;

   my $buffer = Future::Buffer->new(
      fill => sub { Future::IO->sysread( $socket, 8192 ) }

   async sub print_lines
      while(1) {
         my $line = await $buffer->read_until( "\n" );
         chomp $line;

         say "Got a line: $line";



Objects in this class provide a string buffer, on which operations return Future instances which will complete when data is available. Data can be inserted into the buffer either in a push-based manner by calling the write method, or in a pull-based manner by providing it with a fill callback by which it can request data itself. This flexibility allows the buffer to act as an adapter between push- and pull-based providers and consumers.

Each read-like method returns a Future which will complete once there are enough bytes in the buffer to satisfy the required condition. The buffer behaves somewhat like a pipe, where bytes provided at the writing end (either by the write method or the fill callback) are eventually consumed at the reading end by one of the read futures.

Multiple read futures can remain pending at once, and will be completed in the order they were created when more data is eventually available. Thus, any call to the write method to provide more data can potentially result in multiple futures becoming ready.



   $buffer = Future::Buffer->new( %args )

Returns a new Future::Buffer instance.

Takes the following named arguments:

fill => CODE
   $f = $fill->()

      $data = $f->get

Optional callback which the buffer will invoke when it needs more data.

Any read futures which are waiting on the fill future are constructed by using the fill future as a prototype, ensuring they have the correct type.



   $len = $buffer->length

Returns the length of the currently-stored data; that is, data that has been provided by write calls or the fill callback but not yet consumed by a read future.


   $empty = $buffer->is_empty

Returns true if the stored length is zero.


   $f = $buffer->write( $data )

Appends to the stored data, invoking any pending read futures that are outstanding and can now complete.

Currently this method returns an already-completed Future. Some later version may implement a buffer maximum size, and choose not to complete this future until there is enough space to accept the new data. For now it is safe for the caller to ignore the return value, but it may become not so.


   $f = $buffer->read_atmost( $len )

      $data = $f->get

Returns a future which will complete when there is some data available in the buffer and will yield up too the given length. Note that, analogous to calling the read IO method on a filehandle, this can still complete and yield a shorter length if less is currently available.


   $f = $buffer->read_exactly( $len )

      $data = $f->get

Returns a future which will complete when there is enough data available in the buffer to yield exactly the length given.


   $f = $buffer->read_until( $pattern )

      $data = $f->get

Returns a future which will complete when the buffer contains a match for the given pattern (which may either be a plain string or a compiled Regexp). The future will yield the contents of the buffer up to and including this match.

For example, a readline-like operation can be performed by

   $f = $buffer->read_until( "\x0d\x0a" );


  • An "on-read" event, taking maybe inspiration from IO::Async::Stream. This would allow both pull- and push-based consumers.

  • Size limitation. Allow an upper bound of stored data, make write calls return pending futures until buffer can accept it. Needs consideration of unbounded read_until though.

  • Consider some read + unpack assistance, to allow nice handling of binary protocols by unpacking out of the buffer directly.

  • Consider what happens at EOF. Add a close method for producers to call. Understand what fill would do there. Have all the pending read futures yield an empty list maybe?


Paul Evans <leonerd@leonerd.org.uk>

Inspired by Ryu::Buffer by Tom Molesworth <TEAM@cpan.org>