1Scope::Upper(3) User Contributed Perl Documentation Scope::Upper(3)
2
6 Scope::Upper - Act on upper scopes.
7
9 Version 0.10
10
12 package X;
13 use Scope::Upper qw/reap localize localize_elem localize_delete :words/;
15 sub desc { shift->{desc} }
17 sub set_tag {
19 my ($desc) = @_;
20 # First localize $x so that it gets destroyed last
22 localize '$x' => bless({ desc => $desc }, __PACKAGE__) => UP; # one scope up
23 reap sub {
25 my $pkg = caller;
26 my $x = do { no strict 'refs'; ${$pkg.'::x'} }; # Get the $x in the scope
27 print $x->desc . ": done\n";
28 } => SCOPE 1; # same as UP here
29 localize_elem '%SIG', '__WARN__' => sub {
31 my $pkg = caller;
32 my $x = do { no strict 'refs'; ${$pkg.'::x'} }; # Get the $x in the scope
33 CORE::warn($x->desc . ': ' . join('', @_));
34 } => UP CALLER 0; # same as UP here
35 # delete last @ARGV element
37 localize_delete '@ARGV', -1 => UP SUB HERE; # same as UP here
38 }
39 package Y;
41 {
43 X::set_tag('pie');
44 # $x is now a X object, and @ARGV has one element less
45 warn 'what'; # warns "pie: what at ..."
46 ...
47 } # "pie: done" is printed
48 package Z;
50 use Scope::Upper qw/unwind want_at :words/;
52 sub try (&) {
54 my @result = shift->();
55 my $cx = SUB UP SUB;
56 unwind +(want_at($cx) ? @result : scalar @result) => $cx;
57 }
58 ...
60 sub zap {
62 try {
63 return @things; # returns to try() and then outside zap()
64 # not reached
65 }
66 # not reached
67 }
68 my @what = zap(); # @what contains @things
70
72 This module lets you defer actions at run-time that will take place
73 when the control flow returns into an upper scope. Currently, you can:
74 · hook an upper scope end with "reap" ;
76 · localize variables, array/hash values or deletions of elements in
78 higher contexts with respectively "localize", "localize_elem" and
79 "localize_delete" ;
80 · return values immediately to an upper level with "unwind", and know
82 which context was in use then with "want_at".
83
85 In all those functions, $context refers to the target scope.
86 You have to use one or a combination of "WORDS" to build the $context
88 passed to these functions. This is needed in order to ensure that the
89 module still works when your program is ran in the debugger. The only
90 thing you can assume is that it is an absolute indicator of the frame,
91 which means that you can safely store it at some point and use it when
92 needed, and it will still denote the original scope.
93 "reap $callback, $context"
95 Add a destructor that calls $callback (in void context) when the upper
96 scope represented by $context ends.
97 "localize $what, $value, $context"
99 A "local" delayed to the time of first return into the upper scope
100 denoted by $context. $what can be :
101 · A glob, in which case $value can either be a glob or a reference.
103 "localize" follows then the same syntax as "local *x = $value".
104 For example, if $value is a scalar reference, then the "SCALAR"
105 slot of the glob will be set to $$value - just like "local *x = \1"
106 sets $x to 1.
107 · A string beginning with a sigil, representing the symbol to
109 localize and to assign to. If the sigil is '$', "localize" follows
110 the same syntax as "local $x = $value", i.e. $value isn't
111 dereferenced. For example,
112 localize '$x', \'foo' => HERE;
114 will set $x to a reference to the string 'foo'. Other sigils ('@',
116 '%', '&' and '*') require $value to be a reference of the
117 corresponding type.
118 When the symbol is given by a string, it is resolved when the
120 actual localization takes place and not when "localize" is called.
121 This means that
122 sub tag { localize '$x', $_[0] => UP }
124 will localize in the caller's namespace.
