1Contextual::Return(3) User Contributed Perl DocumentationContextual::Return(3)
2
6 Contextual::Return - Create context-sensitive return values
7
9 This document describes Contextual::Return version 0.004014
10
12 use Contextual::Return;
13 use Carp;
14 sub foo {
16 return
17 SCALAR { 'thirty-twelve' }
18 LIST { 1,2,3 }
19 BOOL { 1 }
21 NUM { 7*6 }
22 STR { 'forty-two' }
23 HASHREF { {name => 'foo', value => 99} }
25 ARRAYREF { [3,2,1] }
26 GLOBREF { \*STDOUT }
28 CODEREF { croak "Don't use this result as code!"; }
29 ;
30 }
31 # and later...
33 if (my $foo = foo()) {
35 for my $count (1..$foo) {
36 print "$count: $foo is:\n"
37 . " array: @{$foo}\n"
38 . " hash: $foo->{name} => $foo->{value}\n"
39 ;
40 }
41 print {$foo} $foo->();
42 }
43
45 Usually, when you need to create a subroutine that returns different
46 values in different contexts (list, scalar, or void), you write
47 something like:
48 sub get_server_status {
50 my ($server_ID) = @_;
51 # Acquire server data somehow...
53 my %server_data = _ascertain_server_status($server_ID);
54 # Return different components of that data,
56 # depending on call context...
57 if (wantarray()) {
58 return @server_data{ qw(name uptime load users) };
59 }
60 if (defined wantarray()) {
61 return $server_data{load};
62 }
63 if (!defined wantarray()) {
64 carp 'Useless use of get_server_status() in void context';
65 return;
66 }
67 else {
68 croak q{Bad context! No biscuit!};
69 }
70 }
71 That works okay, but the code could certainly be more readable. In its
73 simplest usage, this module makes that code more readable by providing
74 three subroutines--LIST(), SCALAR(), VOID()--that are true only when
75 the current subroutine is called in the corresponding context:
76 use Contextual::Return;
78 sub get_server_status {
80 my ($server_ID) = @_;
81 # Acquire server data somehow...
83 my %server_data = _ascertain_server_status($server_ID);
84 # Return different components of that data
86 # depending on call context...
87 if (LIST) { return @server_data{ qw(name uptime load users) } }
88 if (SCALAR) { return $server_data{load} }
89 if (VOID) { print "$server_data{load}\n" }
90 else { croak q{Bad context! No biscuit!} }
91 }
92 Contextual returns
94 Those three subroutines can also be used in another way: as labels on a
95 series of contextual return blocks (collectively known as a contextual
96 return sequence). When a context sequence is returned, it automatically
97 selects the appropriate contextual return block for the calling
98 context. So the previous example could be written even more cleanly
99 as:
100 use Contextual::Return;
102 sub get_server_status {
104 my ($server_ID) = @_;
105 # Acquire server data somehow...
107 my %server_data = _ascertain_server_status($server_ID);
108 # Return different components of that data
110 # depending on call context...
111 return (
112 LIST { return @server_data{ qw(name uptime load users) } }
113 SCALAR { return $server_data{load} }
114 VOID { print "$server_data{load}\n" }
115 DEFAULT { croak q{Bad context! No biscuit!} }
116 );
117 }
118 The context sequence automatically selects the appropriate block for
120 each call context.
121 Lazy contextual return values
123 "LIST" and "VOID" blocks are always executed during the "return"
124 statement. However, scalar return blocks ("SCALAR", "STR", "NUM",
125 "BOOL", etc.) blocks are not. Instead, returning any of scalar block
126 types causes the subroutine to return an object that lazily evaluates
127 that block only when the return value is used.
128 This means that returning a "SCALAR" block is a convenient way to
130 implement a subroutine with a lazy return value. For example:
131 sub digest {
133 return SCALAR {
134 my ($text) = @_;
135 md5($text);
136 }
137 }
138 my $digest = digest($text);
140 print $digest; # md5() called only when $digest used as string
142 To better document this usage, the "SCALAR" block has a synonym:
144 "LAZY".
145 sub digest {
147 return LAZY {
148 my ($text) = @_;
149 md5($text);
150 }
151 }
152 Active contextual return values
154 Once a return value has been lazily evaluated in a given context, the
155 resulting value is cached, and thereafter reused in that same context.
156 However, you can specify that, rather than being cached, the value
158 should be re-evaluated every time the value is used:
159 sub make_counter {
161 my $counter = 0;
162 return ACTIVE
163 SCALAR { ++$counter }
164 ARRAYREF { [1..$counter] }
165 }
166 my $idx = make_counter();
168 print "$idx\n"; # 1
170 print "$idx\n"; # 2
171 print "[@$idx]\n"; # [1 2]
172 print "$idx\n"; # 3
173 print "[@$idx]\n"; # [1 2 3]
174 Semi-lazy contextual return values
176 Sometimes, single or repeated lazy evaluation of a scalar return value
177 in different contexts isn't what you really want. Sometimes what you
178 really want is for the return value to be lazily evaluated once only
179 (the first time it's used in any context), and then for that first
180 value to be reused whenever the return value is subsequently
181 reevaluated in any other context.
182 To get that behaviour, you can use the "FIXED" modifier, which causes
184 the return value to morph itself into the actual value the first time
185 it is used. For example:
186 sub lazy {
188 return
189 SCALAR { 42 }
190 ARRAYREF { [ 1, 2, 3 ] }
191 ;
192 }
193 my $lazy = lazy();
195 print $lazy + 1; # 43
196 print "@{$lazy}"; # 1 2 3
197 sub semilazy {
200 return FIXED
201 SCALAR { 42 }
202 ARRAYREF { [ 1, 2, 3 ] }
203 ;
204 }
205 my $semi = semilazy();
207 print $semi + 1; # 43
208 print "@{$semi}"; # die q{Can't use string ("42") as an ARRAY ref}
209 Finer distinctions of scalar context
211 Because the scalar values returned from a context sequence are lazily
212 evaluated, it becomes possible to be more specific about what kind of
213 scalar value should be returned: a boolean, a number, or a string. To
214 support those distinctions, Contextual::Return provides four extra
215 context blocks: "NUM", "STR", "BOOL", and "PUREBOOL":
216 sub get_server_status {
218 my ($server_ID) = @_;
219 # Acquire server data somehow...
221 my %server_data = _ascertain_server_status($server_ID);
222 # Return different components of that data
224 # depending on call context...
225 return (
226 LIST { @server_data{ qw(name uptime load users) } }
227 PUREBOOL { $_ = $server_data{uptime}; $server_data{uptime} > 0 }
228 BOOL { $server_data{uptime} > 0 }
229 NUM { $server_data{load} }
230 STR { "$server_data{name}: $server_data{uptime}" }
231 VOID { print "$server_data{load}\n" }
232 DEFAULT { croak q{Bad context! No biscuit!} }
233 );
234 }
235 With these in place, the object returned from a scalar-context call to
237 get_server_status() now behaves differently, depending on how it's
238 used. For example:
239 if ( my $status = get_server_status() ) { # BOOL: True if uptime > 0
241 $load_distribution[$status]++; # INT: Evaluates to load value
242 print "$status\n"; # STR: Prints "name: uptime"
243 }
244 if (get_server_status()) { # PUREBOOL: also sets $_;
246 print; # ...which is then used here
247 }
248 Boolean vs Pure Boolean contexts
250 There is a special subset of boolean contexts where the return value is
252 being used and immediately thrown away. For example, in the loop:
253 while (get_data()) {
255 ...
256 }
257 the value returned by get_data() is tested for truth and then
259 discarded. This is known as "pure boolean context". In contrast, in
260 the loop:
261 while (my $data = get_data()) {
263 ...
264 }
265 the value returned by get_data() is first assigned to $data, then
267 tested for truth. Because of the assignment, the return value is not
268 discarded after the boolean test. This is ordinary "boolean context".
