2 Scope::Upper - Act on upper scopes.
8 "reap", "localize", "localize_elem", "localize_delete" and "WORDS" :
13 reap localize localize_elem localize_delete
18 my ($class, $name) = @_;
20 localize '$tag' => bless({ name => $name }, $class) => UP;
22 reap { print Scope->tag->name, ": end\n" } UP;
25 # Get the tag stored in the caller namespace
28 my $pkg = __PACKAGE__;
29 $pkg = caller $l++ while $pkg eq __PACKAGE__;
35 sub name { shift->{name} }
37 # Locally capture warnings and reprint them with the name prefixed
39 localize_elem '%SIG', '__WARN__' => sub {
40 print Scope->tag->name, ': ', @_;
46 for (reverse 0 .. $#INC) {
47 # First UP is the for loop, second is the sub boundary
48 localize_delete '@INC', $_ => UP UP;
57 Scope->new("top"); # initializes $UserLand::tag
61 my $one = 1 + undef; # prints "top: Use of uninitialized value..."
66 print $@; # prints "Can't locate Cwd.pm in @INC
67 } # (@INC contains:) at..."
69 require Cwd; # loads Cwd.pm
72 } # prints "top: done"
74 "unwind" and "want_at" :
78 use Scope::Upper qw<unwind want_at :words>;
81 my @result = shift->();
82 my $cx = SUB UP; # Point to the sub above this one
83 unwind +(want_at($cx) ? @result : scalar @result) => $cx;
90 my @things = qw<a b c>;
91 return @things; # returns to try() and then outside zap()
97 my @stuff = zap(); # @stuff contains qw<a b c>
98 my $stuff = zap(); # $stuff contains 3
104 use Scope::Upper qw<uplevel CALLER>;
112 my $sub = (caller 0)[3];
113 print "$_[0] from $sub()";
117 target('hello'); # "hello from Uplevel::target()"
119 "uid" and "validate_uid" :
121 use Scope::Upper qw<uid validate_uid>;
128 if ($uid eq uid(UP)) { # yes
131 if (validate_uid($uid)) { # yes
137 if (validate_uid($uid)) { # no
142 This module lets you defer actions *at run-time* that will take place
143 when the control flow returns into an upper scope. Currently, you can:
145 * hook an upper scope end with "reap" ;
147 * localize variables, array/hash values or deletions of elements in
148 higher contexts with respectively "localize", "localize_elem" and
151 * return values immediately to an upper level with "unwind", and know
152 which context was in use then with "want_at" ;
154 * execute a subroutine in the setting of an upper subroutine stack
155 frame with "uplevel" ;
157 * uniquely identify contextes with "uid" and "validate_uid".
160 In all those functions, $context refers to the target scope.
162 You have to use one or a combination of "WORDS" to build the $context
163 passed to these functions. This is needed in order to ensure that the
164 module still works when your program is ran in the debugger. The only
165 thing you can assume is that it is an *absolute* indicator of the frame,
166 which means that you can safely store it at some point and use it when
167 needed, and it will still denote the original scope.
171 reap { ... } $context;
172 &reap($callback, $context);
174 Adds a destructor that calls $callback (in void context) when the upper
175 scope represented by $context ends.
178 localize $what, $value;
179 localize $what, $value, $context;
181 Introduces a "local" delayed to the time of first return into the upper
182 scope denoted by $context. $what can be :
184 * A glob, in which case $value can either be a glob or a reference.
