8 Scope::Upper - Act on upper scopes.
25 use Scope::Upper qw/reap localize localize_elem localize_delete :words/;
27 sub desc { shift->{desc} }
32 # First localize $x so that it gets destroyed last
33 localize '$x' => bless({ desc => $desc }, __PACKAGE__) => UP; # one scope up
37 my $x = do { no strict 'refs'; ${$pkg.'::x'} }; # Get the $x in the scope
38 print $x->desc . ": done\n";
39 } => SCOPE 1; # same as UP here
41 localize_elem '%SIG', '__WARN__' => sub {
43 my $x = do { no strict 'refs'; ${$pkg.'::x'} }; # Get the $x in the scope
44 CORE::warn($x->desc . ': ' . join('', @_));
45 } => UP CALLER 0; # same as UP here
47 # delete last @ARGV element
48 localize_delete '@ARGV', -1 => UP SUB HERE; # same as UP here
55 # $x is now a X object, and @ARGV has one element less
56 warn 'what'; # warns "pie: what at ..."
58 } # "pie: done" is printed
62 use Scope::Upper qw/unwind want_at :words/;
65 my @result = shift->();
67 unwind +(want_at($cx) ? @result : scalar @result) => $cx;
74 return @things; # returns to try() and then outside zap()
80 my @what = zap(); # @what contains @things
84 This module lets you defer actions I<at run-time> that will take place when the control flow returns into an upper scope.
91 hook an upper scope end with L</reap> ;
95 localize variables, array/hash values or deletions of elements in higher contexts with respectively L</localize>, L</localize_elem> and L</localize_delete> ;
99 return values immediately to an upper level with L</unwind>, and know which context was in use then with L</want_at>.
105 In all those functions, C<$context> refers to the target scope.
107 You have to use one or a combination of L</WORDS> to build the C<$context> passed to these functions.
108 This is needed in order to ensure that the module still works when your program is ran in the debugger.
109 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.
115 XSLoader::load(__PACKAGE__, $VERSION);
118 =head2 C<reap $callback, $context>
120 Add a destructor that calls C<$callback> (in void context) when the upper scope represented by C<$context> ends.
122 =head2 C<localize $what, $value, $context>
124 A C<local> delayed to the time of first return into the upper scope denoted by C<$context>.
131 A glob, in which case C<$value> can either be a glob or a reference.
132 L</localize> follows then the same syntax as C<local *x = $value>.
133 For example, if C<$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<$x> to C<1>.
137 A string beginning with a sigil, representing the symbol to localize and to assign to.
138 If the sigil is C<'$'>, L</localize> follows the same syntax as C<local $x = $value>, i.e. C<$value> isn't dereferenced.
141 localize '$x', \'foo' => HERE;
143 will set C<$x> to a reference to the string C<'foo'>.
144 Other sigils (C<'@'>, C<'%'>, C<'&'> and C<'*'>) require C<$value> to be a reference of the corresponding type.
146 When the symbol is given by a string, it is resolved when the actual localization takes place and not when C<localize> is called.
147 Thus, if the symbol name is not qualified, it will refer to the variable in the package where the localization actually takes place and not in the one where the C<localize> call was compiled.
152 sub new { localize '$tag', $_[0] => UP }
163 will localize C<$Tool::tag> and not C<$Scope::tag>.
167 =head2 C<localize_elem $what, $key, $value, $context>
169 Similar to L</localize> but for array and hash elements.
170 If C<$what> is a glob, the slot to fill is determined from which type of reference C<$value> is ; otherwise it's inferred from the sigil.
171 C<$key> is either an array index or a hash key, depending of which kind of variable you localize.
173 =head2 C<localize_delete $what, $key, $context>
175 Similiar to L</localize>, but for deleting variables or array/hash elements.
182 A glob, in which case C<$key> is ignored and the call is equivalent to C<local *x>.
186 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}>.
190 A string beginning with C<'&'>, which more or less does C<undef &func> in the upper scope.
191 It's actually more powerful, as C<&func> won't even C<exists> anymore.
196 =head2 C<unwind @values, $context>
198 Returns C<@values> I<from> the context pointed by C<$context>, i.e. from the subroutine, eval or format just above C<$context>, and immediately restart the program flow at this point - thus effectively returning to (or from, depending on how you see it) an upper context.
200 The upper context isn't coerced onto C<@values>, which is hence always evaluated in list context.
204 my @a = ('a' .. 'z');
209 will set C<$num> to C<'z'>.
210 You can use L</want_at> to handle these cases.
212 =head2 C<want_at $context>
214 Like C<wantarray>, but for the subroutine/eval/format just above C<$context>.
216 The previous example can then be "corrected" :
219 my @a = ('a' .. 'z');
220 unwind +(want_at(HERE) ? @a : scalar @a) => HERE;
224 will rightfully set C<$num> to C<26>.