269 In Perl, pure boolean context is often associated with a special side-
271 effect, that does not occur in regular boolean contexts. For example:
272 while (<>) {...} # $_ set as side-effect of pure boolean context
274 while ($v = <>) {...} # $_ NOT set in ordinary boolean context
276 Contextual::Return supports this with a special subcase of "BOOL" named
278 <PUREBOOL>. In pure boolean contexts, Contextual::Return will call a
279 "PUREBOOL" handler if one has been defined, or fall back to a "BOOL" or
280 "SCALAR" handler if no "PUREBOOL" handler exists. In ordinary boolean
281 contexts only the "BOOL" or "SCALAR" handlers are tried, even if a
282 "PUREBOOL" handler is also defined.
283 Typically "PUREBOOL" handlers are set up to have some side-effect (most
285 commonly: setting $_ or <$@>), like so:
286 sub get_data {
288 my ($succeeded, @data) = _go_and_get_data();
289 return
291 PUREBOOL { $_ = $data[0]; $succeeded; }
292 BOOL { $succeeded; }
293 SCALAR { $data[0]; }
294 LIST { @data; }
295 }
296 However, there is no requirement that they have side-effects. For
298 example, they can also be used to implement "look-but-don't-retrieve-
299 yet" checking:
300 sub get_data {
302 my $data;
303 return
304 PUREBOOL { _check_for_but_dont_get_data(); }
305 BOOL { defined( $data ||= _go_and_get_data() ); }
306 REF { $data ||= _go_and_get_data(); }
307 }
308 Self-reference within handlers
310 Any handler can refer to the contextual return object it is part of, by
311 calling the RETOBJ() function. This is particularly useful for
312 "PUREBOOL" and "LIST" handlers. For example:
313 return
315 PUREBOOL { $_ = RETOBJ; next handler; }
316 BOOL { !$failed; }
317 DEFAULT { $data; };
318 Referential contexts
320 The other major kind of scalar return value is a reference.
321 Contextual::Return provides contextual return blocks that allow you to
322 specify what to (lazily) return when the return value of a subroutine
323 is used as a reference to a scalar ("SCALARREF {...}"), to an array
324 ("ARRAYREF {...}"), to a hash ("HASHREF {...}"), to a subroutine
325 ("CODEREF {...}"), or to a typeglob ("GLOBREF {...}").
326 For example, the server status subroutine shown earlier could be
328 extended to allow it to return a hash reference, thereby supporting
329 "named return values":
330 sub get_server_status {
332 my ($server_ID) = @_;
333 # Acquire server data somehow...
335 my %server_data = _ascertain_server_status($server_ID);
336 # Return different components of that data
338 # depending on call context...
339 return (
340 LIST { @server_data{ qw(name uptime load users) } }
341 BOOL { $server_data{uptime} > 0 }
342 NUM { $server_data{load} }
343 STR { "$server_data{name}: $server_data{uptime}" }
344 VOID { print "$server_data{load}\n" }
345 HASHREF { return \%server_data }
346 DEFAULT { croak q{Bad context! No biscuit!} }
347 );
348 }
349 # and later...
351 my $users = get_server_status->{users};
353 # or, lazily...
356 my $server = get_server_status();
358 print "$server->{name} load = $server->{load}\n";
360 Interpolative referential contexts
362 The "SCALARREF {...}" and "ARRAYREF {...}" context blocks are
363 especially useful when you need to interpolate a subroutine into
364 strings. For example, if you have a subroutine like:
365 sub get_todo_tasks {
367 return (
368 SCALAR { scalar @todo_list } # How many?
369 LIST { @todo_list } # What are they?
370 );
371 }
372 # and later...
374 print "There are ", scalar(get_todo_tasks()), " tasks:\n",
376 get_todo_tasks();
377 then you could make it much easier to interpolate calls to that
379 subroutine by adding:
380 sub get_todo_tasks {
382 return (
383 SCALAR { scalar @todo_list } # How many?
384 LIST { @todo_list } # What are they?
385 SCALARREF { \scalar @todo_list } # Ref to how many
387 ARRAYREF { \@todo_list } # Ref to them
388 );
389 }
390 # and then...
392 print "There are ${get_todo_tasks()} tasks:\n@{get_todo_tasks()}";
394 In fact, this behaviour is so useful that it's the default. If you
396 don't provide an explicit "SCALARREF {...}" block, Contextual::Return
397 automatically provides an implicit one that simply returns a reference
398 to whatever would have been returned in scalar context. Likewise, if
399 no "ARRAYREF {...}" block is specified, the module supplies one that
400 returns the list-context return value wrapped up in an array reference.
401 So you could just write:
403 sub get_todo_tasks {
405 return (
406 SCALAR { scalar @todo_list } # How many?
407 LIST { @todo_list } # What are they?
408 );
409 }
410 # and still do this...
412 print "There are ${get_todo_tasks()} tasks:\n@{get_todo_tasks()}";
414 Fallback contexts
416 As the previous sections imply, the "BOOL {...}", "NUM {...}", "STR
417 {...}", and various "*REF {...}" blocks, are special cases of the
418 general "SCALAR {...}" context block. If a subroutine is called in one
419 of these specialized contexts but does not use the corresponding
420 context block, then the more general "SCALAR {...}" block is used
421 instead (if it has been specified).
422 So, for example:
424 sub read_value_from {
426 my ($fh) = @_;
427 my $value = <$fh>;
429 chomp $value;
430 return (
432 BOOL { defined $value }
433 SCALAR { $value }
434 );
435 }
436 ensures that the read_value_from() subroutine returns true in boolean
438 contexts if the read was successful. But, because no specific "NUM
439 {...}" or "STR {...}" return behaviours were specified, the subroutine
440 falls back on using its generic "SCALAR {...}" block in all other
441 scalar contexts.
442 Another way to think about this behaviour is that the various kinds of
444 scalar context blocks form a hierarchy:
445 SCALAR
447 ^
448 |
449 |--< BOOL
450 |
451 |--< NUM
452 |
453 `--< STR
454 Contextual::Return uses this hierarchical relationship to choose the
456 most specific context block available to handle any particular return
457 context, working its way up the tree from the specific type it needs,
458 to the more general type, if that's all that is available.
459 There are two slight complications to this picture. The first is that
461 Perl treats strings and numbers as interconvertable so the diagram (and
462 the Contextual::Return module) also has to allow these interconversions
463 as a fallback strategy:
464 SCALAR
466 ^
467 |
468 |--< BOOL
469 |
470 |--< NUM
471 | : ^
472 | v :
473 `--< STR
474 The dotted lines are meant to indicate that this intraconversion is
476 secondary to the main hierarchical fallback. That is, in a numeric
477 context, a "STR {...}" block will only be used if there is no "NUM
478 {...}" block and no "SCALAR {...}" block. In other words, the generic
479 context type is always used in preference to string<->number
480 conversion.
481 The second slight complication is that the above diagram only shows a
483 small part of the complete hierarchy of contexts supported by
484 Contextual::Return. The full fallback hierarchy (including dotted
485 interconversions) is:
486 DEFAULT
488 ^
489 |
490 |--< VOID
491 |
492 `--< NONVOID
493 ^
494 |
495 |--< VALUE <...............
496 | ^ :
497 | | :
498 | |--< SCALAR <.......:...
499 | | ^ :
500 | | | :
501 | | |--< BOOL :
502 | | | ^ :
503 | | | | :
504 | | | PUREBOOL :
505 | | | :
506 | | |--< NUM <..:.
507 | | | : ^ :
508 | | | v : :
509 | | `--< STR <....:..
510 | | :
511 | | ::
512 | `--< LIST ................: :
513 | : ^ :
514 | : : :
515 `--- REF : : :
516 ^ : : :
517 | v : :
518 |--< ARRAYREF :
519 | :
520 |--< SCALARREF .............:
521 |
522 |--< HASHREF
523 |
524 |--< CODEREF
525 |
526 |--< GLOBREF
527 |
528 `--< OBJREF <....... METHOD
529 ^
530 :........... BLESSED
531 As before, each dashed arrow represents a fallback relationship. That
533 is, if the required context specifier isn't available, the arrows are
534 followed until a more generic one is found. The dotted arrows again
535 represent the interconversion of return values, which is attempted only
536 after the normal hierarchical fallback fails.
