8 Scope::Upper - Act on upper scopes.
23 L</reap>, L</localize>, L</localize_elem>, L</localize_delete> and L</WORDS> :
28 reap localize localize_elem localize_delete
33 my ($class, $name) = @_;
35 localize '$tag' => bless({ name => $name }, $class) => UP;
37 reap { print Scope->tag->name, ": end\n" } UP;
40 # Get the tag stored in the caller namespace
43 my $pkg = __PACKAGE__;
44 $pkg = caller $l++ while $pkg eq __PACKAGE__;
50 sub name { shift->{name} }
52 # Locally capture warnings and reprint them with the name prefixed
54 localize_elem '%SIG', '__WARN__' => sub {
55 print Scope->tag->name, ': ', @_;
61 for (reverse 0 .. $#INC) {
62 # First UP is the for loop, second is the sub boundary
63 localize_delete '@INC', $_ => UP UP;
72 Scope->new("top"); # initializes $UserLand::tag
76 my $one = 1 + undef; # prints "top: Use of uninitialized value..."
81 print $@; # prints "Can't locate Cwd.pm in @INC
82 } # (@INC contains:) at..."
84 require Cwd; # loads Cwd.pm
87 } # prints "top: done"
89 L</unwind> and L</want_at> :
93 use Scope::Upper qw<unwind want_at :words>;
96 my @result = shift->();
97 my $cx = SUB UP; # Point to the sub above this one
98 unwind +(want_at($cx) ? @result : scalar @result) => $cx;
105 my @things = qw<a b c>;
106 return @things; # returns to try() and then outside zap()
112 my @stuff = zap(); # @stuff contains qw<a b c>
113 my $stuff = zap(); # $stuff contains 3
119 use Scope::Upper qw<uplevel CALLER>;
127 my $sub = (caller 0)[3];
128 print "$_[0] from $sub()";
132 target('hello'); # "hello from Uplevel::target()"
134 L</uid> and L</validate_uid> :
136 use Scope::Upper qw<uid validate_uid>;
143 if ($uid eq uid(UP)) { # yes
146 if (validate_uid($uid)) { # yes
152 if (validate_uid($uid)) { # no
158 This module lets you defer actions I<at run-time> that will take place when the control flow returns into an upper scope.
165 hook an upper scope end with L</reap> ;
169 localize variables, array/hash values or deletions of elements in higher contexts with respectively L</localize>, L</localize_elem> and L</localize_delete> ;
173 return values immediately to an upper level with L</unwind>, and know which context was in use then with L</want_at> ;
177 execute a subroutine in the setting of an upper subroutine stack frame with L</uplevel> ;
181 uniquely identify contextes with L</uid> and L</validate_uid>.
187 In all those functions, C<$context> refers to the target scope.
189 You have to use one or a combination of L</WORDS> to build the C<$context> passed to these functions.
190 This is needed in order to ensure that the module still works when your program is ran in the debugger.
191 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.
197 XSLoader::load(__PACKAGE__, $VERSION);
200 =head2 C<reap $callback, $context>
202 Adds a destructor that calls C<$callback> (in void context) when the upper scope represented by C<$context> ends.
204 =head2 C<localize $what, $value, $context>
206 Introduces a C<local> delayed to the time of first return into the upper scope denoted by C<$context>.
213 A glob, in which case C<$value> can either be a glob or a reference.
214 L</localize> follows then the same syntax as C<local *x = $value>.
215 For example, if C<$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<$x> to C<1>.
219 A string beginning with a sigil, representing the symbol to localize and to assign to.
220 If the sigil is C<'$'>, L</localize> follows the same syntax as C<local $x = $value>, i.e. C<$value> isn't dereferenced.
223 localize '$x', \'foo' => HERE;
225 will set C<$x> to a reference to the string C<'foo'>.
226 Other sigils (C<'@'>, C<'%'>, C<'&'> and C<'*'>) require C<$value> to be a reference of the corresponding type.
228 When the symbol is given by a string, it is resolved when the actual localization takes place and not when L</localize> is called.
229 Thus, if the symbol name is not qualified, it will refer to the variable in the package where the localization actually takes place and not in the one where the L</localize> call was compiled.
234 sub new { localize '$tag', $_[0] => UP }
245 will localize C<$Tool::tag> and not C<$Scope::tag>.
246 If you want the other behaviour, you just have to specify C<$what> as a glob or a qualified name.
248 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.
249 This situation never arises with C<local> because it only compiles when the localized variable is already declared.
250 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.
254 =head2 C<localize_elem $what, $key, $value, $context>
256 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>.
257 Unlike L</localize>, C<$what> must be a string and the type of localization is inferred from its sigil.
258 The two only valid types are array and hash ; for anything besides those, L</localize_elem> will throw an exception.
259 C<$key> is either an array index or a hash key, depending of which kind of variable you localize.
261 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.
