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(); # $stuff contains 3
114 This module lets you defer actions I<at run-time> that will take place when the control flow returns into an upper scope.
121 hook an upper scope end with L</reap> ;
125 localize variables, array/hash values or deletions of elements in higher contexts with respectively L</localize>, L</localize_elem> and L</localize_delete> ;
129 return values immediately to an upper level with L</unwind>, and know which context was in use then with L</want_at>.
135 In all those functions, C<$context> refers to the target scope.
137 You have to use one or a combination of L</WORDS> to build the C<$context> passed to these functions.
138 This is needed in order to ensure that the module still works when your program is ran in the debugger.
139 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.
145 XSLoader::load(__PACKAGE__, $VERSION);
148 =head2 C<reap $callback, $context>
150 Adds a destructor that calls C<$callback> (in void context) when the upper scope represented by C<$context> ends.
152 =head2 C<localize $what, $value, $context>
154 Introduces a C<local> delayed to the time of first return into the upper scope denoted by C<$context>.
161 A glob, in which case C<$value> can either be a glob or a reference.
162 L</localize> follows then the same syntax as C<local *x = $value>.
163 For example, if C<$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<$x> to C<1>.
167 A string beginning with a sigil, representing the symbol to localize and to assign to.
168 If the sigil is C<'$'>, L</localize> follows the same syntax as C<local $x = $value>, i.e. C<$value> isn't dereferenced.
171 localize '$x', \'foo' => HERE;
173 will set C<$x> to a reference to the string C<'foo'>.
174 Other sigils (C<'@'>, C<'%'>, C<'&'> and C<'*'>) require C<$value> to be a reference of the corresponding type.
176 When the symbol is given by a string, it is resolved when the actual localization takes place and not when C<localize> is called.
177 Thus, if the symbol name is not qualified, it will refer to the variable in the package where the localization actually takes place and not in the one where the C<localize> call was compiled.
182 sub new { localize '$tag', $_[0] => UP }
193 will localize C<$Tool::tag> and not C<$Scope::tag>.
194 If you want the other behaviour, you just have to specify C<$what> as a glob or a qualified name.
196 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.
197 This situation never arises with C<local> because it only compiles when the localized variable is already declared.
198 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.
202 =head2 C<localize_elem $what, $key, $value, $context>
204 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>.
205 Just like for L</localize>, the type of localization is determined from which kind of reference C<$value> is when C<$what> is a glob, and from the sigil when it's a string.
206 C<$key> is either an array index or a hash key, depending of which kind of variable you localize.
208 If C<$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.
210 =head2 C<localize_delete $what, $key, $context>
212 Introduces the deletion of a variable or an array/hash element delayed to the time of first return into the upper scope denoted by C<$context>.
219 A glob, in which case C<$key> is ignored and the call is equivalent to C<local *x>.
223 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}>.
227 A string beginning with C<'&'>, which more or less does C<undef &func> in the upper scope.
228 It's actually more powerful, as C<&func> won't even C<exists> anymore.
233 =head2 C<unwind @values, $context>
235 Returns C<@values> I<from> the context pointed by C<$context>, i.e. from the subroutine, eval or format at or just above C<$context>, and immediately restart the program flow at this point - thus effectively returning to an upper scope.
237 The upper context isn't coerced onto C<@values>, which is hence always evaluated in list context.
241 my @a = ('a' .. 'z');
246 will set C<$num> to C<'z'>.
247 You can use L</want_at> to handle these cases.
249 =head2 C<want_at $context>
251 Like C<wantarray>, but for the subroutine/eval/format at or just above C<$context>.
253 The previous example can then be "corrected" :
256 my @a = ('a' .. 'z');
257 unwind +(want_at(HERE) ? @a : scalar @a) => HERE;
261 will rightfully set C<$num> to C<26>.
265 =head2 C<SU_THREADSAFE>
267 True iff the module could have been built when thread-safety features.
275 Returns the context that currently represents the highest scope.
279 The context of the current scope.
281 =head2 Getting a context from a context
283 For any of those functions, C<$from> is expected to be a context.
284 When omitted, it defaults to the the current context.
288 The context of the scope just above C<$from>.
292 The context of the closest subroutine above C<$from>.
293 Note that C<$from> is returned if it is already a subroutine context ; hence C<SUB SUB == SUB>.
297 The context of the closest eval above C<$from>.
298 Note that C<$from> is returned if it is already an eval context ; hence C<EVAL EVAL == EVAL>.
300 =head2 Getting a context from a level
302 Here, C<$level> should denote a number of scopes above the current one.
303 When omitted, it defaults to C<0> and those functions return the same context as L</HERE>.
305 =head3 C<SCOPE $level>
307 The C<$level>-th upper context, regardless of its type.
309 =head3 C<CALLER $level>
311 The context of the C<$level>-th upper subroutine/eval/format.
312 It kind of corresponds to the context represented by C<caller $level>, but while e.g. C<caller 0> refers to the caller context, C<CALLER 0> will refer to the top scope in the current context.