537 For example, if a subroutine is called in a context that expects a
539 scalar reference, but no "SCALARREF {...}" block is provided, then
540 Contextual::Return tries the following blocks in order:
541 REF {...}
543 NONVOID {...}
544 DEFAULT {...}
545 STR {...} (automatically taking a reference to the result)
546 NUM {...} (automatically taking a reference to the result)
547 SCALAR {...} (automatically taking a reference to the result)
548 VALUE {...} (automatically taking a reference to the result)
549 Likewise, in a list context, if there is no "LIST {...}" context block,
551 the module tries:
552 VALUE {...}
554 NONVOID {...}
555 DEFAULT {...}
556 ARRAYREF {...} (automatically dereferencing the result)
557 STR {...} (treating it as a list of one element)
558 NUM {...} (treating it as a list of one element)
559 SCALAR {...} (treating it as a list of one element)
560 The more generic context blocks are especially useful for intercepting
562 unexpected and undesirable call contexts. For example, to turn off the
563 automatic scalar-ref and array-ref interpolative behaviour described in
564 "Interpolative referential contexts", you could intercept all
565 referential contexts using a generic "REF {...}" context block:
566 sub get_todo_tasks {
568 return (
569 SCALAR { scalar @todo_list } # How many?
570 LIST { @todo_list } # What are they?
571 REF { croak q{get_todo_task() can't be used as a reference} }
573 );
574 }
575 print 'There are ', get_todo_tasks(), '...'; # Still okay
577 print "There are ${get_todo_tasks()}..."; # Throws an exception
578 Treating return values as objects
580 Normally, when a return value is treated as an object (i.e. has a
581 method called on it), Contextual::Return invokes any "OBJREF" handler
582 that was specified in the contextual return list, and delegates the
583 method call to the object returned by that handler.
584 However, you can also be more specific, by specifying a "METHOD"
586 context handler in the contextual return list. The block of this
587 handler is expected to return one or more method-name/method-handler
588 pairs, like so:
589 return
591 METHOD {
592 get_count => sub { my $n = shift; $data[$n]{count} },
593 get_items => sub { my $n = shift; $data[$n]{items} },
594 clear => sub { @data = (); },
595 reset => sub { @data = (); },
596 }
597 Then, whenever one of the specified methods is called on the return
599 value, the corresponding subroutine will be called to implement it.
600 The method handlers must always be subroutine references, but the
602 method-name specifiers may be strings (as in the previous example) or
603 they may be specified generically, as either regexes or array
604 references. Generic method names are used to call the same handler for
605 two or more distinct method names. For example, the previous example
606 could be simplified to:
607 return
609 METHOD {
610 qr/get_(\w+)/ => sub { my $n = shift; $data[$n]{$1} },
611 ['clear','reset'] => sub { @data = (); },
612 }
613 A method name specified by regex will invoke the corresponding handler
615 for any method call request that the regex matches. A method name
616 specified by array ref will invoke the corresponding handler if the
617 method requested matches any of the elements of the array (which may
618 themselves be strings or regexes).
619 When the method handler is invoked, the name of the method requested is
621 passed to the handler in $_, and the method's argument list is passed
622 (as usual) via @_.
623 Note that any methods not explicitly handled by the "METHOD" handlers
625 will still be delegated to the object returned by the "OBJREF" handler
626 (if it is also specified).
627 Not treating return values as objects
629 The use of "OBJREF" and "METHOD" are slightly complicated by the fact
630 that contextual return values are themselves objects.
631 For example, prior to version 0.4.4 of the module, if you passed a
633 contextual return value to Scalar::Util::blessed(), it always returned
634 a true value (namely, the string: 'Contextual::Return::Value'), even if
635 the return value had not specified handlers for "OBJREF" or "METHOD".
636 In other words, the implementation of contextual return values (as
638 objects) was getting in the way of the use of contextual return values
639 (as non-objects).
640 So the module now also provides a "BLESSED" handler, which allows you
642 to explicitly control how contextual return values interact with
643 Scalar::Util::blessed().
644 If $crv is a contextual return value, by default
646 Scalar::Util::blessed($crv) will now only return true if that return
647 value has a "OBJREF", "LAZY", "REF", "SCALAR", "VALUE", "NONVOID", or
648 "DEFAULT" handler that in turn returns a blessed object.
649 However if $crv also provides a "BLESSED" handler, blessed() will
651 return whatever that handler returns.
652 This means:
654 sub simulate_non_object {
656 return BOOL { 1 }
657 NUM { 42 }
658 }
659 sub simulate_real_object {
661 return OBJREF { bless {}, 'My::Class' }
662 BOOL { 1 }
663 NUM { 42 }
664 }
665 sub simulate_faked_object {
667 return BLESSED { 'Foo' }
668 BOOL { 1 }
669 NUM { 42 }
670 }
671 sub simulate_previous_behaviour {
673 return BLESSED { 'Contextual::Return::Value' }
674 BOOL { 1 }
675 NUM { 42 }
676 }
677 say blessed( simulate_non_object() ); # undef
680 say blessed( simulate_real_object() ); # My::Class
681 say blessed( simulate_faked_object() ); # Foo
682 say blessed( simulate_previous_behaviour() ); # Contextual::Return::Value
683 Typically, you either want no "BLESSED" handler (in which case
685 contextual return values pretend not to be blessed objects), or you
686 want "BLESSED { 'Contextual::Return::Value' }" for backwards
687 compatibility with pre-v0.4.7 behaviour.
688 Preventing fallbacks
690 Sometimes fallbacks can be too helpful. Or sometimes you want to impose
692 strict type checking on a return value.
693 Contextual::Returns allows that via the "STRICT" specifier. If you
695 include "STRICT" anywhere in your return statement, the module disables
696 all fallbacks and will therefore through an exception if the return
697 value is used in any way not explicitly specified in the contextual
698 return sequence.
699 For example, to create a subroutine that returns only a string:
701 sub get_name {
703 return STRICT STR { 'Bruce' }
704 }
705 If the return value of the subroutine is used in any other way than as
707 a string, an exception will be thrown.
708 You can still specify handlers for more than a single kind of context
710 when using "STRICT":
711 sub get_name {
713 return STRICT
714 STR { 'Bruce' }
715 BOOL { 0 }
716 }
717 ...but these will still be the only contexts in which the return value
719 can be used:
720 my $n = get_name() ? 1 : 2; # Okay because BOOL handler specified
722 my $n = 'Dr' . get_name(); # Okay because STR handler specified
724 my $n = 1 + get_name(); # Exception thrown because no NUM handler
726 In other words, "STRICT" allows you to impose strict type checking on
728 your contextual return value.
729 Deferring handlers
731 Because the various handlers form a hierarchy, it's possible to
732 implement more specific handlers by falling back on ("deferring to")
733 more general ones. For example, a "PUREBOOL" handler is almost always
734 identical in its basic behaviour to the corresponding "BOOL" handler,
735 except that it adds some side-effect. For example:
736 return
738 PUREBOOL { $_ = $return_val; defined $return_val && $return_val > 0 }
739 BOOL { defined $return_val && $return_val > 0 }
740 SCALAR { $return_val; }
741 So Contextual::Return allows you to have a handler perform some action
743 and then defer to a more general handler to supply the actual return
744 value. To fall back to a more general case in this way, you simply
745 write:
746 next handler;
748 at the end of the handler in question, after which Contextual::Return
750 will find the next-most-specific handler and execute it as well. So the
751 previous example, could be re-written:
752 return
754 PUREBOOL { $_ = $return_val; next handler; }
755 BOOL { defined $return_val && $return_val > 0 }
756 SCALAR { $return_val; }
757 Note that any specific handler can defer to a more general one in this
759 same way. For example, you could provide consistent and maintainable
760 type-checking for a subroutine that returns references by providing
761 "ARRAYREF", "HASHREF", and "SCALARREF" handlers that all defer to a
762 generic "REF" handler, like so:
763 my $retval = _get_ref();
765 return
767 SCALARREF { croak 'Type mismatch' if ref($retval) ne 'SCALAR';
768 next handler;
769 }
770 ARRAYREF { croak 'Type mismatch' if ref($retval) ne 'ARRAY';
771 next handler;
772 }
773 HASHREF { croak 'Type mismatch' if ref($retval) ne 'HASH';
774 next handler;
775 }
776 REF { $retval }
777 If, at a later time, the process of returning a reference became more
779 complex, only the "REF" handler would have to be updated.
