1CPS(3) User Contributed Perl Documentation CPS(3)
2
6 "CPS" - manage flow of control in Continuation-Passing Style
7
9 Note: This module is entirely deprecated now. It is maintained for
10 compatibility for any code still using it, but please consider
11 rewriting to use Future instead, which offers a far neater method
12 of representing asynchronous program and data flow. In addition,
13 Future::AsyncAwait can further improve readability of
14 "Future"-based code by letting it use the familiar kinds of Perl
15 control structure while still being asynchronous.
16 At some later date this entire "CPS" module distribution may be
18 deleted.
19 The functions in this module implement or assist the writing of
21 programs, or parts of them, in Continuation Passing Style (CPS).
22 Briefly, CPS is a style of writing code where the normal call/return
23 mechanism is replaced by explicit "continuations", values passed in to
24 functions which they should invoke, to implement return behaviour. For
25 more detail on CPS, see the SEE ALSO section.
26 What this module implements is not in fact true CPS, as Perl does not
28 natively support the idea of a real continuation (such as is created by
29 a co-routine). Furthermore, for CPS to be efficient in languages that
30 natively support it, their runtimes typically implement a lot of
31 optimisation of CPS code, which the Perl interpreter would be unable to
32 perform. Instead, CODE references are passed around to stand in their
33 place. While not particularly useful for most regular cases, this
34 becomes very useful whenever some form of asynchronous or event-based
35 programming is being used. Continuations passed in to the body function
36 of a control structure can be stored in the event handlers of the
37 asynchronous or event-driven framework, so that when they are invoked
38 later, the code continues, eventually arriving at its final answer at
39 some point in the future.
40 In order for these examples to make sense, a fictional and simple
42 asynchronisation framework has been invented. The exact details of
43 operation should not be important, as it simply stands to illustrate
44 the point. I hope its general intention should be obvious. :)
45 read_stdin_line( \&on_line ); # wait on a line from STDIN, then pass it
47 # to the handler function
48 This module itself provides functions that manage the flow of control
50 through a continuation passing program. They do not directly facilitate
51 the flow of data through a program. That can be managed by lexical
52 variables captured by the closures passed around. See the EXAMPLES
53 section.
54 For CPS versions of data-flow functionals, such as "map" and "grep",
56 see also CPS::Functional.
57
59 use CPS qw( kloop );
60 kloop( sub {
62 my ( $knext, $klast ) = @_;
63 print "Enter a number, or q to quit: ";
65 read_stdin_line( sub {
67 my ( $first ) = @_;
68 chomp $first;
69 return $klast->() if $first eq "q";
71 print "Enter a second number: ";
73 read_stdin_line( sub {
75 my ( $second ) = @_;
76 print "The sum is " . ( $first + $second ) . "\n";
78 $knext->();
80 } );
81 } );
82 },
83 sub { exit }
84 );
85
87 In all of the following functions, the "\&body" function can provide
88 results by invoking its continuation / one of its continuations, either
89 synchronously or asynchronously at some point later (via some event
90 handling or other mechanism); the next invocation of "\&body" will not
91 take place until the previous one exits if it is done synchronously.
92 They all take the prefix "k" before the name of the regular perl
94 keyword or function they aim to replace. It is common in CPS code in
95 other languages, such as Scheme or Haskell, to store a continuation in
96 a variable called "k". This convention is followed here.
97 kloop( \&body, $k )
99 CPS version of perl's "while(true)" loop. Repeatedly calls the "body"
100 code until it indicates the end of the loop, then invoke $k.
101 $body->( $knext, $klast )
103 $knext->()
104 $klast->()
105 $k->()
107 If $knext is invoked, the body will be called again. If $klast is
109 invoked, the continuation $k is invoked.
110 kwhile( \&body, $k )
112 Compatibility synonym for "kloop"; it was renamed after version 0.10.
113 New code should use "kloop" instead.
114 kforeach( \@items, \&body, $k )
116 CPS version of perl's "foreach" loop. Calls the "body" code once for
117 each element in @items, until either the items are exhausted or the
118 "body" invokes its $klast continuation, then invoke $k.
