TITLE

Coroutines for Perl

VERSION

  Maintainer: Tom Scola <tscola@inch.com>
  Date: 4 Aug 2000
  Mailing List: perl6-internals@perl.org
  Number: 27
  Version: 1
  Status: Developing

ABSTRACT

Two motavations for the development of Perl 6 are to bring threading and event-based programming to the core language. Coroutines can be implemented in either a threaded or an event-based manner, are at a much higher level conceptually, and are much easier to program than either of those methods. As implemented in perl, coroutines use programming methods that most perl programmers are already familiar with, making the transition to them simple and natural.

DESCRIPTION

This proposal introduces a new keyword to perl, "co", as a complement to "sub". A subroutine is defined thusly:

  co foo { ... }

Coroutines can also be closures:

  my $x = co { ... }

Unlike coroutines as defined by Knuth, and implemented in laguages such as Simula or Python, perl does not have an explicit "resume" call for invoking coroutines. Instead, perl uses a mechanism that perl programmers are already familiar with: pipes. Note that these are intra-process pipes, and would be implemented differently than kernel pipes.

Coroutines are identified by a prefixed vertical bar. To invoke a coroutine, you write data to or read data from it.

   $y = <|foo>;

   print |$x "hello, world\n";

Inside a coroutine, the meanings of "<>" and the default file descriptor for print, printf, etc. are overloaded.

Coroutines and pipelines would effectively turn Perl into a dataflow programming language, providing yet another paradigm for Perl to encompass.

IMPLEMENTATION

Coroutines can be implemented in their traditional manner, using context switching, or as threads, but would hide the mundane details of threads from the programmer. They would be more suited for producer/consumer type problems rather than massively parallel compuations, but I doubt there is much call for the latter to be implemented in perl.

OPEN ISSUES

coroutine priority: There probably needs to be some mechanism for defining the priority of a coroutine.
reference passing: Is new syntax needed for passing references via pipelines?

REFERENCES

Donald Knuth, The Art of Computer Programming
"Pipes -- Powerful and Elegant Programming Paradigm" http://www.softpanorama.org/Scripting/pipes.shtml
"Coroutines in Python" http://www.nightmare.com/~rushing/copython/