780 Nested handlers
782 Another way of factoring out return behaviour is to nest more specific
783 handlers inside more general ones. For instance, in the final example
784 given in "Boolean vs Pure Boolean contexts":
785 sub get_data {
787 my $data;
788 return
789 PUREBOOL { _check_for_but_dont_get_data(); }
790 BOOL { defined( $data ||= _go_and_get_data() ); }
791 REF { $data ||= _go_and_get_data(); }
792 }
793 you could factor out the repeated calls to _go_and_get_data() like so:
795 sub get_data {
797 return
798 PUREBOOL { _check_for_but_dont_get_data(); }
799 DEFAULT {
800 my $data = _go_and_get_data();
801 BOOL { defined $data; }
803 REF { $data; }
804 }
805 }
806 Here, the "DEFAULT" handler deals with every return context except pure
808 boolean. Within that "DEFAULT" handler, the data is first retrieved,
809 and then two "sub-handlers" deal with the ordinary boolean and
810 referential contexts.
811 Typically nested handlers are used in precisely this way: to optimize
813 for inexpensive special cases (such as pure boolean or integer or void
814 return contexts) and only do extra work for those other cases that
815 require it.
816 Failure contexts
818 Two of the most common ways to specify that a subroutine has failed are
819 to return a false value, or to throw an exception. The
820 Contextual::Return module provides a mechanism that allows the
821 subroutine writer to support both of these mechanisms at the same time,
822 by using the "FAIL" specifier.
823 A return statement of the form:
825 return FAIL;
827 causes the surrounding subroutine to return "undef" (i.e. false) in
829 boolean contexts, and to throw an exception in any other context. For
830 example:
831 use Contextual::Return;
833 sub get_next_val {
835 my $next_val = <>;
836 return FAIL if !defined $next_val;
837 chomp $next_val;
838 return $next_val;
839 }
840 If the "return FAIL" statement is executed, it will either return false
842 in a boolean context:
843 if (my $val = get_next_val()) { # returns undef if no next val
845 print "[$val]\n";
846 }
847 or else throw an exception if the return value is used in any other
849 context:
850 print get_next_val(); # throws exception if no next val
852 my $next_val = get_next_val();
854 print "[$next_val]\n"; # throws exception if no next val
855 The exception that is thrown is of the form:
857 Call to main::get_next_val() failed at demo.pl line 42
859 but you can change that message by providing a block to the "FAIL",
861 like so:
862 return FAIL { "No more data" } if !defined $next_val;
864 in which case, the final value of the block becomes the exception
866 message:
867 No more data at demo.pl line 42
869 A failure value can be interrogated for its error message, by calling
871 its error() method, like so:
872 my $val = get_next_val();
874 if ($val) {
875 print "[$val]\n";
876 }
877 else {
878 print $val->error, "\n";
879 }
880 Configurable failure contexts
882 The default "FAIL" behaviour--false in boolean context, fatal in all
883 others--works well in most situations, but violates the Platinum Rule
884 ("Do unto others as they would have done unto them").
885 So it may be user-friendlier if the user of a module is allowed decide
887 how the module's subroutines should behave on failure. For example, one
888 user might prefer that failing subs always return undef; another might
889 prefer that they always throw an exception; a third might prefer that
890 they always log the problem and return a special Failure object; whilst
891 a fourth user might want to get back 0 in scalar contexts, an empty
892 list in list contexts, and an exception everywhere else.
893 You could create a module that allows the user to specify all these
895 alternatives, like so:
896 package MyModule;
898 use Contextual::Return;
899 use Log::StdLog;
900 sub import {
902 my ($package, @args) = @_;
903 Contextual::Return::FAIL_WITH {
905 ':false' => sub { return undef },
906 ':fatal' => sub { croak @_ },
907 ':filed' => sub {
908 print STDLOG 'Sub ', (caller 1)[3], ' failed';
909 return Failure->new();
910 },
911 ':fussy' => sub {
912 SCALAR { undef }
913 LIST { () }
914 DEFAULT { croak @_ }
915 },
916 }, @args;
917 }
918 This configures Contextual::Return so that, instead of the usual false-
920 or-fatal semantics, every "return FAIL" within MyModule's namespace is
921 implemented by one of the four subroutines specified in the hash that
922 was passed to "FAIL_WITH".
923 Which of those four subs implements the "FAIL" is determined by the
925 arguments passed after the hash (i.e. by the contents of @args).
926 "FAIL_WITH" walks through that list of arguments and compares them
927 against the keys of the hash. If a key matches an argument, the
928 corresponding value is used as the implementation of "FAIL". Note that,
929 if subsequent arguments also match a key, their subroutine overrides
930 the previously installed implementation, so only the final override has
931 any effect. Contextual::Return generates warnings when multiple
932 overrides are specified.
933 All of which mean that, if a user loaded the MyModule module like this:
935 use MyModule qw( :fatal other args here );
937 then every "FAIL" within MyModule would be reconfigured to throw an
939 exception in all circumstances, since the presence of the ':fatal' in
940 the argument list will cause "FAIL_WITH" to select the hash entry whose
941 key is ':fatal'.
942 On the other hand, if they loaded the module:
944 use MyModule qw( :fussy other args here );
946 then each "FAIL" within MyModule would return undef or empty list or
948 throw an exception, depending on context, since that's what the
949 subroutine whose key is ':fussy' does.
950 Many people prefer module interfaces with a "flag => value" format, and
952 "FAIL_WITH" supports this too. For example, if you wanted your module
953 to take a "-fail" flag, whose associated value could be any of
954 "undefined", "exception", "logged", or "context", then you could
955 implement that simply by specifying the flag as the first argument
956 (i.e. before the hash) like so:
957 sub import {
959 my $package = shift;
960 Contextual::Return::FAIL_WITH -fail => {
962 'undefined' => sub { return undef },
963 'exception' => sub { croak @_ },
964 'logged' => sub {
965 print STDLOG 'Sub ', (caller 1)[3], ' failed';
966 return Failure->new();
967 },
968 'context' => sub {
969 SCALAR { undef }
970 LIST { () }
971 DEFAULT { croak @_ }
972 },
973 }, @_;
974 and then load the module:
976 use MyModule qw( other args here ), -fail=>'undefined';
978 or:
980 use MyModule qw( other args here ), -fail=>'exception';
982 In this case, "FAIL_WITH" scans the argument list for a pair of values:
984 its flag string, followed by some other selector value. Then it looks
985 up the selector value in the hash, and installs the corresponding
986 subroutine as its local "FAIL" handler.
987 If this "flagged" interface is used, the user of the module can also
989 specify their own handler directly, by passing a subroutine reference
990 as the selector value instead of a string:
991 use MyModule qw( other args here ), -fail=>sub{ die 'horribly'};
993 If this last example were used, any call to "FAIL" within MyModule
995 would invoke the specified anonymous subroutine (and hence throw a
996 'horribly' exception).
997 Note that, any overriding of a "FAIL" handler is specific to the
999 namespace and file from which the subroutine that calls "FAIL_WITH" is
1000 itself called. Since "FAIL_WITH" is designed to be called from within a
1001 module's import() subroutine, that generally means that the "FAIL"s
1002 within a given module X are only overridden for the current namespace
1003 within the particular file from module X is loaded. This means that two
1004 separate pieces of code (in separate files or separate namespaces) can
1005 each independently override a module's "FAIL" behaviour, without
1006 interfering with each other.
1007 Lvalue contexts
1009 Recent versions of Perl offer (limited) support for lvalue subroutines:
1010 subroutines that return a modifiable variable, rather than a simple
1011 constant value.
