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<$value> to be a reference of the corresponding type.
146 When the symbol is given by a string, it is resolved when the actual localization takes place and not when C<localize> is called.
149 sub tag { localize '$x', $_[0] => UP }
151 will localize in the caller's namespace.
155 =head2 C<localize_elem $what, $key, $value, $context>
157 Similar to L</localize> but for array and hash elements.
158 If C<$what> is a glob, the slot to fill is determined from which type of reference C<$value> is ; otherwise it's inferred from the sigil.
159 C<$key> is either an array index or a hash key, depending of which kind of variable you localize.
161 =head2 C<localize_delete $what, $key, $context>
163 Similiar to L</localize>, but for deleting variables or array/hash elements.
170 A glob, in which case C<$key> is ignored and the call is equivalent to C<local *x>.
174 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}>.
178 A string beginning with C<'&'>, which more or less does C<undef &func> in the upper scope.
179 It's actually more powerful, as C<&func> won't even C<exists> anymore.
184 =head2 C<unwind @values, $context>
186 Returns C<@values> I<from> the context pointed by C<$context>, i.e. from the subroutine, eval or format just above C<$context>, and immediately restart the program flow at this point - thus effectively returning to (or from, depending on how you see it) an upper context.
188 The upper context isn't coerced onto C<@values>, which is hence always evaluated in list context.
192 my @a = ('a' .. 'z');
197 will set C<$num> to C<'z'>.
198 You can use L</want_at> to handle these cases.
200 =head2 C<want_at $context>
202 Like C<wantarray>, but for the subroutine/eval/format just above C<$context>.
204 The previous example can then be "corrected" :
207 my @a = ('a' .. 'z');
208 unwind +(want_at(HERE) ? @a : scalar @a) => HERE;
212 will rightfully set C<$num> to C<26>.
216 =head2 C<SU_THREADSAFE>
218 True iff the module could have been built when thread-safety features.
226 Returns the context that currently represents the highest scope.
230 The context of the current scope.
232 =head2 Getting a context from a context
234 For any of those functions, C<$from> is expected to be a context.
235 When omitted, it defaults to the the current context.
239 The context of the scope just above C<$from>.
243 The context of the closest subroutine above C<$from>.
244 Note that C<$from> is returned if it is already a subroutine context ; hence C<SUB SUB == SUB>.
248 The context of the closest eval above C<$from>.
249 Note that C<$from> is returned if it is already an eval context ; hence C<EVAL EVAL == EVAL>.
251 =head2 Getting a context from a level
253 Here, C<$level> should denote a number of scopes above the current one.
254 When omitted, it defaults to C<0> and those functions return the same context as L</HERE>.
256 =head3 C<SCOPE $level>
258 The C<$level>-th upper context, regardless of its type.
260 =head3 C<CALLER $level>
262 The context of the C<$level>-th upper subroutine/eval/format.
263 It kind of corresponds to the context represented by C<caller $level>, but while e.g. C<caller 0> refers to the caller context, C<CALLER 0> will refer to the top scope in the current context.
267 Where L</reap> fires depending on the C<$cxt> :
273 reap \&cleanup => $cxt;
275 } # $cxt = SCOPE(0), or HERE
277 }->(); # $cxt = SCOPE(1), or UP, or SUB, or CALLER, or CALLER(0)
279 }; # $cxt = SCOPE(2), or UP UP, or UP SUB, or EVAL, or CALLER(1)
281 }->(); # $cxt = SCOPE(3), or SUB UP SUB, or SUB EVAL, or CALLER(2)
284 Where L</localize>, L</localize_elem> and L</localize_delete> act depending on the C<$cxt> :
290 localize '$x' => 1 => $cxt;
291 # $cxt = SCOPE(0), or HERE
294 # $cxt = SCOPE(1), or UP, or SUB, or CALLER, or CALLER(0)
297 # $cxt = SCOPE(2), or UP UP, or UP SUB, or EVAL, or CALLER(1)
300 # $cxt = SCOPE(3), or SUB UP SUB, or SUB EVAL, or CALLER(2)
303 # $cxt = SCOPE(4), UP SUB UP SUB, or UP SUB EVAL, or UP CALLER(2), or TOP
306 Where L</unwind> and L</want_at> point to depending on the C<$cxt>:
312 unwind @things => $cxt;
316 }->(); # $cxt = SCOPE(0 .. 1), or HERE, or UP, or SUB, or CALLER(0)
318 }; # $cxt = SCOPE(2), or UP UP, or UP SUB, or EVAL, or CALLER(1)
320 }->(); # $cxt = SCOPE(3), or SUB UP SUB, or SUB EVAL, or CALLER(2)
325 The functions L</reap>, L</localize>, L</localize_elem>, L</localize_delete>, L</unwind> and L</want_at> are only exported on request, either individually or by the tags C<':funcs'> and C<':all'>.
327 The constant L</SU_THREADSAFE> is also only exported on request, individually or by the tags C<':consts'> and C<':all'>.
329 Same goes for the words L</TOP>, L</HERE>, L</UP>, L</SUB>, L</EVAL>, L</SCOPE> and L</CALLER> that are only exported on request, individually or by the tags C<':words'> and C<':all'>.
333 use base qw/Exporter/;
337 funcs => [ qw/reap localize localize_elem localize_delete unwind want_at/ ],
338 words => [ qw/TOP HERE UP SUB EVAL SCOPE CALLER/ ],
339 consts => [ qw/SU_THREADSAFE/ ],
341 our @EXPORT_OK = map { @$_ } values %EXPORT_TAGS;
342 $EXPORT_TAGS{'all'} = [ @EXPORT_OK ];
346 Be careful that local variables are restored in the reverse order in which they were localized.
347 Consider those examples:
351 reap sub { print $x } => HERE;
359 reap sub { $x = 2 } => HERE;
364 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>.
366 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.
367 This is an hopeless case because C<BEGIN> blocks are executed once while localizing constructs should do their job at each run.
368 However, it's possible to hook the end of the current scope compilation with L<B::Hooks::EndOfScope>.
370 Some rare oddities may still happen when running inside the debugger.
371 It may help to use a perl higher than 5.8.9 or 5.10.0, as they contain some context-related fixes.
375 L<XSLoader> (standard since perl 5.006).
379 L<Alias>, L<Hook::Scope>, L<Scope::Guard>, L<Guard>.
381 L<Continuation::Escape> is a thin wrapper around L<Scope::Upper> that gives you a continuation passing style interface to L</unwind>.
382 It's easier to use, but it requires you to have control over the scope where you want to return.
386 Vincent Pit, C<< <perl at profvince.com> >>, L<http://www.profvince.com>.
388 You can contact me by mail or on C<irc.perl.org> (vincent).
392 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>. I will be notified, and then you'll automatically be notified of progress on your bug as I make changes.
396 You can find documentation for this module with the perldoc command.
400 Tests code coverage report is available at L<http://www.profvince.com/perl/cover/Scope-Upper>.
402 =head1 ACKNOWLEDGEMENTS
404 Inspired by Ricardo Signes.
406 Thanks to Shawn M. Moore for motivation.
408 =head1 COPYRIGHT & LICENSE
410 Copyright 2008,2009,2010 Vincent Pit, all rights reserved.
412 This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
416 1; # End of Scope::Upper