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 $sub()";
129 target('hello'); # "hello from Uplevel::target()"
133 This module lets you defer actions I<at run-time> that will take place when the control flow returns into an upper scope.
140 hook an upper scope end with L</reap> ;
144 localize variables, array/hash values or deletions of elements in higher contexts with respectively L</localize>, L</localize_elem> and L</localize_delete> ;
148 return values immediately to an upper level with L</unwind>, and know which context was in use then with L</want_at> ;
152 execute a subroutine in the setting of an upper subroutine stack frame with L</uplevel>.
158 In all those functions, C<$context> refers to the target scope.
160 You have to use one or a combination of L</WORDS> to build the C<$context> passed to these functions.
161 This is needed in order to ensure that the module still works when your program is ran in the debugger.
162 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.
168 XSLoader::load(__PACKAGE__, $VERSION);
171 =head2 C<reap $callback, $context>
173 Adds a destructor that calls C<$callback> (in void context) when the upper scope represented by C<$context> ends.
175 =head2 C<localize $what, $value, $context>
177 Introduces a C<local> delayed to the time of first return into the upper scope denoted by C<$context>.
184 A glob, in which case C<$value> can either be a glob or a reference.
185 L</localize> follows then the same syntax as C<local *x = $value>.
186 For example, if C<$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<$x> to C<1>.
190 A string beginning with a sigil, representing the symbol to localize and to assign to.
191 If the sigil is C<'$'>, L</localize> follows the same syntax as C<local $x = $value>, i.e. C<$value> isn't dereferenced.
194 localize '$x', \'foo' => HERE;
196 will set C<$x> to a reference to the string C<'foo'>.
197 Other sigils (C<'@'>, C<'%'>, C<'&'> and C<'*'>) require C<$value> to be a reference of the corresponding type.
199 When the symbol is given by a string, it is resolved when the actual localization takes place and not when L</localize> is called.
200 Thus, if the symbol name is not qualified, it will refer to the variable in the package where the localization actually takes place and not in the one where the L</localize> call was compiled.
205 sub new { localize '$tag', $_[0] => UP }
216 will localize C<$Tool::tag> and not C<$Scope::tag>.
217 If you want the other behaviour, you just have to specify C<$what> as a glob or a qualified name.
219 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.
220 This situation never arises with C<local> because it only compiles when the localized variable is already declared.
221 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.
225 =head2 C<localize_elem $what, $key, $value, $context>
227 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>.
228 Unlike L</localize>, C<$what> must be a string and the type of localization is inferred from its sigil.
229 The two only valid types are array and hash ; for anything besides those, L</localize_elem> will throw an exception.
230 C<$key> is either an array index or a hash key, depending of which kind of variable you localize.
232 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.
234 =head2 C<localize_delete $what, $key, $context>
236 Introduces the deletion of a variable or an array/hash element delayed to the time of first return into the upper scope denoted by C<$context>.
243 A glob, in which case C<$key> is ignored and the call is equivalent to C<local *x>.
247 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}>.
251 A string beginning with C<'&'>, which more or less does C<undef &func> in the upper scope.
252 It's actually more powerful, as C<&func> won't even C<exists> anymore.
257 =head2 C<unwind @values, $context>
259 Returns C<@values> I<from> the context pointed by C<$context>, i.e. from the subroutine, eval or format at or just above C<$context>, and immediately restart the program flow at this point - thus effectively returning to an upper scope.
261 The upper context isn't coerced onto C<@values>, which is hence always evaluated in list context.
265 my @a = ('a' .. 'z');
270 will set C<$num> to C<'z'>.
271 You can use L</want_at> to handle these cases.
273 =head2 C<want_at $context>
275 Like C<wantarray>, but for the subroutine/eval/format at or just above C<$context>.
277 The previous example can then be "corrected" :
280 my @a = ('a' .. 'z');
281 unwind +(want_at(HERE) ? @a : scalar @a) => HERE;
285 will rightfully set C<$num> to C<26>.
287 =head2 C<uplevel $code, @args, $context>
289 Executes the code reference C<$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.
290 The code is executed in the context of the C<uplevel> call, and what it returns is returned as-is by C<uplevel>.
302 my @inverses = target(1, 2, 4); # @inverses contains (0, 0.5, 0.25)
303 my $count = target(1, 2, 4); # $count is 3
305 L<Sub::Uplevel> also implements a pure-Perl version of C<uplevel>.
306 Both are identical, with the following caveats :
312 The L<Sub::Uplevel> implementation of C<uplevel> may execute a code reference in the context of B<any> upper stack frame.
313 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.
317 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.
325 uplevel { die 'wut' } CALLER(2); # for Scope::Upper
326 # uplevel(3, sub { die 'wut' }) # for Sub::Uplevel
329 print "inner block: $@";
333 print "outer block: $@";
335 will print "inner block: wut..." with L<Sub::Uplevel> and "outer block: wut..." with L<Scope::Upper>.