185 "localize" follows then the same syntax as "local *x = $value". For
186 example, if $value is a scalar reference, then the "SCALAR" slot of
187 the glob will be set to $$value - just like "local *x = \1" sets $x
190 * A string beginning with a sigil, representing the symbol to localize
191 and to assign to. If the sigil is '$', "localize" follows the same
192 syntax as "local $x = $value", i.e. $value isn't dereferenced. For
195 localize '$x', \'foo' => HERE;
197 will set $x to a reference to the string 'foo'. Other sigils ('@',
198 '%', '&' and '*') require $value to be a reference of the
201 When the symbol is given by a string, it is resolved when the actual
202 localization takes place and not when "localize" is called. Thus, if
203 the symbol name is not qualified, it will refer to the variable in
204 the package where the localization actually takes place and not in
205 the one where the "localize" call was compiled. For example,
209 sub new { localize '$tag', $_[0] => UP }
220 will localize $Tool::tag and not $Scope::tag. If you want the other
221 behaviour, you just have to specify $what as a glob or a qualified
224 Note that if $what is a string denoting a variable that wasn't
225 declared beforehand, the relevant slot will be vivified as needed
226 and won't be deleted from the glob when the localization ends. This
227 situation never arises with "local" because it only compiles when
228 the localized variable is already declared. Although I believe it
229 shouldn't be a problem as glob slots definedness is pretty much an
230 implementation detail, this behaviour may change in the future if
234 localize_elem $what, $key, $value;
235 localize_elem $what, $key, $value, $context;
237 Introduces a "local $what[$key] = $value" or "local $what{$key} =
238 $value" delayed to the time of first return into the upper scope denoted
239 by $context. Unlike "localize", $what must be a string and the type of
240 localization is inferred from its sigil. The two only valid types are
241 array and hash ; for anything besides those, "localize_elem" will throw
242 an exception. $key is either an array index or a hash key, depending of
243 which kind of variable you localize.
245 If $what is a string pointing to an undeclared variable, the variable
246 will be vivified as soon as the localization occurs and emptied when it
247 ends, although it will still exist in its glob.
249 "localize_delete $what, $key, $context"
250 Introduces the deletion of a variable or an array/hash element delayed
251 to the time of first return into the upper scope denoted by $context.
254 * A glob, in which case $key is ignored and the call is equivalent to
257 * A string beginning with '@' or '%', for which the call is equivalent
258 to respectiveley "local $a[$key]; delete $a[$key]" and "local
259 $h{$key}; delete $h{$key}".
261 * A string beginning with '&', which more or less does "undef &func"
262 in the upper scope. It's actually more powerful, as &func won't even
263 "exists" anymore. $key is ignored.
267 unwind @values, $context;
269 Returns @values *from* the context pointed by $context, i.e. from the
270 subroutine, eval or format at or just above $context, and immediately
271 restart the program flow at this point - thus effectively returning to
274 The upper context isn't coerced onto @values, which is hence always
275 evaluated in list context. This means that
278 my @a = ('a' .. 'z');
283 will set $num to 'z'. You can use "want_at" to handle these cases.
287 my $want = want_at $context;
289 Like "wantarray", but for the subroutine/eval/format at or just above
292 The previous example can then be "corrected" :
295 my @a = ('a' .. 'z');
296 unwind +(want_at(HERE) ? @a : scalar @a) => HERE;
300 will rightfully set $num to 26.
302 "uplevel $code, @args, $context"
303 my @ret = uplevel { ...; return @ret };
304 my @ret = uplevel { my @args = @_; ...; return @ret } @args;
305 my @ret = uplevel { ... } @args, $context;
306 my @ret = &uplevel($callback, @args, $context);
308 Executes the code reference $code with arguments @args as if it were
309 located at the subroutine stack frame pointed by $context, effectively
310 fooling "caller" and "die" into believing that the call actually
311 happened higher in the stack. The code is executed in the context of the
312 "uplevel" call, and what it returns is returned as-is by "uplevel".
324 my @inverses = target(1, 2, 4); # @inverses contains (0, 0.5, 0.25)
325 my $count = target(1, 2, 4); # $count is 3
327 Sub::Uplevel also implements a pure-Perl version of "uplevel". Both are
328 identical, with the following caveats :
330 * The Sub::Uplevel implementation of "uplevel" may execute a code
331 reference in the context of any upper stack frame. The Scope::Upper
332 version can only uplevel to a subroutine stack frame, and will croak
333 if you try to target an "eval" or a format.
