8 Scope::Upper - Act on upper scopes.
23 L</reap>, L</localize>, L</localize_elem>, L</localize_delete> and L</WORDS> :
28 reap localize localize_elem localize_delete
33 my ($class, $name) = @_;
35 localize '$tag' => bless({ name => $name }, $class) => UP;
37 reap { print Scope->tag->name, ": end\n" } UP;
40 # Get the tag stored in the caller namespace
43 my $pkg = __PACKAGE__;
44 $pkg = caller $l++ while $pkg eq __PACKAGE__;
50 sub name { shift->{name} }
52 # Locally capture warnings and reprint them with the name prefixed
54 localize_elem '%SIG', '__WARN__' => sub {
55 print Scope->tag->name, ': ', @_;
61 for (reverse 0 .. $#INC) {
62 # First UP is the for loop, second is the sub boundary
63 localize_delete '@INC', $_ => UP UP;
72 Scope->new("top"); # initializes $UserLand::tag
76 my $one = 1 + undef; # prints "top: Use of uninitialized value..."
81 print $@; # prints "Can't locate Cwd.pm in @INC
82 } # (@INC contains:) at..."
84 require Cwd; # loads Cwd.pm
87 } # prints "top: done"
89 L</unwind> and L</want_at> :
93 use Scope::Upper qw<unwind want_at :words>;
96 my @result = shift->();
97 my $cx = SUB UP; # Point to the sub above this one
98 unwind +(want_at($cx) ? @result : scalar @result) => $cx;
105 my @things = qw<a b c>;
106 return @things; # returns to try() and then outside zap()
112 my @stuff = zap(); # @stuff contains qw<a b c>
113 my $stuff = zap(); # $stuff contains 3
119 use Scope::Upper qw<uplevel CALLER>;
127 my $sub = (caller 0)[3];
128 print "$_[0] from $sub()";
132 target('hello'); # "hello from Uplevel::target()"
134 L</uid> and L</validate_uid> :
136 use Scope::Upper qw<uid validate_uid>;
143 if ($uid eq uid(UP)) { # yes
146 if (validate_uid($uid)) { # yes
152 if (validate_uid($uid)) { # no
158 This module lets you defer actions I<at run-time> that will take place when the control flow returns into an upper scope.
165 hook an upper scope end with L</reap> ;
169 localize variables, array/hash values or deletions of elements in higher contexts with respectively L</localize>, L</localize_elem> and L</localize_delete> ;
173 return values immediately to an upper level with L</unwind>, L</yield> and L</leave>, and know which context was in use then with L</want_at> ;
177 execute a subroutine in the setting of an upper subroutine stack frame with L</uplevel> ;
181 uniquely identify contextes with L</uid> and L</validate_uid>.
187 In all those functions, C<$context> refers to the target scope.
189 You have to use one or a combination of L</WORDS> to build the C<$context> passed to these functions.
190 This is needed in order to ensure that the module still works when your program is ran in the debugger.
191 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.
197 XSLoader::load(__PACKAGE__, $VERSION);
203 reap { ... } $context;
204 &reap($callback, $context);
206 Adds a destructor that calls C<$callback> (in void context) when the upper scope represented by C<$context> ends.
210 localize $what, $value;
211 localize $what, $value, $context;
213 Introduces a C<local> delayed to the time of first return into the upper scope denoted by C<$context>.
220 A glob, in which case C<$value> can either be a glob or a reference.
221 L</localize> follows then the same syntax as C<local *x = $value>.
222 For example, if C<$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<$x> to C<1>.
226 A string beginning with a sigil, representing the symbol to localize and to assign to.
227 If the sigil is C<'$'>, L</localize> follows the same syntax as C<local $x = $value>, i.e. C<$value> isn't dereferenced.
230 localize '$x', \'foo' => HERE;
232 will set C<$x> to a reference to the string C<'foo'>.
233 Other sigils (C<'@'>, C<'%'>, C<'&'> and C<'*'>) require C<$value> to be a reference of the corresponding type.
235 When the symbol is given by a string, it is resolved when the actual localization takes place and not when L</localize> is called.
236 Thus, if the symbol name is not qualified, it will refer to the variable in the package where the localization actually takes place and not in the one where the L</localize> call was compiled.