1012 Contextual::Return can make it easier to create such subroutines,
1014 within the limitations imposed by Perl itself. The limitations that
1015 Perl places on lvalue subs are:
1016 1. The subroutine must be declared with an ":lvalue" attribute:
1018 sub foo :lvalue {...}
1020 2. The subroutine must not return via an explicit "return". Instead,
1022 the last statement must evaluate to a variable, or must be a call
1023 to another lvalue subroutine call.
1024 my ($foo, $baz);
1026 sub foo :lvalue {
1028 $foo; # last statement evals to a var
1029 }
1030 sub bar :lvalue {
1032 foo(); # last statement is lvalue sub call
1033 }
1034 sub baz :lvalue {
1036 my ($arg) = @_;
1037 $arg > 0 # last statement evals...
1039 ? $baz # ...to a var
1040 : bar(); # ...or to an lvalue sub call
1041 }
1042 Thereafter, any call to the lvalue subroutine produces a result that
1044 can be assigned to:
1045 baz(0) = 42; # same as: $baz = 42
1047 baz(1) = 84; # same as: bar() = 84
1049 # which is the same as: foo() = 84
1050 # which is the same as: $foo = 84
1051 Ultimately, every lvalue subroutine must return a scalar variable,
1053 which is then used as the lvalue of the assignment (or whatever other
1054 lvalue operation is applied to the subroutine call). Unfortunately,
1055 because the subroutine has to return this variable before the
1056 assignment can take place, there is no way that a normal lvalue
1057 subroutine can get access to the value that will eventually be assigned
1058 to its return value.
1059 This is occasionally annoying, so the Contextual::Return module offers
1061 a solution: in addition to all the context blocks described above, it
1062 provides three special contextual return blocks specifically for use in
1063 lvalue subroutines: "LVALUE", "RVALUE", and "NVALUE".
1064 Using these blocks you can specify what happens when an lvalue
1066 subroutine is used in lvalue and non-lvalue (rvalue) context. For
1067 example:
1068 my $verbosity_level = 1;
1070 # Verbosity values must be between 0 and 5...
1072 sub verbosity :lvalue {
1073 LVALUE { $verbosity_level = max(0, min($_, 5)) }
1074 RVALUE { $verbosity_level }
1075 }
1076 The "LVALUE" block is executed whenever "verbosity" is called as an
1078 lvalue:
1079 verbosity() = 7;
1081 The block has access to the value being assigned, which is passed to it
1083 as $_. So, in the above example, the assigned value of 7 would be
1084 aliased to $_ within the "LVALUE" block, would be reduced to 5 by the
1085 "min-of-max" expression, and then assigned to $verbosity_level.
1086 (If you need to access the caller's $_, it's also still available: as
1088 $CALLER::_.)
1089 When the subroutine isn't used as an lvalue:
1091 print verbosity();
1093 the "RVALUE" block is executed instead and its final value returned.
1095 Within an "RVALUE" block you can use any of the other features of
1096 Contextual::Return. For example:
1097 sub verbosity :lvalue {
1099 LVALUE { $verbosity_level = int max(0, min($_, 5)) }
1100 RVALUE {
1101 NUM { $verbosity_level }
1102 STR { $description[$verbosity_level] }
1103 BOOL { $verbosity_level > 2 }
1104 }
1105 }
1106 but the context sequence must be nested inside an "RVALUE" block.
1108 You can also specify what an lvalue subroutine should do when it is
1110 used neither as an lvalue nor as an rvalue (i.e. in void context), by
1111 using an "NVALUE" block:
1112 sub verbosity :lvalue {
1114 my ($level) = @_;
1115 NVALUE { $verbosity_level = int max(0, min($level, 5)) }
1117 LVALUE { $verbosity_level = int max(0, min($_, 5)) }
1118 RVALUE {
1119 NUM { $verbosity_level }
1120 STR { $description[$verbosity_level] }
1121 BOOL { $verbosity_level > 2 }
1122 }
1123 }
1124 In this example, a call to verbosity() in void context sets the
1126 verbosity level to whatever argument is passed to the subroutine:
1127 verbosity(1);
1129 Note that you cannot get the same effect by nesting a "VOID" block
1131 within an "RVALUE" block:
1132 LVALUE { $verbosity_level = int max(0, min($_, 5)) }
1134 RVALUE {
1135 NUM { $verbosity_level }
1136 STR { $description[$verbosity_level] }
1137 BOOL { $verbosity_level > 2 }
1138 VOID { $verbosity_level = $level } # Wrong!
1139 }
1140 That's because, in a void context the return value is never evaluated,
1142 so it is never treated as an rvalue, which means the "RVALUE" block
1143 never executes.
1144 Result blocks
1146 Occasionally, it's convenient to calculate a return value before the
1147 end of a contextual return block. For example, you may need to clean up
1148 external resources involved in the calculation after it's complete.
1149 Typically, this requirement produces a slightly awkward code sequence
1150 like this:
1151 return
1153 VALUE {
1154 $db->start_work();
1155 my $result = $db->retrieve_query($query);
1156 $db->commit();
1157 $result;
1158 }
1159 Such code sequences become considerably more awkward when you want the
1161 return value to be context sensitive, in which case you have to write
1162 either:
1163 return
1165 LIST {
1166 $db->start_work();
1167 my @result = $db->retrieve_query($query);
1168 $db->commit();
1169 @result;
1170 }
1171 SCALAR {
1172 $db->start_work();
1173 my $result = $db->retrieve_query($query);
1174 $db->commit();
1175 $result;
1176 }
1177 or, worse:
1179 return
1181 VALUE {
1182 $db->start_work();
1183 my $result = LIST ? [$db->retrieve_query($query)]
1184 : $db->retrieve_query($query);
1185 $db->commit();
1186 LIST ? @{$result} : $result;
1187 }
1188 To avoid these infelicities, Contextual::Return provides a second way
1190 of setting the result of a context block; a way that doesn't require
1191 that the result be the last statement in the block:
1192 return
1194 LIST {
1195 $db->start_work();
1196 RESULT { $db->retrieve_query($query) };
1197 $db->commit();
1198 }
1199 SCALAR {
1200 $db->start_work();
1201 RESULT { $db->retrieve_query($query) };
1202 $db->commit();
1203 }
1204 The presence of a "RESULT" block inside a contextual return block
1206 causes that block to return the value of the final statement of the
1207 "RESULT" block as the handler's return value, rather than returning the
1208 value of the handler's own final statement. In other words, the
1209 presence of a "RESULT" block overrides the normal return value of a
1210 context handler.
1211 Better still, the "RESULT" block always evaluates its final statement
1213 in the same context as the surrounding "return", so you can just write:
1214 return
1216 VALUE {
1217 $db->start_work();
1218 RESULT { $db->retrieve_query($query) };
1219 $db->commit();
1220 }
1221 and the retrieve_query() method will be called in the appropriate
1223 context in all cases.
1224 A "RESULT" block can appear anywhere inside any contextual return
1226 block, but may not be used outside a context block. That is, this is an
1227 error:
1228 if ($db->closed) {
1230 RESULT { undef }; # Error: not in a context block
1231 }
1232 return
1233 VALUE {
1234 $db->start_work();
1235 RESULT { $db->retrieve_query($query) };
1236 $db->commit();
1237 }
1238 Post-handler clean-up
1240 If a subroutine uses an external resource, it's often necessary to
1241 close or clean-up that resource after the subroutine ends...regardless
1242 of whether the subroutine exits normally or via an exception.
1243 Typically, this is done by encapsulating the resource in a lexically
1245 scoped object whose destructor does the clean-up. However, if the
1246 clean-up doesn't involve deallocation of an object (as in the
1247 "$db->commit()" example in the previous section), it can be annoying to
1248 have to create a class and allocate a container object, merely to
1249 mediate the clean-up.
1250 To make it easier to manage such resources, Contextual::Return supplies
1252 a special labelled block: the "RECOVER" block. If a "RECOVER" block is
1253 specified as part of a contextual return sequence, that block is
1254 executed after any context handler, even if the context handler exits
1255 via an exception.
