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CPS(3)		      User Contributed Perl Documentation		CPS(3)

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.

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> 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>

perl v5.18.2			  2014-05-14				CPS(3)
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