126 "localize_elem $what, $key, $value, $context"
128 Similar to "localize" but for array and hash elements. If $what is a
129 glob, the slot to fill is determined from which type of reference
130 $value is ; otherwise it's inferred from the sigil. $key is either an
131 array index or a hash key, depending of which kind of variable you
132 localize.
133 "localize_delete $what, $key, $context"
135 Similiar to "localize", but for deleting variables or array/hash
136 elements. $what can be:
137 · A glob, in which case $key is ignored and the call is equivalent to
139 "local *x".
140 · A string beginning with '@' or '%', for which the call is
142 equivalent to respectiveley "local $a[$key]; delete $a[$key]" and
143 "local $h{$key}; delete $h{$key}".
144 · A string beginning with '&', which more or less does "undef &func"
146 in the upper scope. It's actually more powerful, as &func won't
147 even "exists" anymore. $key is ignored.
148 "unwind @values, $context"
150 Returns @values from the context pointed by $context, i.e. from the
151 subroutine, eval or format just above $context, and immediately restart
152 the program flow at this point - thus effectively returning to (or
153 from, depending on how you see it) an upper context.
154 The upper context isn't coerced onto @values, which is hence always
156 evaluated in list context. This means that
157 my $num = sub {
159 my @a = ('a' .. 'z');
160 unwind @a => HERE;
161 # not reached
162 }->();
163 will set $num to 'z'. You can use "want_at" to handle these cases.
165 "want_at $context"
167 Like "wantarray", but for the subroutine/eval/format just above
168 $context.
169 The previous example can then be "corrected" :
171 my $num = sub {
173 my @a = ('a' .. 'z');
174 unwind +(want_at(HERE) ? @a : scalar @a) => HERE;
175 # not reached
176 }->();
177 will rightfully set $num to 26.
179
181 "SU_THREADSAFE"
182 True iff the module could have been built when thread-safety features.
183
185 Constants
186 "TOP"
187 Returns the context that currently represents the highest scope.
189 "HERE"
191 The context of the current scope.
193 Getting a context from a context
195 For any of those functions, $from is expected to be a context. When
196 omitted, it defaults to the the current context.
197 "UP $from"
199 The context of the scope just above $from.
201 "SUB $from"
203 The context of the closest subroutine above $from. Note that $from is
205 returned if it is already a subroutine context ; hence "SUB SUB ==
206 SUB".
207 "EVAL $from"
209 The context of the closest eval above $from. Note that $from is
211 returned if it is already an eval context ; hence "EVAL EVAL == EVAL".
212 Getting a context from a level
214 Here, $level should denote a number of scopes above the current one.
215 When omitted, it defaults to 0 and those functions return the same
216 context as "HERE".
217 "SCOPE $level"
219 The $level-th upper context, regardless of its type.
221 "CALLER $level"
223 The context of the $level-th upper subroutine/eval/format. It kind of
225 corresponds to the context represented by "caller $level", but while
226 e.g. "caller 0" refers to the caller context, "CALLER 0" will refer to
227 the top scope in the current context.
228 Examples
230 Where "reap" fires depending on the $cxt :
231 sub {
233 eval {
234 sub {
235 {
236 reap \&cleanup => $cxt;
237 ...
238 } # $cxt = SCOPE(0), or HERE
239 ...
240 }->(); # $cxt = SCOPE(1), or UP, or SUB, or CALLER, or CALLER(0)
241 ...
242 }; # $cxt = SCOPE(2), or UP UP, or UP SUB, or EVAL, or CALLER(1)
243 ...
244 }->(); # $cxt = SCOPE(3), or SUB UP SUB, or SUB EVAL, or CALLER(2)
245 ...
246 Where "localize", "localize_elem" and "localize_delete" act depending
248 on the $cxt :
249 sub {
251 eval {
252 sub {
253 {
254 localize '$x' => 1 => $cxt;
255 # $cxt = SCOPE(0), or HERE
256 ...
257 }
258 # $cxt = SCOPE(1), or UP, or SUB, or CALLER, or CALLER(0)
259 ...