339 L<Sub::Uplevel> globally overrides the Perl keyword C<caller>, while L<Scope::Upper> does not.
343 A simple wrapper lets you mimic the interface of L<Sub::Uplevel/uplevel> :
350 my $cxt = Scope::Upper::CALLER($frame);
351 &Scope::Upper::uplevel($code => @_ => $cxt);
354 Albeit the three exceptions listed above, it passes all the tests of L<Sub::Uplevel>.
358 =head2 C<SU_THREADSAFE>
360 True iff the module could have been built when thread-safety features.
368 Returns the context that currently represents the highest scope.
372 The context of the current scope.
374 =head2 Getting a context from a context
376 For any of those functions, C<$from> is expected to be a context.
377 When omitted, it defaults to the the current context.
381 The context of the scope just above C<$from>.
385 The context of the closest subroutine above C<$from>.
386 Note that C<$from> is returned if it is already a subroutine context ; hence C<SUB SUB == SUB>.
390 The context of the closest eval above C<$from>.
391 Note that C<$from> is returned if it is already an eval context ; hence C<EVAL EVAL == EVAL>.
393 =head2 Getting a context from a level
395 Here, C<$level> should denote a number of scopes above the current one.
396 When omitted, it defaults to C<0> and those functions return the same context as L</HERE>.
398 =head3 C<SCOPE $level>
400 The C<$level>-th upper context, regardless of its type.
402 =head3 C<CALLER $level>
404 The context of the C<$level>-th upper subroutine/eval/format.
405 It kind of corresponds to the context represented by C<caller $level>, but while e.g. C<caller 0> refers to the caller context, C<CALLER 0> will refer to the top scope in the current context.
409 Where L</reap> fires depending on the C<$cxt> :
415 reap \&cleanup => $cxt;
417 } # $cxt = SCOPE(0), or HERE
419 }->(); # $cxt = SCOPE(1), or UP, or SUB, or CALLER, or CALLER(0)
421 }; # $cxt = SCOPE(2), or UP UP, or UP SUB, or EVAL, or CALLER(1)
423 }->(); # $cxt = SCOPE(3), or SUB UP SUB, or SUB EVAL, or CALLER(2)
426 Where L</localize>, L</localize_elem> and L</localize_delete> act depending on the C<$cxt> :
432 localize '$x' => 1 => $cxt;
433 # $cxt = SCOPE(0), or HERE
436 # $cxt = SCOPE(1), or UP, or SUB, or CALLER, or CALLER(0)
439 # $cxt = SCOPE(2), or UP UP, or UP SUB, or EVAL, or CALLER(1)
442 # $cxt = SCOPE(3), or SUB UP SUB, or SUB EVAL, or CALLER(2)
445 # $cxt = SCOPE(4), UP SUB UP SUB, or UP SUB EVAL, or UP CALLER(2), or TOP
448 Where L</unwind>, L</want_at> and L</uplevel> point to depending on the C<$cxt>:
454 unwind @things => $cxt; # or uplevel { ... } $cxt;
458 }->(); # $cxt = SCOPE(0 .. 1), or HERE, or UP, or SUB, or CALLER(0)
460 }; # $cxt = SCOPE(2), or UP UP, or UP SUB, or EVAL, or CALLER(1) (*)
462 }->(); # $cxt = SCOPE(3), or SUB UP SUB, or SUB EVAL, or CALLER(2)
465 # (*) Note that uplevel() will croak if you pass that scope frame,
466 # because it cannot target eval scopes.
470 The functions L</reap>, L</localize>, L</localize_elem>, L</localize_delete>, L</unwind>, L</want_at> and L</uplevel> are only exported on request, either individually or by the tags C<':funcs'> and C<':all'>.
472 The constant L</SU_THREADSAFE> is also only exported on request, individually or by the tags C<':consts'> and C<':all'>.
474 Same goes for the words L</TOP>, L</HERE>, L</UP>, L</SUB>, L</EVAL>, L</SCOPE> and L</CALLER> that are only exported on request, individually or by the tags C<':words'> and C<':all'>.
478 use base qw<Exporter>;
484 localize localize_elem localize_delete
488 words => [ qw<TOP HERE UP SUB EVAL SCOPE CALLER> ],
489 consts => [ qw<SU_THREADSAFE> ],
491 our @EXPORT_OK = map { @$_ } values %EXPORT_TAGS;
492 $EXPORT_TAGS{'all'} = [ @EXPORT_OK ];
496 Be careful that local variables are restored in the reverse order in which they were localized.
497 Consider those examples:
501 reap sub { print $x } => HERE;
509 reap sub { $x = 2 } => HERE;
514 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>.
516 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.
517 This is an hopeless case because C<BEGIN> blocks are executed once while localizing constructs should do their job at each run.
518 However, it's possible to hook the end of the current scope compilation with L<B::Hooks::EndOfScope>.
520 Some rare oddities may still happen when running inside the debugger.
521 It may help to use a perl higher than 5.8.9 or 5.10.0, as they contain some context-related fixes.
523 Calling C<goto> to replace an L</uplevel>'d code frame does not work when a custom runloop is used or when debugging flags are set with C<perl -D>.
524 In those two 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.
526 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.
527 Despite this shortcoming, this XS version of L</uplevel> should still run way faster than the pure-Perl version from L<Sub::Uplevel>.
531 L<XSLoader> (standard since perl 5.006).
535 L<perlfunc/local>, L<perlsub/"Temporary Values via local()">.
537 L<Alias>, L<Hook::Scope>, L<Scope::Guard>, L<Guard>.
541 L<Continuation::Escape> is a thin wrapper around L<Scope::Upper> that gives you a continuation passing style interface to L</unwind>.
542 It's easier to use, but it requires you to have control over the scope where you want to return.
548 Vincent Pit, C<< <perl at profvince.com> >>, L<http://www.profvince.com>.
550 You can contact me by mail or on C<irc.perl.org> (vincent).
554 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>.
555 I will be notified, and then you'll automatically be notified of progress on your bug as I make changes.
559 You can find documentation for this module with the perldoc command.
563 Tests code coverage report is available at L<http://www.profvince.com/perl/cover/Scope-Upper>.
565 =head1 ACKNOWLEDGEMENTS
567 Inspired by Ricardo Signes.
569 Thanks to Shawn M. Moore for motivation.
571 =head1 COPYRIGHT & LICENSE
573 Copyright 2008,2009,2010,2011 Vincent Pit, all rights reserved.
575 This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
579 1; # End of Scope::Upper