263 =head2 C<localize_delete $what, $key, $context>
265 Introduces the deletion of a variable or an array/hash element delayed to the time of first return into the upper scope denoted by C<$context>.
272 A glob, in which case C<$key> is ignored and the call is equivalent to C<local *x>.
276 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}>.
280 A string beginning with C<'&'>, which more or less does C<undef &func> in the upper scope.
281 It's actually more powerful, as C<&func> won't even C<exists> anymore.
286 =head2 C<unwind @values, $context>
288 Returns C<@values> I<from> the context pointed by C<$context>, i.e. from the subroutine, eval or format at or just above C<$context>, and immediately restart the program flow at this point - thus effectively returning to an upper scope.
290 The upper context isn't coerced onto C<@values>, which is hence always evaluated in list context.
294 my @a = ('a' .. 'z');
299 will set C<$num> to C<'z'>.
300 You can use L</want_at> to handle these cases.
302 =head2 C<want_at $context>
304 Like C<wantarray>, but for the subroutine/eval/format at or just above C<$context>.
306 The previous example can then be "corrected" :
309 my @a = ('a' .. 'z');
310 unwind +(want_at(HERE) ? @a : scalar @a) => HERE;
314 will rightfully set C<$num> to C<26>.
316 =head2 C<uplevel $code, @args, $context>
318 Executes the code reference C<$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.
319 The code is executed in the context of the C<uplevel> call, and what it returns is returned as-is by C<uplevel>.
331 my @inverses = target(1, 2, 4); # @inverses contains (0, 0.5, 0.25)
332 my $count = target(1, 2, 4); # $count is 3
334 L<Sub::Uplevel> also implements a pure-Perl version of C<uplevel>.
335 Both are identical, with the following caveats :
341 The L<Sub::Uplevel> implementation of C<uplevel> may execute a code reference in the context of B<any> upper stack frame.
342 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.
346 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.
354 uplevel { die 'wut' } CALLER(2); # for Scope::Upper
355 # uplevel(3, sub { die 'wut' }) # for Sub::Uplevel
358 print "inner block: $@";
362 print "outer block: $@";
364 will print "inner block: wut..." with L<Sub::Uplevel> and "outer block: wut..." with L<Scope::Upper>.
368 L<Sub::Uplevel> globally overrides the Perl keyword C<caller>, while L<Scope::Upper> does not.
372 A simple wrapper lets you mimic the interface of L<Sub::Uplevel/uplevel> :
379 my $cxt = Scope::Upper::CALLER($frame);
380 &Scope::Upper::uplevel($code => @_ => $cxt);
383 Albeit the three exceptions listed above, it passes all the tests of L<Sub::Uplevel>.
385 =head2 C<uid $context>
387 Returns an unique identifier (UID) for the context (or dynamic scope) pointed by C<$context>, or for the current context if C<$context> is omitted.
388 This UID will only be valid for the life time of the context it represents, and another UID will be generated next time the same scope is executed.
394 if ($uid eq uid()) { # yes, this is the same context
398 if ($uid eq uid()) { # no, we are one scope below
401 if ($uid eq uid(UP)) { # yes, UP points to the same scope as $uid
407 # $uid is now invalid
410 if ($uid eq uid()) { # no, this is another block
415 For example, each loop iteration gets its own UID :
424 # %uids has 5 entries
426 The UIDs are not guaranteed to be numbers, so you must use the C<eq> operator to compare them.
428 To check whether a given UID is valid, you can use the L</validate_uid> function.
430 =head2 C<validate_uid $uid>
432 Returns true if and only if C<$uid> is the UID of a currently valid context (that is, it designates a scope that is higher than the current one in the call stack).
438 if (validate_uid($uid)) { # yes
442 if (validate_uid($uid)) { # yes
448 if (validate_uid($uid)) { # no
454 =head2 C<SU_THREADSAFE>
456 True iff the module could have been built when thread-safety features.
464 Returns the context that currently represents the highest scope.
468 The context of the current scope.
470 =head2 Getting a context from a context
472 For any of those functions, C<$from> is expected to be a context.
473 When omitted, it defaults to the the current context.
477 The context of the scope just above C<$from>.
481 The context of the closest subroutine above C<$from>.
482 Note that C<$from> is returned if it is already a subroutine context ; hence C<SUB SUB == SUB>.
486 The context of the closest eval above C<$from>.
487 Note that C<$from> is returned if it is already an eval context ; hence C<EVAL EVAL == EVAL>.
489 =head2 Getting a context from a level
491 Here, C<$level> should denote a number of scopes above the current one.
492 When omitted, it defaults to C<0> and those functions return the same context as L</HERE>.
494 =head3 C<SCOPE $level>
496 The C<$level>-th upper context, regardless of its type.
498 =head3 C<CALLER $level>
500 The context of the C<$level>-th upper subroutine/eval/format.
501 It kind of corresponds to the context represented by C<caller $level>, but while e.g. C<caller 0> refers to the caller context, C<CALLER 0> will refer to the top scope in the current context.
