8 Scope::Upper - Act on upper scopes.
23 L</reap>, L</localize>, L</localize_elem>, L</localize_delete> and L</WORDS> :
27 use Scope::Upper qw<reap localize localize_elem localize_delete :words>;
30 my ($class, $name) = @_;
32 localize '$tag' => bless({ name => $name }, $class) => UP;
34 reap { print Scope->tag->name, ": end\n" } UP;
37 # Get the tag stored in the caller namespace
40 my $pkg = __PACKAGE__;
41 $pkg = caller $l++ while $pkg eq __PACKAGE__;
47 sub name { shift->{name} }
49 # Locally capture warnings and reprint them with the name prefixed
51 localize_elem '%SIG', '__WARN__' => sub {
52 print Scope->tag->name, ': ', @_;
58 for (reverse 0 .. $#INC) {
59 # First UP is the for loop, second is the sub boundary
60 localize_delete '@INC', $_ => UP UP;
69 Scope->new("top"); # initializes $UserLand::tag
73 my $one = 1 + undef; # prints "top: Use of uninitialized value..."
78 print $@; # prints "Can't locate Cwd.pm in @INC (@INC contains:) at..."
81 require Cwd; # loads Cwd.pm
84 } # prints "top: done"
86 L</unwind> and L</want_at> :
90 use Scope::Upper qw<unwind want_at :words>;
93 my @result = shift->();
94 my $cx = SUB UP; # Point to the sub above this one
95 unwind +(want_at($cx) ? @result : scalar @result) => $cx;
102 my @things = qw<a b c>;
103 return @things; # returns to try() and then outside zap()
109 my @stuff = zap(); # @stuff contains qw<a b c>
110 my $stuff = zap(); # $stuff contains 3
116 use Scope::Upper qw<uplevel CALLER>;
124 my $sub = (caller 0)[3];
125 print "$_[0] from $sub()";
129 target('hello'); # "hello from Uplevel::target()"
131 L</uid> and L</validate_uid> :
133 use Scope::Upper qw<uid validate_uid>;
140 if ($uid eq uid(UP)) { # yes
143 if (validate_uid($uid)) { # yes
149 if (validate_uid($uid)) { # no
155 This module lets you defer actions I<at run-time> that will take place when the control flow returns into an upper scope.
162 hook an upper scope end with L</reap> ;
166 localize variables, array/hash values or deletions of elements in higher contexts with respectively L</localize>, L</localize_elem> and L</localize_delete> ;
170 return values immediately to an upper level with L</unwind>, and know which context was in use then with L</want_at> ;
174 execute a subroutine in the setting of an upper subroutine stack frame with L</uplevel> ;
178 uniquely identify contextes with L</uid> and L</validate_uid>.
184 In all those functions, C<$context> refers to the target scope.
186 You have to use one or a combination of L</WORDS> to build the C<$context> passed to these functions.
187 This is needed in order to ensure that the module still works when your program is ran in the debugger.
188 The only thing you can assume is that it is an I<absolute> indicator of the frame, which means that you can safely store it at some point and use it when needed, and it will still denote the original scope.
194 XSLoader::load(__PACKAGE__, $VERSION);
197 =head2 C<reap $callback, $context>
199 Adds a destructor that calls C<$callback> (in void context) when the upper scope represented by C<$context> ends.
201 =head2 C<localize $what, $value, $context>
203 Introduces a C<local> delayed to the time of first return into the upper scope denoted by C<$context>.
210 A glob, in which case C<$value> can either be a glob or a reference.
211 L</localize> follows then the same syntax as C<local *x = $value>.
212 For example, if C<$value> is a scalar reference, then the C<SCALAR> slot of the glob will be set to C<$$value> - just like C<local *x = \1> sets C<$x> to C<1>.
216 A string beginning with a sigil, representing the symbol to localize and to assign to.
217 If the sigil is C<'$'>, L</localize> follows the same syntax as C<local $x = $value>, i.e. C<$value> isn't dereferenced.
220 localize '$x', \'foo' => HERE;
222 will set C<$x> to a reference to the string C<'foo'>.
223 Other sigils (C<'@'>, C<'%'>, C<'&'> and C<'*'>) require C<$value> to be a reference of the corresponding type.
225 When the symbol is given by a string, it is resolved when the actual localization takes place and not when L</localize> is called.