119 $body->( $item, $knext, $klast )
121 $knext->()
122 $klast->()
123 $k->()
125 kdescendd( $root, \&body, $k )
127 CPS version of recursive descent on a tree-like structure, defined by a
128 function, "body", which when given a node in the tree, yields a list of
129 child nodes.
130 $body->( $node, $kmore )
132 $kmore->( @child_nodes )
133 $k->()
135 The first value to be passed into "body" is $root.
137 At each iteration, a node is given to the "body" function, and it is
139 expected to pass a list of child nodes into its $kmore continuation.
140 These will then be iterated over, in the order given. The tree-like
141 structure is visited depth-first, descending fully into one subtree of
142 a node before moving on to the next.
143 This function does not provide a way for the body to accumulate a
145 resultant data structure to pass into its own continuation. The body is
146 executed simply for its side-effects and its continuation is invoked
147 with no arguments. A variable of some sort should be shared between the
148 body and the continuation if this is required.
149 kdescendb( $root, \&body, $k )
151 A breadth-first variation of "kdescendd". This function visits each
152 child node of the parent, before iterating over all of these nodes's
153 children, recursively until the bottom of the tree.
154 kpar( @bodies, $k )
156 This CPS function takes a list of function bodies and calls them all
157 immediately. Each is given its own continuation. Once every body has
158 invoked its continuation, the main continuation $k is invoked.
159 $body->( $kdone )
161 $kdone->()
162 $k->()
164 This allows running multiple operations in parallel, and waiting for
166 them all to complete before continuing. It provides in a CPS form
167 functionality similar to that provided in a more object-oriented
168 fashion by modules such as Async::MergePoint or Event::Join.
169 kpareach( \@items, \&body, $k )
171 This CPS function takes a list of items and a function body, and calls
172 the body immediately once for each item in the list. Each invocation is
173 given its own continuation. Once every body has invoked its
174 continuation, the main continuation $k is invoked.
175 $body->( $item, $kdone )
177 $kdone->()
178 $k->()
180 This is similar to "kforeach", except that the body is started
182 concurrently for all items in the list list, rather than each item
183 waiting for the previous to finish.
184 kseq( @bodies, $k )
186 This CPS function takes a list of function bodies and calls them each,
187 one at a time in sequence. Each is given a continuation to invoke,
188 which will cause the next body to be invoked. When the last body has
189 invoked its continuation, the main continuation $k is invoked.
190 $body->( $kdone )
192 $kdone->()
193 $k->()
195 A benefit of this is that it allows a long operation that uses many
197 continuation "pauses", to be written without code indenting further and
198 further to the right. Another is that it allows easy skipping of
199 conditional parts of a computation, which would otherwise be tricky to
200 write in a CPS form. See the EXAMPLES section.
201
203 All of the above functions are implemented using a loop which
204 repeatedly calls the body function until some terminating condition. By
205 controlling the way this loop re-invokes itself, a program can control
206 the behaviour of the functions.
207 For every one of the above functions, there also exists a variant which
209 takes a CPS::Governor object as its first argument. These functions use
210 the governor object to control their iteration.
211 kloop( \&body, $k )
213 gkloop( $gov, \&body, $k )
214 kforeach( \@items, \&body, $k )
216 gkforeach( $gov, \@items, \&body, $k )
217 etc...
219 In this way, other governor objects can be constructed which have
221 different running properties; such as interleaving iterations of their
222 loop with other IO activity in an event-driven framework, or giving
223 rate-limitation control on the speed of iteration of the loop.
224
226 These function names do not begin with "k" because they are not
227 themselves CPS primatives, but may be useful in CPS-oriented code.
228 $kfunc = liftk { BLOCK }
230 $kfunc = liftk( \&func )
231 Returns a new CODE reference to a CPS-wrapped version of the code block
232 or passed CODE reference. When $kfunc is invoked, the function &func is
233 called in list context, being passed all the arguments given to $kfunc
234 apart from the last, expected to be its continuation. When &func
235 returns, the result is passed into the continuation.
