NAME¶
"CPS" - manage flow of control in Continuation-Passing Style
OVERVIEW¶
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. For backward compatibility with
previous versions, the functions in "CPS::Functional" are also
exported here; and "kunfold" is renamed to "kgenerate".
This may be removed in a later version.
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¶
- •
- CPS::Functional - functional utilities in Continuation-Passing Style
- •
- http://en.wikipedia.org/wiki/Continuation-passing_style
<http://en.wikipedia.org/wiki/Continuation-passing_style> on
wikipedia
- •
- Coro - co-routines in Perl
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>