226 Thus, if the symbol name is not qualified, it will refer to the variable in the package where the localization actually takes place and not in the one where the L</localize> call was compiled.
231 sub new { localize '$tag', $_[0] => UP }
242 will localize C<$Tool::tag> and not C<$Scope::tag>.
243 If you want the other behaviour, you just have to specify C<$what> as a glob or a qualified name.
245 Note that if C<$what> is a string denoting a variable that wasn't declared beforehand, the relevant slot will be vivified as needed and won't be deleted from the glob when the localization ends.
246 This situation never arises with C<local> because it only compiles when the localized variable is already declared.
247 Although I believe it shouldn't be a problem as glob slots definedness is pretty much an implementation detail, this behaviour may change in the future if proved harmful.
251 =head2 C<localize_elem $what, $key, $value, $context>
253 Introduces a C<local $what[$key] = $value> or C<local $what{$key} = $value> delayed to the time of first return into the upper scope denoted by C<$context>.
254 Unlike L</localize>, C<$what> must be a string and the type of localization is inferred from its sigil.
255 The two only valid types are array and hash ; for anything besides those, L</localize_elem> will throw an exception.
256 C<$key> is either an array index or a hash key, depending of which kind of variable you localize.
258 If C<$what> is a string pointing to an undeclared variable, the variable will be vivified as soon as the localization occurs and emptied when it ends, although it will still exist in its glob.
260 =head2 C<localize_delete $what, $key, $context>
262 Introduces the deletion of a variable or an array/hash element delayed to the time of first return into the upper scope denoted by C<$context>.
269 A glob, in which case C<$key> is ignored and the call is equivalent to C<local *x>.
273 A string beginning with C<'@'> or C<'%'>, for which the call is equivalent to respectiveley C<local $a[$key]; delete $a[$key]> and C<local $h{$key}; delete $h{$key}>.
277 A string beginning with C<'&'>, which more or less does C<undef &func> in the upper scope.
278 It's actually more powerful, as C<&func> won't even C<exists> anymore.
283 =head2 C<unwind @values, $context>
285 Returns C<@values> I<from> the context pointed by C<$context>, i.e. from the subroutine, eval or format at or just above C<$context>, and immediately restart the program flow at this point - thus effectively returning to an upper scope.
287 The upper context isn't coerced onto C<@values>, which is hence always evaluated in list context.
291 my @a = ('a' .. 'z');
296 will set C<$num> to C<'z'>.
297 You can use L</want_at> to handle these cases.
299 =head2 C<want_at $context>
301 Like C<wantarray>, but for the subroutine/eval/format at or just above C<$context>.
303 The previous example can then be "corrected" :
306 my @a = ('a' .. 'z');
307 unwind +(want_at(HERE) ? @a : scalar @a) => HERE;
311 will rightfully set C<$num> to C<26>.
313 =head2 C<uplevel $code, @args, $context>
315 Executes the code reference C<$code> with arguments C<@args> as if it were located at the subroutine stack frame pointed by C<$context>, effectively fooling C<caller> and C<die> into believing that the call actually happened higher in the stack.
316 The code is executed in the context of the C<uplevel> call, and what it returns is returned as-is by C<uplevel>.
328 my @inverses = target(1, 2, 4); # @inverses contains (0, 0.5, 0.25)
329 my $count = target(1, 2, 4); # $count is 3
331 L<Sub::Uplevel> also implements a pure-Perl version of C<uplevel>.
332 Both are identical, with the following caveats :
338 The L<Sub::Uplevel> implementation of C<uplevel> may execute a code reference in the context of B<any> upper stack frame.
339 The L<Scope::Upper> version can only uplevel to a B<subroutine> stack frame, and will croak if you try to target an C<eval> or a format.
343 Exceptions thrown from the code called by this version of C<uplevel> will not be caught by C<eval> blocks between the target frame and the uplevel call, while they will for L<Sub::Uplevel>'s version.
351 uplevel { die 'wut' } CALLER(2); # for Scope::Upper
352 # uplevel(3, sub { die 'wut' }) # for Sub::Uplevel
355 print "inner block: $@";
359 print "outer block: $@";
361 will print "inner block: wut..." with L<Sub::Uplevel> and "outer block: wut..." with L<Scope::Upper>.