1256 So, for example, you could implement a simple commit-or-revert policy
1258 like so:
1259 return
1261 LIST { $db->retrieve_all($query) }
1262 SCALAR { $db->retrieve_next($query) }
1263 RECOVER {
1264 if ($@) {
1265 $db->revert();
1266 }
1267 else {
1268 $db->commit();
1269 }
1270 }
1271 The presence of a "RECOVER" block also intercepts all exceptions thrown
1273 in any other context block in the same contextual return sequence. Any
1274 such exception is passed into the "RECOVER" block in the usual manner:
1275 via the $@ variable. The exception may be rethrown out of the "RECOVER"
1276 block by calling "die":
1277 return
1279 LIST { $db->retrieve_all($query) }
1280 DEFAULT { croak "Invalid call (not in list context)" }
1281 RECOVER {
1282 die $@ if $@; # Propagate any exception
1283 $db->commit(); # Otherwise commit the changes
1284 }
1285 A "RECOVER" block can also access or replace the returned value, by
1287 invoking a "RESULT" block. For example:
1288 return
1290 LIST { attempt_to_generate_list_for(@_) }
1291 SCALAR { attempt_to_generate_count_for(@_) }
1292 RECOVER {
1293 if ($@) { # On any exception...
1294 warn "Replacing return value. Previously: ", RESULT;
1295 RESULT { undef } # ...return undef
1296 }
1297 }
1298 Post-return clean-up
1300 Occasionally it's necessary to defer the clean-up of resources until
1301 after the return value has been used. Once again, this is usually done
1302 by returning an object with a suitable destructor.
1303 Using Contextual::Return you can get the same effect, by providing a
1305 "CLEANUP" block in the contextual return sequence:
1306 return
1308 LIST { $db->retrieve_all($query) }
1309 SCALAR { $db->retrieve_next($query) }
1310 CLEANUP { $db->commit() }
1311 In this example, the "commit" method call is only performed after the
1313 return value has been used by the caller. Note that this is quite
1314 different from using a "RECOVER" block, which is called as the
1315 subroutine returns its value; a "CLEANUP" is called when the returned
1316 value is garbage collected.
1317 A "CLEANUP" block is useful for controlling resources allocated to
1319 support an "ACTIVE" return value. For example:
1320 my %file;
1322 # Return an active value that is always the next line from a file...
1324 sub readline_from {
1325 my ($file_name) = @_;
1326 # Open the file, if not already open...
1328 if (!$file{$file_name}) {
1329 open $file{$file_name}{handle}, '<', $file_name;
1330 }
1331 # Track how many active return values are using this file...
1333 $file{$file_name}{count}++;
1334 return ACTIVE
1336 # Evaluating the return value returns the next line...
1337 VALUE { readline $file{$file_name}{handle} }
1338 # Once the active value is finished with, clean up the filehandle...
1340 CLEANUP {
1341 delete $file{$file_name}
1342 if --$file{$file_name}{count} == 0;
1343 }
1344 }
1345 Debugging contextual return values
1347 Contextual return values are implemented as opaque objects (using the
1348 "inside-out" technique). This means that passing such values to
1349 Data::Dumper produces an uninformative output like:
1350 $VAR1 = bless( do{\(my $o = undef)}, 'Contextual::Return::Value' );
1352 So the module provides two methods that allow contextual return values
1354 to be correctly reported: either directly, or when dumped by
1355 Data::Dumper.
1356 To dump a contextual return value directly, call the module's DUMP()
1358 method explicitly and print the result:
1359 print $crv->Contextual::Return::DUMP();
1361 This produces an output something like:
1363 [
1365 { FROM => 'main::foo' },
1366 { NO_HANDLER => [ 'VOID', 'CODEREF', 'HASHREF', 'GLOBREF' ] },
1367 { FALLBACKS => [ 'VALUE' ] },
1368 { LIST => [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ] },
1369 { STR => '<<<Throws exception: Died at demo.pl line 7.>>>' },
1370 { NUM => 42 },
1371 { BOOL => -1 },
1372 { SCALARREF => '<<<self-reference>>>' },
1373 { ARRAYREF => [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ] },
1374 ];
1375 The "FROM" hash entry names the subroutine that produced the return
1377 value. The "NO_HANDLER" hash entry lists those contexts for which no
1378 handler was defined (and which would therefore normally produce "can't
1379 call" exceptions such as: "Can't call main::foo in VOID context"). The
1380 "FALLBACKS" hash entry lists any "generic" contexts such as "VALUE",
1381 "NONVOID", "REF", "DEFAULT", etc. that the contextual return value can
1382 also handle. After these, all the remaining hash entries are actual
1383 contexts in which the return value could successfully be evaluated, and
1384 the value it would produce in each of those contexts.
1385 The Data::Dumper module also has a mechanism by which you can tell it
1387 how to produce a similar listing automatically whenever a contextual
1388 return value is passed to its "Dumper" method. Data::Dumper allows you
1389 to register a "freezer" method, that is called prior to dumping, and
1390 which can be used to adapt an opaque object to make it dumpable.
1391 Contextual::Return provides just such a method
1392 (Contextual::Return::FREEZE()) for you to register, like so:
1393 use Data::Dumper 'Dumper';
1395 local $Data::Dumper::Freezer = 'Contextual::Return::FREEZE';
1397 print Dumper $foo;
1399 The output is then precisely the same as Contextual::Return::DUMP()
1401 would produce.
1402 Note that, with both of the above dumping mechanisms, it is essential
1404 to use the full name of the method. That is:
1405 print $crv->Contextual::Return::DUMP();
1407 rather than:
1409 print $crv->DUMP();
1411 This is because the shorter version is interpreted as calling the
1413 DUMP() method on the object returned by the return value's "OBJREF"
1414 context block (see "Scalar reference contexts")
1415 For the same reason, you must write:
1417 local $Data::Dumper::Freezer = 'Contextual::Return::FREEZE';
1419 not:
1421 local $Data::Dumper::Freezer = 'FREEZE';
1423 Namespace controls
1425 By default the module exports a large number of return context markers:
1426 DEFAULT REF LAZY
1428 VOID SCALARREF FIXED
1429 NONVOID ARRAYREF ACTIVE
1430 LIST CODEREF RESULT
1431 SCALAR HASHREF RECOVER
1432 VALUE GLOBREF CLEANUP
1433 STR OBJREF RVALUE
1434 NUM METHOD LVALUE
1435 BOOL NVALUE
1436 PUREBOOL
1437 These are exported as subroutines, and so can conflict with existing
1439 subroutines in your namespace, or with subroutines imported from other
1440 modules.
1441 Contextual::Return allows you to control which contextual return blocks
1443 are exported into any namespace that uses the module. It also allows
1444 you to rename blocks to avoid namespace conflicts with existing
1445 subroutines.
1446 Both these features are controlled by passing arguments to the "use"
1448 statement that loads the module as follows:
1449 • Any string passed as an argument to "use Contextual::Return",
1451 exports only the block name it specifies;
1452 • Any regex passed as an argument to "use Contextual::Return" exports
1454 every block name it matches;
1455 • Any array ref (recursively) exports each of its elements
1457 • Any string that appears immediately after one of the above three
1459 specifiers, and which is not itself a block name, renames the
1460 handlers exported by that preceding specifier by filtering each
1461 handler name through sprintf()
1462 That is, you can specify handlers to be exported by exact name (as a
1464 string), by general pattern (as a regex), or collectively (in an
1465 array). And after any of these export specifications, you can append a
1466 template in which any '%s' will be replaced by the original name of the
1467 handler. For example:
1468 # Selectively export specific sets of handlers...
1470 use Contextual::Return qr/[NLR]VALUE/;
1471 use Contextual::Return qr/.*REF/;
1472 # Selective export specific sets and add a suffix to each...
1474 use Contextual::Return qr/[NLR]VALUE/ => '%s_CONTEXT';
1475 # Selective export specific sets and add a prefix to each...
1477 use Contextual::Return qr/.*REF/ => 'CR_%s';
1478 # Export a list of handlers...
