Ubuntu Manpage: "CPS" - manage flow of control in Continuation-Passing Style

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
        );

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>