260 }->();
261 # $cxt = SCOPE(2), or UP UP, or UP SUB, or EVAL, or CALLER(1)
262 ...
263 };
264 # $cxt = SCOPE(3), or SUB UP SUB, or SUB EVAL, or CALLER(2)
265 ...
266 }->();
267 # $cxt = SCOPE(4), UP SUB UP SUB, or UP SUB EVAL, or UP CALLER(2), or TOP
268 ...
269 Where "unwind" and "want_at" point to depending on the $cxt:
271 sub {
273 eval {
274 sub {
275 {
276 unwind @things => $cxt;
277 ...
278 }
279 ...
280 }->(); # $cxt = SCOPE(0 .. 1), or HERE, or UP, or SUB, or CALLER(0)
281 ...
282 }; # $cxt = SCOPE(2), or UP UP, or UP SUB, or EVAL, or CALLER(1)
283 ...
284 }->(); # $cxt = SCOPE(3), or SUB UP SUB, or SUB EVAL, or CALLER(2)
285 ...
286
288 The functions "reap", "localize", "localize_elem", "localize_delete",
289 "unwind" and "want_at" are only exported on request, either
290 individually or by the tags ':funcs' and ':all'.
291 The constant "SU_THREADSAFE" is also only exported on request,
293 individually or by the tags ':consts' and ':all'.
294 Same goes for the words "TOP", "HERE", "UP", "SUB", "EVAL", "SCOPE" and
296 "CALLER" that are only exported on request, individually or by the tags
297 ':words' and ':all'.
298
300 Be careful that local variables are restored in the reverse order in
301 which they were localized. Consider those examples:
302 local $x = 0;
304 {
305 reap sub { print $x } => HERE;
306 local $x = 1;
307 ...
308 }
309 # prints '0'
310 ...
311 {
312 local $x = 1;
313 reap sub { $x = 2 } => HERE;
314 ...
315 }
316 # $x is 0
317 The first case is "solved" by moving the "local" before the "reap", and
319 the second by using "localize" instead of "reap".
320 The effects of "reap", "localize" and "localize_elem" can't cross
322 "BEGIN" blocks, hence calling those functions in "import" is deemed to
323 be useless. This is an hopeless case because "BEGIN" blocks are
324 executed once while localizing constructs should do their job at each
325 run. However, it's possible to hook the end of the current scope
326 compilation with B::Hooks::EndOfScope.
327 Some rare oddities may still happen when running inside the debugger.
329 It may help to use a perl higher than 5.8.9 or 5.10.0, as they contain
330 some context-related fixes.
331
333 XSLoader (standard since perl 5.006).
334
336 Alias, Hook::Scope, Scope::Guard, Guard.
337 Continuation::Escape is a thin wrapper around Scope::Upper that gives
339 you a continuation passing style interface to "unwind". It's easier to
340 use, but it requires you to have control over the scope where you want
341 to return.
342
344 Vincent Pit, "<perl at profvince.com>", <http://www.profvince.com>.
345 You can contact me by mail or on "irc.perl.org" (vincent).
347
349 Please report any bugs or feature requests to "bug-scope-upper at
350 rt.cpan.org", or through the web interface at
351 http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Scope-Upper
352 <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Scope-Upper>. I will
353 be notified, and then you'll automatically be notified of progress on
354 your bug as I make changes.
355
357 You can find documentation for this module with the perldoc command.
358 perldoc Scope::Upper
360 Tests code coverage report is available at
362 http://www.profvince.com/perl/cover/Scope-Upper
363 <http://www.profvince.com/perl/cover/Scope-Upper>.
364
366 Inspired by Ricardo Signes.
367 Thanks to Shawn M. Moore for motivation.
369
371 Copyright 2008,2009,2010 Vincent Pit, all rights reserved.
372 This program is free software; you can redistribute it and/or modify it
374 under the same terms as Perl itself.
375perl v5.12.0 2010-01-18 Scope::Upper(3)