1480 use Contextual::Return 'NUM', 'STR', 'BOOL' ;
1481 use Contextual::Return qw< NUM STR BOOL >;
1482 use Contextual::Return ['NUM', 'STR', 'BOOL'];
1483 # Export a list of handlers, renaming them individually...
1485 use Contextual::Return NUM => 'NUMERIC', STR => 'TEXT', BOOL => 'CR_%s';
1486 # Export a list of handlers, renaming them collectively...
1488 use Contextual::Return ['NUM', 'STR', 'BOOL'] => '%s_CONTEXT';
1489 # Mixed exports and renames...
1491 use Contextual::Return (
1492 STR => 'TEXT',
1493 ['NUM', 'BOOL'] => 'CR_%s',
1494 ['LIST', 'SCALAR', 'VOID', qr/^[NLR]VALUE/] => '%s_CONTEXT',
1495 );
1496
1498 Context tests
1499 LIST()
1500 Returns true if the current subroutine was called in list context.
1501 A cleaner way of writing: wantarray()
1502 SCALAR()
1504 Returns true if the current subroutine was called in scalar
1505 context. A cleaner way of writing: "defined wantarray() && !
1506 wantarray()"
1507 VOID()
1509 Returns true if the current subroutine was called in void context.
1510 A cleaner way of writing: "!defined wantarray()"
1511 NONVOID()
1513 Returns true if the current subroutine was called in list or scalar
1514 context. A cleaner way of writing: "defined wantarray()"
1515 Standard contexts
1517 "LIST {...}"
1518 The block specifies what the context sequence should evaluate to
1519 when called in list context.
1520 "SCALAR {...}"
1522 The block specifies what the context sequence should evaluate to in
1523 scalar contexts, unless some more-specific specifier scalar context
1524 specifier (see below) also occurs in the same context sequence.
1525 "VOID {...}"
1527 The block specifies what the context sequence should do when called
1528 in void context.
1529 Scalar value contexts
1531 "BOOL {...}"
1532 The block specifies what the context sequence should evaluate to
1533 when treated as a boolean value.
1534 "NUM {...}"
1536 The block specifies what the context sequence should evaluate to
1537 when treated as a numeric value.
1538 "STR {...}"
1540 The block specifies what the context sequence should evaluate to
1541 when treated as a string value.
1542 "LAZY {...}"
1544 Another name for "SCALAR {...}". Usefully self-documenting when the
1545 primary purpose of the contextual return is to defer evaluation of
1546 the return value until it's actually required.
1547 Scalar reference contexts
1549 "SCALARREF {...}"
1550 The block specifies what the context sequence should evaluate to
1551 when treated as a reference to a scalar.
1552 "ARRAYREF {...}"
1554 The block specifies what the context sequence should evaluate to
1555 when treated as a reference to an array.
1556 "HASHREF {...}"
1558 The block specifies what the context sequence should evaluate to
1559 when treated as a reference to a hash.
1560 Note that a common error here is to write:
1562 HASHREF { a=>1, b=>2, c=>3 }
1564 The curly braces there are a block, not a hash constructor, so the
1566 block doesn't return a hash reference and the interpreter throws an
1567 exception. What's needed is:
1568 HASHREF { {a=>1, b=>2, c=>3} }
1570 in which the inner braces are a hash constructor.
1572 "CODEREF {...}"
1574 The block specifies what the context sequence should evaluate to
1575 when treated as a reference to a subroutine.
1576 "GLOBREF {...}"
1578 The block specifies what the context sequence should evaluate to
1579 when treated as a reference to a typeglob.
1580 "OBJREF {...}"
1582 The block specifies what the context sequence should evaluate to
1583 when treated as a reference to an object.
1584 "METHOD {...}"
1586 The block can be used to specify particular handlers for specific
1587 method calls when the return value is treated as an object
1588 reference. It should return a list of methodname/methodbody pairs.
1589 Each method name can be specified as a string, a regex, or an array
1590 of strings or regexes. The method bodies must be specified as
1591 subroutine references (usually anonymous subs). The first method
1592 name that matches the actual method call selects the corresponding
1593 handler, which is then called.
1594 Generic contexts
1596 "VALUE {...}"
1597 The block specifies what the context sequence should evaluate to
1598 when treated as a non-referential value (as a boolean, numeric,
1599 string, scalar, or list). Only used if there is no more-specific
1600 value context specifier in the context sequence.
1601 "REF {...}"
1603 The block specifies what the context sequence should evaluate to
1604 when treated as a reference of any kind. Only used if there is no
1605 more-specific referential context specifier in the context
1606 sequence.
1607 "NONVOID {...}"
1609 The block specifies what the context sequence should evaluate to
1610 when used in a non-void context of any kind. Only used if there is
1611 no more-specific context specifier in the context sequence.
1612 "DEFAULT {...}"
1614 The block specifies what the context sequence should evaluate to
1615 when used in a void or non-void context of any kind. Only used if
1616 there is no more-specific context specifier in the context
1617 sequence.
1618 Failure context
1620 "FAIL"
1621 This block is executed unconditionally and is used to indicate
1622 failure. In a Boolean context it return false. In all other
1623 contexts it throws an exception consisting of the final evaluated
1624 value of the block.
1625 That is, using "FAIL":
1627 return FAIL { "Could not defenestrate the widget" }
1629 is exactly equivalent to writing:
1631 return BOOL { 0 } DEFAULT { croak "Could not defenestrate the
1633 widget" }
1634 except that the reporting of errors is a little smarter under
1636 "FAIL".
1637 If "FAIL" is called without specifying a block:
1639 return FAIL;
1641 it is equivalent to:
1643 return FAIL { croak "Call to <subname> failed" }
1645 (where "<subname>" is replaced with the name of the surrounding
1647 subroutine).
1648 Note that, because "FAIL" implicitly covers every possible return
1650 context, it cannot be chained with other context specifiers.
1651 "Contextual::Return::FAIL_WITH"
1653 This subroutine is not exported, but may be called directly to
1654 reconfigure "FAIL" behaviour in the caller's namespace.
1655 The subroutine is called with an optional string (the flag),
1657 followed by a mandatory hash reference (the configurations hash),
1658 followed by a list of zero-or-more strings (the selector list). The
1659 values of the configurations hash must all be subroutine
1660 references.
1661 If the optional flag is specified, "FAIL_WITH" searches the
1663 selector list looking for that string, then uses the following item
1664 in the selector list as its selector value. If that selector value
1665 is a string, "FAIL_WITH" looks up that key in the hash, and
1666 installs the corresponding subroutine as the namespace's "FAIL"
1667 handler (an exception is thrown if the selector string is not a
1668 valid key of the configurations hash). If the selector value is a
1669 subroutine reference, "FAIL_WITH" installs that subroutine as the
1670 "FAIL" handler.
1671 If the optional flag is not specified, "FAIL_WITH" searches the
1673 entire selector list looking for the last element that matches any
1674 key in the configurations hash. It then looks up that key in the
1675 hash, and installs the corresponding subroutine as the namespace's
1676 "FAIL" handler.
1677 See "Configurable failure contexts" for examples of using this
1679 feature.
1680 Lvalue contexts
1682 "LVALUE"
1683 This block is executed when the result of an ":lvalue" subroutine
1684 is assigned to. The assigned value is passed to the block as $_. To
1685 access the caller's $_ value, use $CALLER::_.
1686 "RVALUE"
1688 This block is executed when the result of an ":lvalue" subroutine
1689 is used as an rvalue. The final value that is evaluated in the
1690 block becomes the rvalue.
1691 "NVALUE"
1693 This block is executed when an ":lvalue" subroutine is evaluated in
1694 void context.
1695 Explicit result blocks
1697 "RESULT"
1698 This block may only appear inside a context handler block. It
1699 causes the surrounding handler to return the final value of the
1700 "RESULT"'s block, rather than the final value of the handler's own
1701 block. This override occurs regardless of the location to the
1702 "RESULT" block within the handler.
1703 If called without a trailing "{...}", it simply returns the current
1705 result value in scalar contexts, or the list of result values in
1706 list context.