241 sub new { localize '$tag', $_[0] => UP }
252 will localize C<$Tool::tag> and not C<$Scope::tag>.
253 If you want the other behaviour, you just have to specify C<$what> as a glob or a qualified name.
255 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.
256 This situation never arises with C<local> because it only compiles when the localized variable is already declared.
257 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.
261 =head2 C<localize_elem>
263 localize_elem $what, $key, $value;
264 localize_elem $what, $key, $value, $context;
266 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>.
267 Unlike L</localize>, C<$what> must be a string and the type of localization is inferred from its sigil.
268 The two only valid types are array and hash ; for anything besides those, L</localize_elem> will throw an exception.
269 C<$key> is either an array index or a hash key, depending of which kind of variable you localize.
271 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.
273 =head2 C<localize_delete>
275 localize_delete $what, $key;
276 localize_delete $what, $key, $context;
278 Introduces the deletion of a variable or an array/hash element delayed to the time of first return into the upper scope denoted by C<$context>.
285 A glob, in which case C<$key> is ignored and the call is equivalent to C<local *x>.
289 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}>.
293 A string beginning with C<'&'>, which more or less does C<undef &func> in the upper scope.
294 It's actually more powerful, as C<&func> won't even C<exists> anymore.
302 unwind @values, $context;
304 Returns C<@values> I<from> the subroutine, eval or format context pointed by or just above C<$context>, and immediately restart the program flow at this point - thus effectively returning C<@values> to an upper scope.
305 If C<@values> is empty, then the C<$context> parameter is optional and defaults to the current context (making the call equivalent to a bare C<return;>) ; otherwise it is mandatory.
307 The upper context isn't coerced onto C<@values>, which is hence always evaluated in list context.
311 my @a = ('a' .. 'z');
316 will set C<$num> to C<'z'>.
317 You can use L</want_at> to handle these cases.
322 yield @values, $context;
324 Returns C<@values> I<from> the context pointed by or just above C<$context>, and immediately restart the program flow at this point.
325 If C<@values> is empty, then the C<$context> parameter is optional and defaults to the current context ; otherwise it is mandatory.
327 L</yield> differs from L</unwind> in that it can target I<any> upper scope (besides a C<s///e> substitution context) and not necessarily a sub, an eval or a format.
328 Hence you can use it to return values from a C<do> or a C<map> block :
332 eval { require Time::HiRes } or yield time() => HERE;
337 yield if $seen{$_}++; # returns the empty list from the block
341 Like for L</unwind>, the upper context isn't coerced onto C<@values>.
348 Immediately returns C<@values> from the current block, whatever it may be (besides a C<s///e> substitution context).
349 C<leave> is actually a synonym for C<unwind HERE>, while C<leave @values> is a synonym for C<yield @values, HERE>.
354 my $want = want_at $context;
356 Like L<perlfunc/wantarray>, but for the subroutine, eval or format context located at or just above C<$context>.
358 It can be used to revise the example showed in L</unwind> :
361 my @a = ('a' .. 'z');
362 unwind +(want_at(HERE) ? @a : scalar @a) => HERE;
366 will rightfully set C<$num> to C<26>.
370 my @ret = uplevel { ...; return @ret };
371 my @ret = uplevel { my @args = @_; ...; return @ret } @args, $context;
372 my @ret = &uplevel($callback, @args, $context);
374 Executes the code reference C<$callback> 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.
375 The code is executed in the context of the C<uplevel> call, and what it returns is returned as-is by C<uplevel>.
387 my @inverses = target(1, 2, 4); # @inverses contains (0, 0.5, 0.25)
388 my $count = target(1, 2, 4); # $count is 3
390 Note that if C<@args> is empty, then the C<$context> parameter is optional and defaults to the current context ; otherwise it is mandatory.
392 L<Sub::Uplevel> also implements a pure-Perl version of C<uplevel>.
393 Both are identical, with the following caveats :
399 The L<Sub::Uplevel> implementation of C<uplevel> may execute a code reference in the context of B<any> upper stack frame.
400 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.
404 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.