335 * Exceptions thrown from the code called by this version of "uplevel"
336 will not be caught by "eval" blocks between the target frame and the
337 uplevel call, while they will for Sub::Uplevel's version. This means
345 uplevel { die 'wut' } CALLER(2); # for Scope::Upper
346 # uplevel(3, sub { die 'wut' }) # for Sub::Uplevel
349 print "inner block: $@";
353 print "outer block: $@";
355 will print "inner block: wut..." with Sub::Uplevel and "outer block:
356 wut..." with Scope::Upper.
358 * Sub::Uplevel globally overrides the Perl keyword "caller", while
359 Scope::Upper does not.
361 A simple wrapper lets you mimic the interface of "uplevel" in
369 my $cxt = Scope::Upper::CALLER($frame);
370 &Scope::Upper::uplevel($code => @_ => $cxt);
373 Albeit the three exceptions listed above, it passes all the tests of
378 my $uid = uid $context;
380 Returns an unique identifier (UID) for the context (or dynamic scope)
381 pointed by $context, or for the current context if $context is omitted.
382 This UID will only be valid for the life time of the context it
383 represents, and another UID will be generated next time the same scope
390 if ($uid eq uid()) { # yes, this is the same context
394 if ($uid eq uid()) { # no, we are one scope below
397 if ($uid eq uid(UP)) { # yes, UP points to the same scope as $uid
403 # $uid is now invalid
406 if ($uid eq uid()) { # no, this is another block
411 For example, each loop iteration gets its own UID :
420 # %uids has 5 entries
422 The UIDs are not guaranteed to be numbers, so you must use the "eq"
423 operator to compare them.
425 To check whether a given UID is valid, you can use the "validate_uid"
429 my $is_valid = validate_uid $uid;
431 Returns true if and only if $uid is the UID of a currently valid context
432 (that is, it designates a scope that is higher than the current one in
439 if (validate_uid($uid)) { # yes
443 if (validate_uid($uid)) { # yes
449 if (validate_uid($uid)) { # no
455 True iff the module could have been built when thread-safety features.
460 my $top_context = TOP;
462 Returns the context that currently represents the highest scope.
465 my $current_context = HERE;
467 The context of the current scope.
469 Getting a context from a context
470 For any of those functions, $from is expected to be a context. When
471 omitted, it defaults to the the current context.
474 my $upper_context = UP;
475 my $upper_context = UP $from;
477 The context of the scope just above $from.
480 my $sub_context = SUB;
481 my $sub_context = SUB $from;
483 The context of the closest subroutine above $from. Note that $from is
484 returned if it is already a subroutine context ; hence "SUB SUB == SUB".
487 my $eval_context = EVAL;
488 my $eval_context = EVAL $from;
490 The context of the closest eval above $from. Note that $from is returned
491 if it is already an eval context ; hence "EVAL EVAL == EVAL".
493 Getting a context from a level
494 Here, $level should denote a number of scopes above the current one.
495 When omitted, it defaults to 0 and those functions return the same
500 my $context = SCOPE $level;
502 The $level-th upper context, regardless of its type.
505 my $context = CALLER;
506 my $context = CALLER $level;
508 The context of the $level-th upper subroutine/eval/format. It kind of
509 corresponds to the context represented by "caller $level", but while
510 e.g. "caller 0" refers to the caller context, "CALLER 0" will refer to
511 the top scope in the current context.
514 Where "reap" fires depending on the $cxt :
520 reap \&cleanup => $cxt;
522 } # $cxt = SCOPE(0) = HERE
524 }->(); # $cxt = SCOPE(1) = UP = SUB = CALLER(0)
526 }; # $cxt = SCOPE(2) = UP UP = UP SUB = EVAL = CALLER(1)
528 }->(); # $cxt = SCOPE(3) = SUB UP SUB = SUB EVAL = CALLER(2)
531 Where "localize", "localize_elem" and "localize_delete" act depending on
538 localize '$x' => 1 => $cxt;
539 # $cxt = SCOPE(0) = HERE
542 # $cxt = SCOPE(1) = UP = SUB = CALLER(0)
545 # $cxt = SCOPE(2) = UP UP = UP SUB = EVAL = CALLER(1)
548 # $cxt = SCOPE(3) = SUB UP SUB = SUB EVAL = CALLER(2)
551 # $cxt = SCOPE(4), UP SUB UP SUB = UP SUB EVAL = UP CALLER(2) = TOP
554 Where "unwind", "want_at" and "uplevel" point to depending on the $cxt:
560 unwind @things => $cxt; # or uplevel { ... } $cxt;
564 }->(); # $cxt = SCOPE(0) = SCOPE(1) = HERE = UP = SUB = CALLER(0)
566 }; # $cxt = SCOPE(2) = UP UP = UP SUB = EVAL = CALLER(1) (*)
568 }->(); # $cxt = SCOPE(3) = SUB UP SUB = SUB EVAL = CALLER(2)
571 # (*) Note that uplevel() will croak if you pass that scope frame,
572 # because it cannot target eval scopes.