236 $kfunc->( @func_args, $k )
238 $k->( @func_ret )
239 The following are equivalent
241 print func( 1, 2, 3 );
243 my $kfunc = liftk( \&func );
245 $kfunc->( 1, 2, 3, sub { print @_ } );
246 Note that the returned wrapper function only has one continuation slot
248 in its arguments. It therefore cannot be used as the body for
249 "kloop()", "kforeach()" or "kgenerate()", because these pass two
250 continuations. There does not exist a "natural" way to lift a normal
251 call/return function into a CPS function which requires more than one
252 continuation, because there is no way to distinguish the different
253 named returns.
254 $func = dropk { BLOCK } $kfunc
256 $func = dropk $waitfunc, $kfunc
257 Returns a new CODE reference to a plain call/return version of the
258 passed CPS-style CODE reference. When the returned ("dropped") function
259 is called, it invokes the passed CPS function, then waits for it to
260 invoke its continuation. When it does, the list that was passed to the
261 continuation is returned by the dropped function. If called in scalar
262 context, only the first value in the list is returned.
263 $kfunc->( @func_args, $k )
265 $k->( @func_ret )
266 $waitfunc->()
268 @func_ret = $func->( @func_args )
270 Given the following trivial CPS function:
272 $kadd = sub { $_[2]->( $_[0] + $_[1] ) };
274 The following are equivalent
276 $kadd->( 10, 20, sub { print "The total is $_[0]\n" } );
278 $add = dropk { } $kadd;
280 print "The total is ".$add->( 10, 20 )."\n";
281 In the general case the CPS function hasn't yet invoked its
283 continuation by the time it returns (such as would be the case when
284 using any sort of asynchronisation or event-driven framework). For
285 "dropk" to actually work in this situation, it requires a way to run
286 the event framework, to cause it to process events until the
287 continuation has been invoked.
288 This is provided by the block, or the first passed CODE reference. When
290 the returned function is invoked, it repeatedly calls the block or wait
291 function, until the CPS function has invoked its continuation.
292
294 Returning Data From Functions
295 No facilities are provided directly to return data from CPS body
296 functions in "kloop", "kpar" and "kseq". Instead, normal lexical
297 variable capture may be used here.
298 my $bat;
300 my $ball;
301 kpar(
303 sub {
304 my ( $k ) = @_;
305 get_bat( on_bat => sub { $bat = shift; goto &$k } );
306 },
307 sub {
308 my ( $k ) = @_;
309 serve_ball( on_ball => sub { $ball = shift; goto &$k } );
310 },
311 sub {
313 $bat->hit( $ball );
314 },
315 );
316 The body function can set the value of a variable that it and its final
318 continuation both capture.
319 Using "kseq" For Conditionals
321 Consider the call/return style of code
322 A();
324 if( $maybe ) {
325 B();
326 }
327 C();
328 We cannot easily write this in CPS form without naming C twice
330 kA( sub {
332 $maybe ?
333 kB( sub { kC() } ) :
334 kC();
335 } );
336 While not so problematic here, it could get awkward if C were in fact a
338 large code block, or if more than a single conditional were employed in
339 the logic; a likely scenario. A further issue is that the logical
340 structure becomes much harder to read.
341 Using "kseq" allows us to name the continuation so each arm of "kmaybe"
343 can invoke it indirectly.
344 kseq(
346 \&kA,
347 sub { my $k = shift; $maybe ? kB( $k ) : goto &$k; },
348 \&kC
349 );
350
352 • Future - represent an operation awaiting completion
353 • Future::AsyncAwait - deferred subroutine syntax for futures
355 • CPS::Functional - functional utilities in Continuation-Passing
357 Style
358 • <http://en.wikipedia.org/wiki/Continuation-passing_style> on
360 wikipedia
361
363 Matt S. Trout (mst) <mst@shadowcat.co.uk> - for the inspiration of
364 "kpareach" and with apologies to for naming of the said. ;)
365
367 Paul Evans <leonerd@leonerd.org.uk>
368perl v5.32.1 2021-01-26 CPS(3)