505 Where L</reap> fires depending on the C<$cxt> :
511 reap \&cleanup => $cxt;
513 } # $cxt = SCOPE(0) = HERE
515 }->(); # $cxt = SCOPE(1) = UP = SUB = CALLER(0)
517 }; # $cxt = SCOPE(2) = UP UP = UP SUB = EVAL = CALLER(1)
519 }->(); # $cxt = SCOPE(3) = SUB UP SUB = SUB EVAL = CALLER(2)
522 Where L</localize>, L</localize_elem> and L</localize_delete> act depending on the C<$cxt> :
528 localize '$x' => 1 => $cxt;
529 # $cxt = SCOPE(0) = HERE
532 # $cxt = SCOPE(1) = UP = SUB = CALLER(0)
535 # $cxt = SCOPE(2) = UP UP = UP SUB = EVAL = CALLER(1)
538 # $cxt = SCOPE(3) = SUB UP SUB = SUB EVAL = CALLER(2)
541 # $cxt = SCOPE(4), UP SUB UP SUB = UP SUB EVAL = UP CALLER(2) = TOP
544 Where L</unwind>, L</want_at> and L</uplevel> point to depending on the C<$cxt>:
550 unwind @things => $cxt; # or uplevel { ... } $cxt;
554 }->(); # $cxt = SCOPE(0) = SCOPE(1) = HERE = UP = SUB = CALLER(0)
556 }; # $cxt = SCOPE(2) = UP UP = UP SUB = EVAL = CALLER(1) (*)
558 }->(); # $cxt = SCOPE(3) = SUB UP SUB = SUB EVAL = CALLER(2)
561 # (*) Note that uplevel() will croak if you pass that scope frame,
562 # because it cannot target eval scopes.
566 The functions L</reap>, L</localize>, L</localize_elem>, L</localize_delete>, L</unwind>, L</want_at> and L</uplevel> are only exported on request, either individually or by the tags C<':funcs'> and C<':all'>.
568 The constant L</SU_THREADSAFE> is also only exported on request, individually or by the tags C<':consts'> and C<':all'>.
570 Same goes for the words L</TOP>, L</HERE>, L</UP>, L</SUB>, L</EVAL>, L</SCOPE> and L</CALLER> that are only exported on request, individually or by the tags C<':words'> and C<':all'>.
574 use base qw<Exporter>;
580 localize localize_elem localize_delete
585 words => [ qw<TOP HERE UP SUB EVAL SCOPE CALLER> ],
586 consts => [ qw<SU_THREADSAFE> ],
588 our @EXPORT_OK = map { @$_ } values %EXPORT_TAGS;
589 $EXPORT_TAGS{'all'} = [ @EXPORT_OK ];
593 Be careful that local variables are restored in the reverse order in which they were localized.
594 Consider those examples:
598 reap sub { print $x } => HERE;
606 reap sub { $x = 2 } => HERE;
611 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>.
613 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.
614 This is an hopeless case because C<BEGIN> blocks are executed once while localizing constructs should do their job at each run.
615 However, it's possible to hook the end of the current scope compilation with L<B::Hooks::EndOfScope>.
617 Some rare oddities may still happen when running inside the debugger.
618 It may help to use a perl higher than 5.8.9 or 5.10.0, as they contain some context-related fixes.
620 Calling C<goto> to replace an L</uplevel>'d code frame does not work :
626 for a C<perl> older than the 5.8 series ;
630 for a C<DEBUGGING> C<perl> run with debugging flags set (as in C<perl -D ...>) ;
634 when the runloop callback is replaced by another module.
638 In those three 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.
640 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.
641 Despite this shortcoming, this XS version of L</uplevel> should still run way faster than the pure-Perl version from L<Sub::Uplevel>.
645 L<XSLoader> (standard since perl 5.006).
649 L<perlfunc/local>, L<perlsub/"Temporary Values via local()">.
651 L<Alias>, L<Hook::Scope>, L<Scope::Guard>, L<Guard>.
655 L<Continuation::Escape> is a thin wrapper around L<Scope::Upper> that gives you a continuation passing style interface to L</unwind>.
656 It's easier to use, but it requires you to have control over the scope where you want to return.
662 Vincent Pit, C<< <perl at profvince.com> >>, L<http://www.profvince.com>.
664 You can contact me by mail or on C<irc.perl.org> (vincent).
668 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>.
669 I will be notified, and then you'll automatically be notified of progress on your bug as I make changes.
673 You can find documentation for this module with the perldoc command.
677 Tests code coverage report is available at L<http://www.profvince.com/perl/cover/Scope-Upper>.
679 =head1 ACKNOWLEDGEMENTS
681 Inspired by Ricardo Signes.
683 Thanks to Shawn M. Moore for motivation.
685 =head1 COPYRIGHT & LICENSE
687 Copyright 2008,2009,2010,2011 Vincent Pit, all rights reserved.
689 This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
693 1; # End of Scope::Upper