575 The functions "reap", "localize", "localize_elem", "localize_delete",
576 "unwind", "want_at" and "uplevel" are only exported on request, either
577 individually or by the tags ':funcs' and ':all'.
579 The constant "SU_THREADSAFE" is also only exported on request,
580 individually or by the tags ':consts' and ':all'.
582 Same goes for the words "TOP", "HERE", "UP", "SUB", "EVAL", "SCOPE" and
583 "CALLER" that are only exported on request, individually or by the tags
587 Be careful that local variables are restored in the reverse order in
588 which they were localized. Consider those examples:
592 reap sub { print $x } => HERE;
600 reap sub { $x = 2 } => HERE;
605 The first case is "solved" by moving the "local" before the "reap", and
606 the second by using "localize" instead of "reap".
608 The effects of "reap", "localize" and "localize_elem" can't cross
609 "BEGIN" blocks, hence calling those functions in "import" is deemed to
610 be useless. This is an hopeless case because "BEGIN" blocks are executed
611 once while localizing constructs should do their job at each run.
612 However, it's possible to hook the end of the current scope compilation
613 with B::Hooks::EndOfScope.
615 Some rare oddities may still happen when running inside the debugger. It
616 may help to use a perl higher than 5.8.9 or 5.10.0, as they contain some
617 context-related fixes.
619 Calling "goto" to replace an "uplevel"'d code frame does not work :
621 * for a "perl" older than the 5.8 series ;
623 * for a "DEBUGGING" "perl" run with debugging flags set (as in "perl
626 * when the runloop callback is replaced by another module.
628 In those three cases, "uplevel" will look for a "goto &sub" statement in
629 its callback and, if there is one, throw an exception before executing
632 Moreover, in order to handle "goto" statements properly, "uplevel"
633 currently has to suffer a run-time overhead proportional to the size of
634 the the callback in every case (with a small ratio), and proportional to
635 the size of all the code executed as the result of the "uplevel" call
636 (including subroutine calls inside the callback) when a "goto" statement
637 is found in the "uplevel" callback. Despite this shortcoming, this XS
638 version of "uplevel" should still run way faster than the pure-Perl
639 version from Sub::Uplevel.
642 XSLoader (standard since perl 5.006).
645 "local" in perlfunc, "Temporary Values via local()" in perlsub.
647 Alias, Hook::Scope, Scope::Guard, Guard.
651 Continuation::Escape is a thin wrapper around Scope::Upper that gives
652 you a continuation passing style interface to "unwind". It's easier to
653 use, but it requires you to have control over the scope where you want
659 Vincent Pit, "<perl at profvince.com>", <http://www.profvince.com>.
661 You can contact me by mail or on "irc.perl.org" (vincent).
664 Please report any bugs or feature requests to "bug-scope-upper at
665 rt.cpan.org", or through the web interface at
666 <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Scope-Upper>. I will be
667 notified, and then you'll automatically be notified of progress on your
668 bug as I make changes.
671 You can find documentation for this module with the perldoc command.
675 Tests code coverage report is available at
676 <http://www.profvince.com/perl/cover/Scope-Upper>.
679 Inspired by Ricardo Signes.
681 Thanks to Shawn M. Moore for motivation.
684 Copyright 2008,2009,2010,2011,2012 Vincent Pit, all rights reserved.
686 This program is free software; you can redistribute it and/or modify it
687 under the same terms as Perl itself.