228 =head2 C<SU_THREADSAFE>
230 True iff the module could have been built when thread-safety features.
238 Returns the context that currently represents the highest scope.
242 The context of the current scope.
244 =head2 Getting a context from a context
246 For any of those functions, C<$from> is expected to be a context.
247 When omitted, it defaults to the the current context.
251 The context of the scope just above C<$from>.
255 The context of the closest subroutine above C<$from>.
256 Note that C<$from> is returned if it is already a subroutine context ; hence C<SUB SUB == SUB>.
260 The context of the closest eval above C<$from>.
261 Note that C<$from> is returned if it is already an eval context ; hence C<EVAL EVAL == EVAL>.
263 =head2 Getting a context from a level
265 Here, C<$level> should denote a number of scopes above the current one.
266 When omitted, it defaults to C<0> and those functions return the same context as L</HERE>.
268 =head3 C<SCOPE $level>
270 The C<$level>-th upper context, regardless of its type.
272 =head3 C<CALLER $level>
274 The context of the C<$level>-th upper subroutine/eval/format.
275 It kind of corresponds to the context represented by C<caller $level>, but while e.g. C<caller 0> refers to the caller context, C<CALLER 0> will refer to the top scope in the current context.
279 Where L</reap> fires depending on the C<$cxt> :
285 reap \&cleanup => $cxt;
287 } # $cxt = SCOPE(0), or HERE
289 }->(); # $cxt = SCOPE(1), or UP, or SUB, or CALLER, or CALLER(0)
291 }; # $cxt = SCOPE(2), or UP UP, or UP SUB, or EVAL, or CALLER(1)
293 }->(); # $cxt = SCOPE(3), or SUB UP SUB, or SUB EVAL, or CALLER(2)
296 Where L</localize>, L</localize_elem> and L</localize_delete> act depending on the C<$cxt> :
302 localize '$x' => 1 => $cxt;
303 # $cxt = SCOPE(0), or HERE
306 # $cxt = SCOPE(1), or UP, or SUB, or CALLER, or CALLER(0)
309 # $cxt = SCOPE(2), or UP UP, or UP SUB, or EVAL, or CALLER(1)
312 # $cxt = SCOPE(3), or SUB UP SUB, or SUB EVAL, or CALLER(2)
315 # $cxt = SCOPE(4), UP SUB UP SUB, or UP SUB EVAL, or UP CALLER(2), or TOP
318 Where L</unwind> and L</want_at> point to depending on the C<$cxt>:
324 unwind @things => $cxt;
328 }->(); # $cxt = SCOPE(0 .. 1), or HERE, or UP, or SUB, or CALLER(0)
330 }; # $cxt = SCOPE(2), or UP UP, or UP SUB, or EVAL, or CALLER(1)
332 }->(); # $cxt = SCOPE(3), or SUB UP SUB, or SUB EVAL, or CALLER(2)
337 The functions L</reap>, L</localize>, L</localize_elem>, L</localize_delete>, L</unwind> and L</want_at> are only exported on request, either individually or by the tags C<':funcs'> and C<':all'>.
339 The constant L</SU_THREADSAFE> is also only exported on request, individually or by the tags C<':consts'> and C<':all'>.
341 Same goes for the words L</TOP>, L</HERE>, L</UP>, L</SUB>, L</EVAL>, L</SCOPE> and L</CALLER> that are only exported on request, individually or by the tags C<':words'> and C<':all'>.
345 use base qw/Exporter/;
349 funcs => [ qw/reap localize localize_elem localize_delete unwind want_at/ ],
350 words => [ qw/TOP HERE UP SUB EVAL SCOPE CALLER/ ],
351 consts => [ qw/SU_THREADSAFE/ ],
353 our @EXPORT_OK = map { @$_ } values %EXPORT_TAGS;
354 $EXPORT_TAGS{'all'} = [ @EXPORT_OK ];
358 Be careful that local variables are restored in the reverse order in which they were localized.
359 Consider those examples:
363 reap sub { print $x } => HERE;
371 reap sub { $x = 2 } => HERE;
376 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>.
378 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.
379 This is an hopeless case because C<BEGIN> blocks are executed once while localizing constructs should do their job at each run.
380 However, it's possible to hook the end of the current scope compilation with L<B::Hooks::EndOfScope>.
382 Some rare oddities may still happen when running inside the debugger.
383 It may help to use a perl higher than 5.8.9 or 5.10.0, as they contain some context-related fixes.
387 L<XSLoader> (standard since perl 5.006).
391 L<Alias>, L<Hook::Scope>, L<Scope::Guard>, L<Guard>.
393 L<Continuation::Escape> is a thin wrapper around L<Scope::Upper> that gives you a continuation passing style interface to L</unwind>.
394 It's easier to use, but it requires you to have control over the scope where you want to return.
398 Vincent Pit, C<< <perl at profvince.com> >>, L<http://www.profvince.com>.
400 You can contact me by mail or on C<irc.perl.org> (vincent).
404 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>.
405 I will be notified, and then you'll automatically be notified of progress on your bug as I make changes.
409 You can find documentation for this module with the perldoc command.
413 Tests code coverage report is available at L<http://www.profvince.com/perl/cover/Scope-Upper>.
415 =head1 ACKNOWLEDGEMENTS
417 Inspired by Ricardo Signes.
419 Thanks to Shawn M. Moore for motivation.
421 =head1 COPYRIGHT & LICENSE
423 Copyright 2008,2009,2010 Vincent Pit, all rights reserved.
425 This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
429 1; # End of Scope::Upper