365 L<Sub::Uplevel> globally overrides the Perl keyword C<caller>, while L<Scope::Upper> does not.
369 A simple wrapper lets you mimic the interface of L<Sub::Uplevel/uplevel> :
376 my $cxt = Scope::Upper::CALLER($frame);
377 &Scope::Upper::uplevel($code => @_ => $cxt);
380 Albeit the three exceptions listed above, it passes all the tests of L<Sub::Uplevel>.
382 =head2 C<uid $context>
384 Returns an unique identifier (UID) for the context (or dynamic scope) pointed by C<$context>, or for the current context if C<$context> is omitted.
385 This UID will only be valid for the life time of the context it represents, and another UID will be generated next time the same scope is executed.
391 if ($uid eq uid()) { # yes, this is the same context
395 if ($uid eq uid()) { # no, we are one scope below
398 if ($uid eq uid(UP)) { # yes, UP points to the same scope as $uid
404 # $uid is now invalid
407 if ($uid eq uid()) { # no, this is another block
412 For example, each loop iteration gets its own UID :
421 # %uids has 5 entries
423 The UIDs are not guaranteed to be numbers, so you must use the C<eq> operator to compare them.
425 To check whether a given UID is valid, you can use the L</validate_uid> function.
427 =head2 C<validate_uid $uid>
429 Returns true if and only if C<$uid> is the UID of a currently valid context (that is, it designates a scope that is higher than the current one in the call stack).
435 if (validate_uid($uid)) { # yes
439 if (validate_uid($uid)) { # yes
445 if (validate_uid($uid)) { # no
451 =head2 C<SU_THREADSAFE>
453 True iff the module could have been built when thread-safety features.
461 Returns the context that currently represents the highest scope.
465 The context of the current scope.
467 =head2 Getting a context from a context
469 For any of those functions, C<$from> is expected to be a context.
470 When omitted, it defaults to the the current context.
474 The context of the scope just above C<$from>.
478 The context of the closest subroutine above C<$from>.
479 Note that C<$from> is returned if it is already a subroutine context ; hence C<SUB SUB == SUB>.
483 The context of the closest eval above C<$from>.
484 Note that C<$from> is returned if it is already an eval context ; hence C<EVAL EVAL == EVAL>.
486 =head2 Getting a context from a level
488 Here, C<$level> should denote a number of scopes above the current one.
489 When omitted, it defaults to C<0> and those functions return the same context as L</HERE>.
491 =head3 C<SCOPE $level>
493 The C<$level>-th upper context, regardless of its type.
495 =head3 C<CALLER $level>
497 The context of the C<$level>-th upper subroutine/eval/format.
498 It kind of corresponds to the context represented by C<caller $level>, but while e.g. C<caller 0> refers to the caller context, C<CALLER 0> will refer to the top scope in the current context.
502 Where L</reap> fires depending on the C<$cxt> :
508 reap \&cleanup => $cxt;
510 } # $cxt = SCOPE(0), or HERE
512 }->(); # $cxt = SCOPE(1), or UP, or SUB, or CALLER, or CALLER(0)
514 }; # $cxt = SCOPE(2), or UP UP, or UP SUB, or EVAL, or CALLER(1)
516 }->(); # $cxt = SCOPE(3), or SUB UP SUB, or SUB EVAL, or CALLER(2)
519 Where L</localize>, L</localize_elem> and L</localize_delete> act depending on the C<$cxt> :
525 localize '$x' => 1 => $cxt;
526 # $cxt = SCOPE(0), or HERE
529 # $cxt = SCOPE(1), or UP, or SUB, or CALLER, or CALLER(0)
532 # $cxt = SCOPE(2), or UP UP, or UP SUB, or EVAL, or CALLER(1)
535 # $cxt = SCOPE(3), or SUB UP SUB, or SUB EVAL, or CALLER(2)
538 # $cxt = SCOPE(4), UP SUB UP SUB, or UP SUB EVAL, or UP CALLER(2), or TOP
541 Where L</unwind>, L</want_at> and L</uplevel> point to depending on the C<$cxt>:
547 unwind @things => $cxt; # or uplevel { ... } $cxt;
551 }->(); # $cxt = SCOPE(0 .. 1), or HERE, or UP, or SUB, or CALLER(0)
553 }; # $cxt = SCOPE(2), or UP UP, or UP SUB, or EVAL, or CALLER(1) (*)
555 }->(); # $cxt = SCOPE(3), or SUB UP SUB, or SUB EVAL, or CALLER(2)
558 # (*) Note that uplevel() will croak if you pass that scope frame,
559 # because it cannot target eval scopes.