316 Where L</reap> fires depending on the C<$cxt> :
322 reap \&cleanup => $cxt;
324 } # $cxt = SCOPE(0), or HERE
326 }->(); # $cxt = SCOPE(1), or UP, or SUB, or CALLER, or CALLER(0)
328 }; # $cxt = SCOPE(2), or UP UP, or UP SUB, or EVAL, or CALLER(1)
330 }->(); # $cxt = SCOPE(3), or SUB UP SUB, or SUB EVAL, or CALLER(2)
333 Where L</localize>, L</localize_elem> and L</localize_delete> act depending on the C<$cxt> :
339 localize '$x' => 1 => $cxt;
340 # $cxt = SCOPE(0), or HERE
343 # $cxt = SCOPE(1), or UP, or SUB, or CALLER, or CALLER(0)
346 # $cxt = SCOPE(2), or UP UP, or UP SUB, or EVAL, or CALLER(1)
349 # $cxt = SCOPE(3), or SUB UP SUB, or SUB EVAL, or CALLER(2)
352 # $cxt = SCOPE(4), UP SUB UP SUB, or UP SUB EVAL, or UP CALLER(2), or TOP
355 Where L</unwind> and L</want_at> point to depending on the C<$cxt>:
361 unwind @things => $cxt;
365 }->(); # $cxt = SCOPE(0 .. 1), or HERE, or UP, or SUB, or CALLER(0)
367 }; # $cxt = SCOPE(2), or UP UP, or UP SUB, or EVAL, or CALLER(1)
369 }->(); # $cxt = SCOPE(3), or SUB UP SUB, or SUB EVAL, or CALLER(2)
374 The functions L</reap>, L</localize>, L</localize_elem>, L</localize_delete>, L</unwind> and L</want_at> are only exported on request, either individually or by the tags C<':funcs'> and C<':all'>.
376 The constant L</SU_THREADSAFE> is also only exported on request, individually or by the tags C<':consts'> and C<':all'>.
378 Same goes for the words L</TOP>, L</HERE>, L</UP>, L</SUB>, L</EVAL>, L</SCOPE> and L</CALLER> that are only exported on request, individually or by the tags C<':words'> and C<':all'>.
382 use base qw/Exporter/;
386 funcs => [ qw/reap localize localize_elem localize_delete unwind want_at/ ],
387 words => [ qw/TOP HERE UP SUB EVAL SCOPE CALLER/ ],
388 consts => [ qw/SU_THREADSAFE/ ],
390 our @EXPORT_OK = map { @$_ } values %EXPORT_TAGS;
391 $EXPORT_TAGS{'all'} = [ @EXPORT_OK ];
395 Be careful that local variables are restored in the reverse order in which they were localized.
396 Consider those examples:
400 reap sub { print $x } => HERE;
408 reap sub { $x = 2 } => HERE;
413 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>.
415 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.
416 This is an hopeless case because C<BEGIN> blocks are executed once while localizing constructs should do their job at each run.
417 However, it's possible to hook the end of the current scope compilation with L<B::Hooks::EndOfScope>.
419 Some rare oddities may still happen when running inside the debugger.
420 It may help to use a perl higher than 5.8.9 or 5.10.0, as they contain some context-related fixes.
424 L<XSLoader> (standard since perl 5.006).
428 L<Alias>, L<Hook::Scope>, L<Scope::Guard>, L<Guard>.
430 L<Continuation::Escape> is a thin wrapper around L<Scope::Upper> that gives you a continuation passing style interface to L</unwind>.
431 It's easier to use, but it requires you to have control over the scope where you want to return.
435 Vincent Pit, C<< <perl at profvince.com> >>, L<http://www.profvince.com>.
437 You can contact me by mail or on C<irc.perl.org> (vincent).
441 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>.
442 I will be notified, and then you'll automatically be notified of progress on your bug as I make changes.
446 You can find documentation for this module with the perldoc command.
450 Tests code coverage report is available at L<http://www.profvince.com/perl/cover/Scope-Upper>.
452 =head1 ACKNOWLEDGEMENTS
454 Inspired by Ricardo Signes.
456 Thanks to Shawn M. Moore for motivation.
458 =head1 COPYRIGHT & LICENSE
460 Copyright 2008,2009,2010 Vincent Pit, all rights reserved.
462 This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
466 1; # End of Scope::Upper