Provided by: libcps-perl_0.19-2_all 
NAME
"CPS" - manage flow of control in Continuation-Passing Style
OVERVIEW
Note: This module is entirely deprecated now. It is maintained for compatibility for any code still
using it, but please consider rewriting to use Future instead, which offers a far neater method of
representing asynchronous program and data flow. In addition, Future::AsyncAwait can further improve
readability of "Future"-based code by letting it use the familiar kinds of Perl control structure
while still being asynchronous.
At some later date this entire "CPS" module distribution may be deleted.
The functions in this module implement or assist the writing of programs, or parts of them, in
Continuation Passing Style (CPS). Briefly, CPS is a style of writing code where the normal call/return
mechanism is replaced by explicit "continuations", values passed in to functions which they should
invoke, to implement return behaviour. For more detail on CPS, see the SEE ALSO section.
What this module implements is not in fact true CPS, as Perl does not natively support the idea of a real
continuation (such as is created by a co-routine). Furthermore, for CPS to be efficient in languages
that natively support it, their runtimes typically implement a lot of optimisation of CPS code, which the
Perl interpreter would be unable to perform. Instead, CODE references are passed around to stand in their
place. While not particularly useful for most regular cases, this becomes very useful whenever some form
of asynchronous or event-based programming is being used. Continuations passed in to the body function of
a control structure can be stored in the event handlers of the asynchronous or event-driven framework, so
that when they are invoked later, the code continues, eventually arriving at its final answer at some
point in the future.
In order for these examples to make sense, a fictional and simple asynchronisation framework has been
invented. The exact details of operation should not be important, as it simply stands to illustrate the
point. I hope its general intention should be obvious. :)
read_stdin_line( \&on_line ); # wait on a line from STDIN, then pass it
# to the handler function
This module itself provides functions that manage the flow of control through a continuation passing
program. They do not directly facilitate the flow of data through a program. That can be managed by
lexical variables captured by the closures passed around. See the EXAMPLES section.
For CPS versions of data-flow functionals, such as "map" and "grep", see also CPS::Functional.
SYNOPSIS
use CPS qw( kloop );
kloop( sub {
my ( $knext, $klast ) = @_;
print "Enter a number, or q to quit: ";
read_stdin_line( sub {
my ( $first ) = @_;
chomp $first;
return $klast->() if $first eq "q";
print "Enter a second number: ";
read_stdin_line( sub {
my ( $second ) = @_;
print "The sum is " . ( $first + $second ) . "\n";
$knext->();
} );
} );
},
sub { exit }
);
FUNCTIONS
In all of the following functions, the "\&body" function can provide results by invoking its continuation
/ one of its continuations, either synchronously or asynchronously at some point later (via some event
handling or other mechanism); the next invocation of "\&body" will not take place until the previous one
exits if it is done synchronously.
They all take the prefix "k" before the name of the regular perl keyword or function they aim to replace.
It is common in CPS code in other languages, such as Scheme or Haskell, to store a continuation in a
variable called "k". This convention is followed here.
kloop( \&body, $k )
CPS version of perl's "while(true)" loop. Repeatedly calls the "body" code until it indicates the end of
the loop, then invoke $k.
$body->( $knext, $klast )
$knext->()
$klast->()
$k->()
If $knext is invoked, the body will be called again. If $klast is invoked, the continuation $k is
invoked.
kwhile( \&body, $k )
Compatibility synonym for "kloop"; it was renamed after version 0.10. New code should use "kloop"
instead.
kforeach( \@items, \&body, $k )
CPS version of perl's "foreach" loop. Calls the "body" code once for each element in @items, until either
the items are exhausted or the "body" invokes its $klast continuation, then invoke $k.
$body->( $item, $knext, $klast )
$knext->()
$klast->()
$k->()
kdescendd( $root, \&body, $k )
CPS version of recursive descent on a tree-like structure, defined by a function, "body", which when
given a node in the tree, yields a list of child nodes.
$body->( $node, $kmore )
$kmore->( @child_nodes )
$k->()
The first value to be passed into "body" is $root.
At each iteration, a node is given to the "body" function, and it is expected to pass a list of child
nodes into its $kmore continuation. These will then be iterated over, in the order given. The tree-like
structure is visited depth-first, descending fully into one subtree of a node before moving on to the
next.
This function does not provide a way for the body to accumulate a resultant data structure to pass into
its own continuation. The body is executed simply for its side-effects and its continuation is invoked
with no arguments. A variable of some sort should be shared between the body and the continuation if this
is required.
kdescendb( $root, \&body, $k )
A breadth-first variation of "kdescendd". This function visits each child node of the parent, before
iterating over all of these nodes's children, recursively until the bottom of the tree.
kpar( @bodies, $k )
This CPS function takes a list of function bodies and calls them all immediately. Each is given its own
continuation. Once every body has invoked its continuation, the main continuation $k is invoked.
$body->( $kdone )
$kdone->()
$k->()
This allows running multiple operations in parallel, and waiting for them all to complete before
continuing. It provides in a CPS form functionality similar to that provided in a more object-oriented
fashion by modules such as Async::MergePoint or Event::Join.
kpareach( \@items, \&body, $k )
This CPS function takes a list of items and a function body, and calls the body immediately once for each
item in the list. Each invocation is given its own continuation. Once every body has invoked its
continuation, the main continuation $k is invoked.