563 The functions L</reap>, L</localize>, L</localize_elem>, L</localize_delete>, L</unwind>, L</want_at> and L</uplevel> are only exported on request, either individually or by the tags C<':funcs'> and C<':all'>.
565 The constant L</SU_THREADSAFE> is also only exported on request, individually or by the tags C<':consts'> and C<':all'>.
567 Same goes for the words L</TOP>, L</HERE>, L</UP>, L</SUB>, L</EVAL>, L</SCOPE> and L</CALLER> that are only exported on request, individually or by the tags C<':words'> and C<':all'>.
571 use base qw<Exporter>;
577 localize localize_elem localize_delete
582 words => [ qw<TOP HERE UP SUB EVAL SCOPE CALLER> ],
583 consts => [ qw<SU_THREADSAFE> ],
585 our @EXPORT_OK = map { @$_ } values %EXPORT_TAGS;
586 $EXPORT_TAGS{'all'} = [ @EXPORT_OK ];
590 Be careful that local variables are restored in the reverse order in which they were localized.
591 Consider those examples:
595 reap sub { print $x } => HERE;
603 reap sub { $x = 2 } => HERE;
608 The first case is "solved" by moving the C<local> before the C<reap>, and the second by using L</localize> instead of L</reap>.
610 The effects of L</reap>, L</localize> and L</localize_elem> can't cross C<BEGIN> blocks, hence calling those functions in C<import> is deemed to be useless.
611 This is an hopeless case because C<BEGIN> blocks are executed once while localizing constructs should do their job at each run.
612 However, it's possible to hook the end of the current scope compilation with L<B::Hooks::EndOfScope>.
614 Some rare oddities may still happen when running inside the debugger.
615 It may help to use a perl higher than 5.8.9 or 5.10.0, as they contain some context-related fixes.
617 Calling C<goto> to replace an L</uplevel>'d code frame does not work :
623 for a C<perl> older than the 5.8 series ;
627 for a C<DEBUGGING> C<perl> run with debugging flags set (as in C<perl -D ...>) ;
631 when the runloop callback is replaced by another module.
635 In those three cases, L</uplevel> will look for a C<goto &sub> statement in its callback and, if there is one, throw an exception before executing the code.
637 Moreover, in order to handle C<goto> statements properly, L</uplevel> currently has to suffer a run-time overhead proportional to the size of the the callback in every case (with a small ratio), and proportional to the size of B<all> the code executed as the result of the L</uplevel> call (including subroutine calls inside the callback) when a C<goto> statement is found in the L</uplevel> callback.
638 Despite this shortcoming, this XS version of L</uplevel> should still run way faster than the pure-Perl version from L<Sub::Uplevel>.
642 L<XSLoader> (standard since perl 5.006).
646 L<perlfunc/local>, L<perlsub/"Temporary Values via local()">.
648 L<Alias>, L<Hook::Scope>, L<Scope::Guard>, L<Guard>.
652 L<Continuation::Escape> is a thin wrapper around L<Scope::Upper> that gives you a continuation passing style interface to L</unwind>.
653 It's easier to use, but it requires you to have control over the scope where you want to return.
659 Vincent Pit, C<< <perl at profvince.com> >>, L<http://www.profvince.com>.
661 You can contact me by mail or on C<irc.perl.org> (vincent).
665 Please report any bugs or feature requests to C<bug-scope-upper at rt.cpan.org>, or through the web interface at L<http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Scope-Upper>.
666 I will be notified, and then you'll automatically be notified of progress on your bug as I make changes.
670 You can find documentation for this module with the perldoc command.
674 Tests code coverage report is available at L<http://www.profvince.com/perl/cover/Scope-Upper>.
676 =head1 ACKNOWLEDGEMENTS
678 Inspired by Ricardo Signes.
680 Thanks to Shawn M. Moore for motivation.
682 =head1 COPYRIGHT & LICENSE
684 Copyright 2008,2009,2010,2011 Vincent Pit, all rights reserved.
686 This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
690 1; # End of Scope::Upper