412 uplevel { die 'wut' } CALLER(2); # for Scope::Upper
413 # uplevel(3, sub { die 'wut' }) # for Sub::Uplevel
416 print "inner block: $@";
420 print "outer block: $@";
422 will print "inner block: wut..." with L<Sub::Uplevel> and "outer block: wut..." with L<Scope::Upper>.
426 L<Sub::Uplevel> globally overrides the Perl keyword C<caller>, while L<Scope::Upper> does not.
430 A simple wrapper lets you mimic the interface of L<Sub::Uplevel/uplevel> :
437 my $cxt = Scope::Upper::CALLER($frame);
438 &Scope::Upper::uplevel($code => @_ => $cxt);
441 Albeit the three exceptions listed above, it passes all the tests of L<Sub::Uplevel>.
446 my $uid = uid $context;
448 Returns an unique identifier (UID) for the context (or dynamic scope) pointed by C<$context>, or for the current context if C<$context> is omitted.
449 This UID will only be valid for the life time of the context it represents, and another UID will be generated next time the same scope is executed.
455 if ($uid eq uid()) { # yes, this is the same context
459 if ($uid eq uid()) { # no, we are one scope below
462 if ($uid eq uid(UP)) { # yes, UP points to the same scope as $uid
468 # $uid is now invalid
471 if ($uid eq uid()) { # no, this is another block
476 For example, each loop iteration gets its own UID :
485 # %uids has 5 entries
487 The UIDs are not guaranteed to be numbers, so you must use the C<eq> operator to compare them.
489 To check whether a given UID is valid, you can use the L</validate_uid> function.
491 =head2 C<validate_uid>
493 my $is_valid = validate_uid $uid;
495 Returns true if and only if C<$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).
501 if (validate_uid($uid)) { # yes
505 if (validate_uid($uid)) { # yes
511 if (validate_uid($uid)) { # no
517 =head2 C<SU_THREADSAFE>
519 True iff the module could have been built when thread-safety features.
527 my $top_context = TOP;
529 Returns the context that currently represents the highest scope.
533 my $current_context = HERE;
535 The context of the current scope.
537 =head2 Getting a context from a context
539 For any of those functions, C<$from> is expected to be a context.
540 When omitted, it defaults to the the current context.
544 my $upper_context = UP;
545 my $upper_context = UP $from;
547 The context of the scope just above C<$from>.
551 my $sub_context = SUB;
552 my $sub_context = SUB $from;
554 The context of the closest subroutine above C<$from>.
555 Note that C<$from> is returned if it is already a subroutine context ; hence C<SUB SUB == SUB>.
559 my $eval_context = EVAL;
560 my $eval_context = EVAL $from;
562 The context of the closest eval above C<$from>.
563 Note that C<$from> is returned if it is already an eval context ; hence C<EVAL EVAL == EVAL>.
565 =head2 Getting a context from a level
567 Here, C<$level> should denote a number of scopes above the current one.
568 When omitted, it defaults to C<0> and those functions return the same context as L</HERE>.
573 my $context = SCOPE $level;
575 The C<$level>-th upper context, regardless of its type.
579 my $context = CALLER;
580 my $context = CALLER $level;
582 The context of the C<$level>-th upper subroutine/eval/format.
583 It kind of corresponds to the context represented by C<caller $level>, but while e.g. C<caller 0> refers to the caller context, C<CALLER 0> will refer to the top scope in the current context.
587 Where L</reap> fires depending on the C<$cxt> :
593 reap \&cleanup => $cxt;
595 } # $cxt = SCOPE(0) = HERE
597 }->(); # $cxt = SCOPE(1) = UP = SUB = CALLER(0)
599 }; # $cxt = SCOPE(2) = UP UP = UP SUB = EVAL = CALLER(1)
601 }->(); # $cxt = SCOPE(3) = SUB UP SUB = SUB EVAL = CALLER(2)
604 Where L</localize>, L</localize_elem> and L</localize_delete> act depending on the C<$cxt> :
610 localize '$x' => 1 => $cxt;
611 # $cxt = SCOPE(0) = HERE
614 # $cxt = SCOPE(1) = UP = SUB = CALLER(0)
617 # $cxt = SCOPE(2) = UP UP = UP SUB = EVAL = CALLER(1)
620 # $cxt = SCOPE(3) = SUB UP SUB = SUB EVAL = CALLER(2)
623 # $cxt = SCOPE(4), UP SUB UP SUB = UP SUB EVAL = UP CALLER(2) = TOP
626 Where L</unwind>, L</yield>, L</want_at> and L</uplevel> point to depending on the C<$cxt>:
632 unwind @things => $cxt; # or yield @things => $cxt
633 # or uplevel { ... } $cxt
637 }->(); # $cxt = SCOPE(0) = SCOPE(1) = HERE = UP = SUB = CALLER(0)
639 }; # $cxt = SCOPE(2) = UP UP = UP SUB = EVAL = CALLER(1) (*)
641 }->(); # $cxt = SCOPE(3) = SUB UP SUB = SUB EVAL = CALLER(2)
644 # (*) Note that uplevel() will croak if you pass that scope frame,
645 # because it cannot target eval scopes.