1707 Recovery blocks
1709 "RECOVER"
1710 If present in a context return sequence, this block grabs control
1711 after any context handler returns or exits via an exception. If an
1712 exception was thrown it is passed to the "RECOVER" block via the $@
1713 variable.
1714 Clean-up blocks
1716 "CLEANUP"
1717 If present in a context return sequence, this block grabs control
1718 when a return value is garbage collected.
1719 Modifiers
1721 "FIXED"
1722 This specifies that the scalar value will only be evaluated once,
1723 the first time it is used, and that the value will then morph into
1724 that evaluated value.
1725 "ACTIVE"
1727 This specifies that the scalar value's originating block will be
1728 re- evaluated every time the return value is used.
1729 Debugging support
1731 "$crv->Contextual::Return::DUMP()"
1732 Return a dumpable representation of the return value in all viable
1733 contexts.
1734 "local $Data::Dumper::Freezer = 'Contextual::Return::FREEZE';"
1736 "local $Data::Dumper::Freezer = \&Contextual::Return::FREEZE;"
1737 Configure Data::Dumper to correctly dump a representation of the
1738 contextual return value.
1739
1741 "Can't use %s as export specifier"
1742 In your "use Contextual::Return" statement you specified something
1743 (such as a hash or coderef) that can't be used to select what the
1744 module exports. Make sure the list of selectors includes only
1745 strings, regexes, or references to arrays of strings or regexes.
1746 "use Contextual::Return qr{%s} didn't export anything"
1748 In your "use Contextual::Return" statement you specified a regex to
1749 select which handlers to support, but the regex didn't select any
1750 handlers. Check that the regex you're using actually does match at
1751 least one of the names of the modules many handlers.
1752 "Can't export %s: no such handler"
1754 In your "use Contextual::Return" statement you specified a string
1755 as the name of a context handler to be exported, but the module
1756 doesn't export a handler of that name. Check the spelling for the
1757 requested export.
1758 "Can't call %s in a %s context"
1760 "Can't use return value of %s in a %s context"
1761 The subroutine you called uses a contextual return, but doesn't
1762 specify what to return in the particular context in which you
1763 called it. You either need to change the context in which you're
1764 calling the subroutine, or else add a context block corresponding
1765 to the offending context (or perhaps a "DEFAULT {...}" block).
1766 "Can't call bare %s {...} in %s context"
1768 You specified a handler (such as "VOID {...}" or "LIST {...}")
1769 outside any subroutine, and in a context that it can't handle. Did
1770 you mean to place the handler outside of a subroutine? If so, then
1771 you need to put it in a context it can actually handle. Otherwise,
1772 perhaps you need to replace the trailing block with parens (that
1773 is: VOID() or LIST()).
1774 "Call to %s at %s didn't return a %s reference""
1776 You called the subroutine in a context that expected to get back a
1777 reference of some kind but the subroutine didn't specify the
1778 corresponding "SCALARREF", "ARRAYREF", "HASHREF", "CODEREF",
1779 "GLOBREF", or generic "REF", "NONVOID", or "DEFAULT" handlers. You
1780 need to specify the appropriate one of these handlers in the
1781 subroutine.
1782 "Can't call method '%s' on %s value returned by %s""
1784 You called the subroutine and then tried to call a method on the
1785 return value, but the subroutine returned a classname or object
1786 that doesn't have that method. This probably means that the
1787 subroutine didn't return the classname or object you expected. Or
1788 perhaps you need to specify an "OBJREF {...}" context block.
1789 "Can't install two %s handlers"
1791 You attempted to specify two context blocks of the same name in the
1792 same return context, which is ambiguous. For example:
1793 sub foo: lvalue {
1795 LVALUE { $foo = $_ }
1796 RVALUE { $foo }
1797 LVALUE { $foo = substr($_,1,10) }
1798 }
1799 or:
1801 sub bar {
1803 return
1804 BOOL { 0 }
1805 NUM { 1 }
1806 STR { "two" }
1807 BOOL { 1 };
1808 }
1809 Did you cut-and-paste wrongly, or mislabel one of the blocks?
1811 "Expected a %s block after the %s block but found instead: %s"
1813 If you specify any of "LVALUE", "RVALUE", or "NVALUE", then you can
1814 only specify "LVALUE", "RVALUE", or "NVALUE" blocks in the same
1815 return context. If you need to specify other contexts (like
1816 "BOOL", or "STR", or "REF", etc.), put them inside an "RVALUE"
1817 block. See "Lvalue contexts" for an example.
1818 "Call to %s failed at %s"
1820 This is the default exception that a "FAIL" throws in a non-scalar
1821 context. Which means that the subroutine you called has signalled
1822 failure by throwing an exception, and you didn't catch that
1823 exception. You should either put the call in an "eval {...}" block
1824 or else call the subroutine in boolean context instead.
1825 "Call to %s failed at %s. Attempted to use failure value at %s"
1827 This is the default exception that a "FAIL" throws when a failure
1828 value is captured in a scalar variable and later used in a non-
1829 boolean context. That means that the subroutine you called must
1830 have failed, and you didn't check the return value for that
1831 failure, so when you tried to use that invalid value it killed your
1832 program. You should either put the original call in an "eval {...}"
1833 or else test the return value in a boolean context and avoid using
1834 it if it's false.
1835 "Usage: FAIL_WITH $flag_opt, \%selector, @args"
1837 The "FAIL_WITH" subroutine expects an optional flag, followed by a
1838 reference to a configuration hash, followed by a list or selector
1839 arguments. You gave it something else. See "Configurable Failure
1840 Contexts".
1841 "Selector values must be sub refs"
1843 You passed a configuration hash to "FAIL_WITH" that specified non-
1844 subroutines as possible "FAIL" handlers. Since non-subroutines
1845 can't possibly be handlers, maybe you forgot the "sub" keyword
1846 somewhere?
1847 "Invalid option: %s =" %s>
1849 The "FAIL_WITH" subroutine was passed a flag/selector pair, but the
1850 selector was not one of those allowed by the configuration hash.
1851 "FAIL handler for package %s redefined"
1853 A warning that the "FAIL" handler for a particular package was
1854 reconfigured more than once. Typically that's because the module
1855 was loaded in two places with difference configurations specified.
1856 You can't reasonably expect two different sets of behaviours from
1857 the one module within the one namespace.
1858
1860 Contextual::Return requires no configuration files or environment
1861 variables.
1862
1864 Requires version.pm and Want.pm.
1865
1867 "LVALUE", "RVALUE", and "NVALUE" do not work correctly under the Perl
1868 debugger. This seems to be because the debugger injects code to capture
1869 the return values from subroutines, which interferes destructively with
1870 the optional final arguments that allow "LVALUE", "RVALUE", and
1871 "NVALUE" to cascade within a single return.
1872
1874 No bugs have been reported.
1875
1877 Damian Conway "<DCONWAY@cpan.org>"
1878
1880 Copyright (c) 2005-2011, Damian Conway "<DCONWAY@cpan.org>". All rights
1881 reserved.
1882 This module is free software; you can redistribute it and/or modify it
1884 under the same terms as Perl itself.
1885
1887 BECAUSE THIS SOFTWARE IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY
1888 FOR THE SOFTWARE, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT
1889 WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER
1890 PARTIES PROVIDE THE SOFTWARE "AS IS" WITHOUT WARRANTY OF ANY KIND,
1891 EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
1892 WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE
1893 ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE SOFTWARE IS WITH
1894 YOU. SHOULD THE SOFTWARE PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL
1895 NECESSARY SERVICING, REPAIR, OR CORRECTION.
1896 IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
1898 WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR
1899 REDISTRIBUTE THE SOFTWARE AS PERMITTED BY THE ABOVE LICENCE, BE LIABLE
1900 TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL, OR
1901 CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE
1902 SOFTWARE (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING
1903 RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A
1904 FAILURE OF THE SOFTWARE TO OPERATE WITH ANY OTHER SOFTWARE), EVEN IF
1905 SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH
1906 DAMAGES.
1907perl v5.36.0 2023-01-20 Contextual::Return(3)