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<$value> to be a reference of the corresponding type.
176 When the symbol is given by a string, it is resolved when the actual localization takes place and not when L</localize> is called.
177 Thus, if the symbol name is not qualified, it will refer to the variable in the package where the localization actually takes place and not in the one where the L</localize> call was compiled.
182 sub new { localize '$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 Unlike L</localize>, C<$what> must be a string and the type of localization is inferred from its sigil.
206 The two only valid types are array and hash ; for anything besides those, L</localize_elem> will throw an exception.
207 C<$key> is either an array index or a hash key, depending of which kind of variable you localize.
209 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.
211 =head2 C<localize_delete $what, $key, $context>
213 Introduces the deletion of a variable or an array/hash element delayed to the time of first return into the upper scope denoted by C<$context>.
220 A glob, in which case C<$key> is ignored and the call is equivalent to C<local *x>.
224 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}>.
228 A string beginning with C<'&'>, which more or less does C<undef &func> in the upper scope.
229 It's actually more powerful, as C<&func> won't even C<exists> anymore.
234 =head2 C<unwind @values, $context>
236 Returns C<@values> I<from> the context pointed by C<$context>, i.e. from the subroutine, eval or format at or just above C<$context>, and immediately restart the program flow at this point - thus effectively returning to an upper scope.
238 The upper context isn't coerced onto C<@values>, which is hence always evaluated in list context.
242 my @a = ('a' .. 'z');
247 will set C<$num> to C<'z'>.
248 You can use L</want_at> to handle these cases.
250 =head2 C<want_at $context>
252 Like C<wantarray>, but for the subroutine/eval/format at or just above C<$context>.
254 The previous example can then be "corrected" :
257 my @a = ('a' .. 'z');
258 unwind +(want_at(HERE) ? @a : scalar @a) => HERE;
262 will rightfully set C<$num> to C<26>.
266 =head2 C<SU_THREADSAFE>
268 True iff the module could have been built when thread-safety features.
276 Returns the context that currently represents the highest scope.
280 The context of the current scope.
282 =head2 Getting a context from a context
284 For any of those functions, C<$from> is expected to be a context.
285 When omitted, it defaults to the the current context.
289 The context of the scope just above C<$from>.
293 The context of the closest subroutine above C<$from>.
294 Note that C<$from> is returned if it is already a subroutine context ; hence C<SUB SUB == SUB>.
298 The context of the closest eval above C<$from>.
299 Note that C<$from> is returned if it is already an eval context ; hence C<EVAL EVAL == EVAL>.
301 =head2 Getting a context from a level
303 Here, C<$level> should denote a number of scopes above the current one.
304 When omitted, it defaults to C<0> and those functions return the same context as L</HERE>.
306 =head3 C<SCOPE $level>
308 The C<$level>-th upper context, regardless of its type.
310 =head3 C<CALLER $level>
312 The context of the C<$level>-th upper subroutine/eval/format.
313 It kind of corresponds to the context represented by C<caller $level>, but while e.g. C<caller 0> refers to the caller context, C<CALLER 0> will refer to the top scope in the current context.
317 Where L</reap> fires depending on the C<$cxt> :
323 reap \&cleanup => $cxt;
325 } # $cxt = SCOPE(0), or HERE
327 }->(); # $cxt = SCOPE(1), or UP, or SUB, or CALLER, or CALLER(0)
329 }; # $cxt = SCOPE(2), or UP UP, or UP SUB, or EVAL, or CALLER(1)
331 }->(); # $cxt = SCOPE(3), or SUB UP SUB, or SUB EVAL, or CALLER(2)
334 Where L</localize>, L</localize_elem> and L</localize_delete> act depending on the C<$cxt> :
340 localize '$x' => 1 => $cxt;
341 # $cxt = SCOPE(0), or HERE
344 # $cxt = SCOPE(1), or UP, or SUB, or CALLER, or CALLER(0)
347 # $cxt = SCOPE(2), or UP UP, or UP SUB, or EVAL, or CALLER(1)
350 # $cxt = SCOPE(3), or SUB UP SUB, or SUB EVAL, or CALLER(2)
353 # $cxt = SCOPE(4), UP SUB UP SUB, or UP SUB EVAL, or UP CALLER(2), or TOP
356 Where L</unwind> and L</want_at> point to depending on the C<$cxt>:
362 unwind @things => $cxt;
366 }->(); # $cxt = SCOPE(0 .. 1), or HERE, or UP, or SUB, or CALLER(0)
368 }; # $cxt = SCOPE(2), or UP UP, or UP SUB, or EVAL, or CALLER(1)
370 }->(); # $cxt = SCOPE(3), or SUB UP SUB, or SUB EVAL, or CALLER(2)
375 The functions L</reap>, L</localize>, L</localize_elem>, L</localize_delete>, L</unwind> and L</want_at> are only exported on request, either individually or by the tags C<':funcs'> and C<':all'>.
377 The constant L</SU_THREADSAFE> is also only exported on request, individually or by the tags C<':consts'> and C<':all'>.
379 Same goes for the words L</TOP>, L</HERE>, L</UP>, L</SUB>, L</EVAL>, L</SCOPE> and L</CALLER> that are only exported on request, individually or by the tags C<':words'> and C<':all'>.
383 use base qw/Exporter/;
387 funcs => [ qw/reap localize localize_elem localize_delete unwind want_at/ ],
388 words => [ qw/TOP HERE UP SUB EVAL SCOPE CALLER/ ],
389 consts => [ qw/SU_THREADSAFE/ ],
391 our @EXPORT_OK = map { @$_ } values %EXPORT_TAGS;
392 $EXPORT_TAGS{'all'} = [ @EXPORT_OK ];
396 Be careful that local variables are restored in the reverse order in which they were localized.
397 Consider those examples:
401 reap sub { print $x } => HERE;
409 reap sub { $x = 2 } => HERE;
414 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>.
416 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.
417 This is an hopeless case because C<BEGIN> blocks are executed once while localizing constructs should do their job at each run.
418 However, it's possible to hook the end of the current scope compilation with L<B::Hooks::EndOfScope>.
420 Some rare oddities may still happen when running inside the debugger.
421 It may help to use a perl higher than 5.8.9 or 5.10.0, as they contain some context-related fixes.
425 L<XSLoader> (standard since perl 5.006).
429 L<perlfunc/local>, L<perlsub/"Temporary Values via local()">.
431 L<Alias>, L<Hook::Scope>, L<Scope::Guard>, L<Guard>.
433 L<Continuation::Escape> is a thin wrapper around L<Scope::Upper> that gives you a continuation passing style interface to L</unwind>.
434 It's easier to use, but it requires you to have control over the scope where you want to return.
440 Vincent Pit, C<< <perl at profvince.com> >>, L<http://www.profvince.com>.
442 You can contact me by mail or on C<irc.perl.org> (vincent).
446 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>.
447 I will be notified, and then you'll automatically be notified of progress on your bug as I make changes.
451 You can find documentation for this module with the perldoc command.
455 Tests code coverage report is available at L<http://www.profvince.com/perl/cover/Scope-Upper>.
457 =head1 ACKNOWLEDGEMENTS
459 Inspired by Ricardo Signes.
461 Thanks to Shawn M. Moore for motivation.
463 =head1 COPYRIGHT & LICENSE
465 Copyright 2008,2009,2010 Vincent Pit, all rights reserved.
467 This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
471 1; # End of Scope::Upper