$body->( $item, $kdone )
$kdone->()
$k->()
This is similar to "kforeach", except that the body is started concurrently for all items in the list
list, rather than each item waiting for the previous to finish.
kseq( @bodies, $k )
This CPS function takes a list of function bodies and calls them each, one at a time in sequence. Each is
given a continuation to invoke, which will cause the next body to be invoked. When the last body has
invoked its continuation, the main continuation $k is invoked.
$body->( $kdone )
$kdone->()
$k->()
A benefit of this is that it allows a long operation that uses many continuation "pauses", to be written
without code indenting further and further to the right. Another is that it allows easy skipping of
conditional parts of a computation, which would otherwise be tricky to write in a CPS form. See the
EXAMPLES section.
GOVERNORS
All of the above functions are implemented using a loop which repeatedly calls the body function until
some terminating condition. By controlling the way this loop re-invokes itself, a program can control the
behaviour of the functions.
For every one of the above functions, there also exists a variant which takes a CPS::Governor object as
its first argument. These functions use the governor object to control their iteration.
kloop( \&body, $k )
gkloop( $gov, \&body, $k )
kforeach( \@items, \&body, $k )
gkforeach( $gov, \@items, \&body, $k )
etc...
In this way, other governor objects can be constructed which have different running properties; such as
interleaving iterations of their loop with other IO activity in an event-driven framework, or giving
rate-limitation control on the speed of iteration of the loop.
CPS UTILITIES
These function names do not begin with "k" because they are not themselves CPS primatives, but may be
useful in CPS-oriented code.
$kfunc = liftk { BLOCK }
$kfunc = liftk( \&func )
Returns a new CODE reference to a CPS-wrapped version of the code block or passed CODE reference. When
$kfunc is invoked, the function &func is called in list context, being passed all the arguments given to
$kfunc apart from the last, expected to be its continuation. When &func returns, the result is passed
into the continuation.
$kfunc->( @func_args, $k )
$k->( @func_ret )
The following are equivalent
print func( 1, 2, 3 );
my $kfunc = liftk( \&func );
$kfunc->( 1, 2, 3, sub { print @_ } );
Note that the returned wrapper function only has one continuation slot in its arguments. It therefore
cannot be used as the body for "kloop()", "kforeach()" or "kgenerate()", because these pass two
continuations. There does not exist a "natural" way to lift a normal call/return function into a CPS
function which requires more than one continuation, because there is no way to distinguish the different
named returns.
$func = dropk { BLOCK } $kfunc
$func = dropk $waitfunc, $kfunc
Returns a new CODE reference to a plain call/return version of the passed CPS-style CODE reference. When
the returned ("dropped") function is called, it invokes the passed CPS function, then waits for it to
invoke its continuation. When it does, the list that was passed to the continuation is returned by the
dropped function. If called in scalar context, only the first value in the list is returned.
$kfunc->( @func_args, $k )
$k->( @func_ret )
$waitfunc->()
@func_ret = $func->( @func_args )
Given the following trivial CPS function:
$kadd = sub { $_[2]->( $_[0] + $_[1] ) };
The following are equivalent
$kadd->( 10, 20, sub { print "The total is $_[0]\n" } );
$add = dropk { } $kadd;
print "The total is ".$add->( 10, 20 )."\n";
In the general case the CPS function hasn't yet invoked its continuation by the time it returns (such as
would be the case when using any sort of asynchronisation or event-driven framework). For "dropk" to
actually work in this situation, it requires a way to run the event framework, to cause it to process
events until the continuation has been invoked.
This is provided by the block, or the first passed CODE reference. When the returned function is invoked,
it repeatedly calls the block or wait function, until the CPS function has invoked its continuation.
EXAMPLES
Returning Data From Functions
No facilities are provided directly to return data from CPS body functions in "kloop", "kpar" and "kseq".
Instead, normal lexical variable capture may be used here.
my $bat;
my $ball;
kpar(
sub {
my ( $k ) = @_;
get_bat( on_bat => sub { $bat = shift; goto &$k } );
},
sub {
my ( $k ) = @_;
serve_ball( on_ball => sub { $ball = shift; goto &$k } );
},
sub {
$bat->hit( $ball );
},
);
The body function can set the value of a variable that it and its final continuation both capture.
Using "kseq" For Conditionals
Consider the call/return style of code
A();
if( $maybe ) {
B();
}
C();
We cannot easily write this in CPS form without naming C twice
kA( sub {
$maybe ?
kB( sub { kC() } ) :
kC();
} );
While not so problematic here, it could get awkward if C were in fact a large code block, or if more than
a single conditional were employed in the logic; a likely scenario. A further issue is that the logical
structure becomes much harder to read.
Using "kseq" allows us to name the continuation so each arm of "kmaybe" can invoke it indirectly.
kseq(
\&kA,
sub { my $k = shift; $maybe ? kB( $k ) : goto &$k; },
\&kC
);
SEE ALSO
• Future - represent an operation awaiting completion
• Future::AsyncAwait - deferred subroutine syntax for futures
• CPS::Functional - functional utilities in Continuation-Passing Style
• <http://en.wikipedia.org/wiki/Continuation-passing_style> on wikipedia
ACKNOWLEDGEMENTS
Matt S. Trout (mst) <mst@shadowcat.co.uk> - for the inspiration of "kpareach" and with apologies to for naming of the said. ;)
AUTHOR
Paul Evans <leonerd@leonerd.org.uk>