649 The functions L</reap>, L</localize>, L</localize_elem>, L</localize_delete>, L</unwind>, L</yield>, L</leave>, L</want_at> and L</uplevel> are only exported on request, either individually or by the tags C<':funcs'> and C<':all'>.
651 The constant L</SU_THREADSAFE> is also only exported on request, individually or by the tags C<':consts'> and C<':all'>.
653 Same goes for the words L</TOP>, L</HERE>, L</UP>, L</SUB>, L</EVAL>, L</SCOPE> and L</CALLER> that are only exported on request, individually or by the tags C<':words'> and C<':all'>.
657 use base qw<Exporter>;
663 localize localize_elem localize_delete
669 words => [ qw<TOP HERE UP SUB EVAL SCOPE CALLER> ],
670 consts => [ qw<SU_THREADSAFE> ],
672 our @EXPORT_OK = map { @$_ } values %EXPORT_TAGS;
673 $EXPORT_TAGS{'all'} = [ @EXPORT_OK ];
677 Be careful that local variables are restored in the reverse order in which they were localized.
678 Consider those examples:
682 reap sub { print $x } => HERE;
690 reap sub { $x = 2 } => HERE;
695 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>.
697 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.
698 This is an hopeless case because C<BEGIN> blocks are executed once while localizing constructs should do their job at each run.
699 However, it's possible to hook the end of the current scope compilation with L<B::Hooks::EndOfScope>.
701 Some rare oddities may still happen when running inside the debugger.
702 It may help to use a perl higher than 5.8.9 or 5.10.0, as they contain some context-related fixes.
704 Calling C<goto> to replace an L</uplevel>'d code frame does not work :
710 for a C<perl> older than the 5.8 series ;
714 for a C<DEBUGGING> C<perl> run with debugging flags set (as in C<perl -D ...>) ;
718 when the runloop callback is replaced by another module.
722 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.
724 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.
725 Despite this shortcoming, this XS version of L</uplevel> should still run way faster than the pure-Perl version from L<Sub::Uplevel>.
732 This module may happen to build with a C++ compiler as well, but don't rely on it, as no guarantee is made in this regard.
734 L<XSLoader> (core since perl 5.006).
738 L<perlfunc/local>, L<perlsub/"Temporary Values via local()">.
740 L<Alias>, L<Hook::Scope>, L<Scope::Guard>, L<Guard>.
744 L<Continuation::Escape> is a thin wrapper around L<Scope::Upper> that gives you a continuation passing style interface to L</unwind>.
745 It's easier to use, but it requires you to have control over the scope where you want to return.
751 Vincent Pit, C<< <perl at profvince.com> >>, L<http://www.profvince.com>.
753 You can contact me by mail or on C<irc.perl.org> (vincent).
757 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>.
758 I will be notified, and then you'll automatically be notified of progress on your bug as I make changes.
762 You can find documentation for this module with the perldoc command.
766 Tests code coverage report is available at L<http://www.profvince.com/perl/cover/Scope-Upper>.
768 =head1 ACKNOWLEDGEMENTS
770 Inspired by Ricardo Signes.
772 Thanks to Shawn M. Moore for motivation.
774 =head1 COPYRIGHT & LICENSE
776 Copyright 2008,2009,2010,2011,2012 Vincent Pit, all rights reserved.
778 This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
782 1; # End of Scope::Upper