2010/12/19

Perl - CPS - version 0.11

CPS is a Perl module that provides several utilities for writing programs in Continuation Passing Style.

In a nutshell, CPS is a method of control flow within a program, which uses values passed around as continuations as a replacement of the usual call/return semantics. In Perl, this is done by passing CODE references; calling a function and passing in a CODE reference to be invoked with the result, rather than having the function return it. While at first this doesn't seem to be very useful, most of its power comes from the observation that the function doesn't need to invoke its continuation immediately; if it performs some IO operation or similar, it can perform this in an asynchronous fashion, invoking its continuation later. This style of coding is often associated with nonblocking or asynchronous event-driven programming. It is typical of such event systems as IO::Async.

A typical problem with implementing CPS control flow, is that all of the usual Perl control-flow mechanisms are built for immediate call/return semantics, where the use of CPS gets in the way. The CPS module provides utility functions for implementing control flow in a continuation passing style, by providing a set of functions that replace the usual Perl control-flow keywords. For example the Perl control structure of a foreach loop
foreach my $frob ( @frobs ) {
my $wibble = mangle( $frob );
say "Mangled $frob looks like $wibble";
}
say "All done";
becomes a call to CPS::kforeach
use CPS qw( kforeach );

kforeach( \@frobs, sub {
my ( $frob, $knext ) = @_;
kmangle( $frob, sub {
my ( $wibble ) = @_;
say "Mangled $frob looks like $wibble";
$knext->();
} );
}, sub {
say "All done";
} );
We haven't really gained anything by doing this though. If the process of mangling a frob involves some IO tasks, perhaps talking to some remote server, then we'll spend most of our time waiting for it to respond when we could be sending multiple requests and waiting on their responses. We could likely save some time by running them concurrently.

I gave a talk at LPW2010 about Continuation Passing Style and CPS, the slides of which are available here.

After discussing CPS and IO::Async at LPW, I was talking with mst about his IPC::Command::Multiplex module. He came up with the idea for another control-flow function; a combination of kpar and kforeach, which I called kpareach. In use it looks exactly like kforeach, except that it starts the loop body for each item all at the same time, in parallel, rather than waiting for each one to finish before invoking the next.

This is a new addition to CPS version 0.11, which is now on CPAN. It is also one of the first new control-flow structures that doesn't have a direct plain-Perl counterpart; a demonstration of the usefulness of CPS in event-driven programming.

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