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suppress scalar leak messages for known leaks (from
[perl5.git] / lib / attributes.pm
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09bef843
SB
1package attributes;
2
3$VERSION = 0.01;
4
5#@EXPORT_OK = qw(get reftype);
6#@EXPORT = ();
7
8use strict;
9
10sub croak {
11 require Carp;
12 goto &Carp::croak;
13}
14
15sub carp {
16 require Carp;
17 goto &Carp::carp;
18}
19
20## forward declaration(s) rather than wrapping the bootstrap call in BEGIN{}
21#sub reftype ($) ;
22#sub _fetch_attrs ($) ;
23#sub _guess_stash ($) ;
24#sub _modify_attrs ;
25#sub _warn_reserved () ;
26#
27# The extra trips through newATTRSUB in the interpreter wipe out any savings
28# from avoiding the BEGIN block. Just do the bootstrap now.
29BEGIN { bootstrap }
30
31sub import {
32 @_ > 2 && ref $_[2] or
33 croak 'Usage: use '.__PACKAGE__.' $home_stash, $ref, @attrlist';
34 my (undef,$home_stash,$svref,@attrs) = @_;
35
36 my $svtype = uc reftype($svref);
37 my $pkgmeth;
38 $pkgmeth = UNIVERSAL::can($home_stash, "MODIFY_${svtype}_ATTRIBUTES")
39 if defined $home_stash && $home_stash ne '';
40 my @badattrs;
41 if ($pkgmeth) {
42 my @pkgattrs = _modify_attrs($svref, @attrs);
43 @badattrs = $pkgmeth->($home_stash, $svref, @attrs);
44 if (!@badattrs && @pkgattrs) {
45 return unless _warn_reserved;
46 @pkgattrs = grep { m/\A[[:lower:]]+(?:\z|\()/ } @pkgattrs;
47 if (@pkgattrs) {
48 for my $attr (@pkgattrs) {
49 $attr =~ s/\(.+\z//s;
50 }
51 my $s = ((@pkgattrs == 1) ? '' : 's');
52 carp "$svtype package attribute$s " .
53 "may clash with future reserved word$s: " .
54 join(' , ' , @pkgattrs);
55 }
56 }
57 }
58 else {
59 @badattrs = _modify_attrs($svref, @attrs);
60 }
61 if (@badattrs) {
62 croak "Invalid $svtype attribute" .
63 (( @badattrs == 1 ) ? '' : 's') .
64 ": " .
65 join(' , ', @badattrs);
66 }
67}
68
69sub get ($) {
70 @_ == 1 && ref $_[0] or
71 croak 'Usage: '.__PACKAGE__.'::get $ref';
72 my $svref = shift;
73 my $svtype = uc reftype $svref;
74 my $stash = _guess_stash $svref;
75 $stash = caller unless defined $stash;
76 my $pkgmeth;
77 $pkgmeth = UNIVERSAL::can($stash, "FETCH_${svtype}_ATTRIBUTES")
78 if defined $stash && $stash ne '';
79 return $pkgmeth ?
80 (_fetch_attrs($svref), $pkgmeth->($stash, $svref)) :
81 (_fetch_attrs($svref))
82 ;
83}
84
85#sub export {
86# require Exporter;
87# goto &Exporter::import;
88#}
89#
90#sub require_version { goto &UNIVERSAL::VERSION }
91
921;
93__END__
94#The POD goes here
95
96=head1 NAME
97
98attributes - get/set subroutine or variable attributes
99
100=head1 SYNOPSIS
101
102 sub foo : method ;
103 my ($x,@y,%z) : Bent ;
104 my $s = sub : method { ... };
105
106 use attributes (); # optional, to get subroutine declarations
107 my @attrlist = attributes::get(\&foo);
108
109=head1 DESCRIPTION
110
111Subroutine declarations and definitions may optionally have attribute lists
112associated with them. (Variable C<my> declarations also may, but see the
113warning below.) Perl handles these declarations by passing some information
114about the call site and the thing being declared along with the attribute
115list to this module. In particular, first example above is equivalent to
116the following:
117
118 use attributes __PACKAGE__, \&foo, 'method';
119
120The second example in the synopsis does something equivalent to this:
121
122 use attributes __PACKAGE__, \$x, 'Bent';
123 use attributes __PACKAGE__, \@y, 'Bent';
124 use attributes __PACKAGE__, \%z, 'Bent';
125
126Yes, that's three invocations.
127
128B<WARNING>: attribute declarations for variables are an I<experimental>
129feature. The semantics of such declarations could change or be removed
130in future versions. They are present for purposes of experimentation
131with what the semantics ought to be. Do not rely on the current
132implementation of this feature.
133
134There are only a few attributes currently handled by Perl itself (or
135directly by this module, depending on how you look at it.) However,
136package-specific attributes are allowed by an extension mechanism.
137(See L<"Package-specific Attribute Handling"> below.)
138
139The setting of attributes happens at compile time. An attempt to set
140an unrecognized attribute is a fatal error. (The error is trappable, but
141it still stops the compilation within that C<eval>.) Setting an attribute
142with a name that's all lowercase letters that's not a built-in attribute
143(such as "foo")
144will result in a warning with B<-w> or C<use warnings 'reserved'>.
145
146=head2 Built-in Attributes
147
148The following are the built-in attributes for subroutines:
149
150=over 4
151
152=item locked
153
154Setting this attribute is only meaningful when the subroutine or
155method is to be called by multiple threads. When set on a method
156subroutine (i.e., one marked with the B<method> attribute below),
157Perl ensures that any invocation of it implicitly locks its first
158argument before execution. When set on a non-method subroutine,
159Perl ensures that a lock is taken on the subroutine itself before
160execution. The semantics of the lock are exactly those of one
161explicitly taken with the C<lock> operator immediately after the
162subroutine is entered.
163
164=item method
165
166Indicates that the referenced subroutine is a method.
167This has a meaning when taken together with the B<locked> attribute,
168as described there. It also means that a subroutine so marked
169will not trigger the "Ambiguous call resolved as CORE::%s" warning.
170
171=back
172
173There are no built-in attributes for anything other than subroutines.
174
175=head2 Available Subroutines
176
177The following subroutines are available for general use once this module
178has been loaded:
179
180=over 4
181
182=item get
183
184This routine expects a single parameter--a reference to a
185subroutine or variable. It returns a list of attributes, which may be
186empty. If passed invalid arguments, it uses die() (via L<Carp::croak|Carp>)
187to raise a fatal exception. If it can find an appropriate package name
188for a class method lookup, it will include the results from a
189C<FETCH_I<type>_ATTRIBUTES> call in its return list, as described in
190L"Package-specific Attribute Handling"> below.
191Otherwise, only L<built-in attributes|"Built-in Attributes"> will be returned.
192
193=item reftype
194
195This routine expects a single parameter--a reference to a subroutine or
196variable. It returns the built-in type of the referenced variable,
197ignoring any package into which it might have been blessed.
198This can be useful for determining the I<type> value which forms part of
199the method names described in L"Package-specific Attribute Handling"> below.
200
201=back
202
203Note that these routines are I<not> exported. This is primarily because
204the C<use> mechanism which would normally import them is already in use
205by Perl itself to implement the C<sub : attributes> syntax.
206
207=head2 Package-specific Attribute Handling
208
209B<WARNING>: the mechanisms described here are still experimental. Do not
210rely on the current implementation. In particular, there is no provision
211for applying package attributes to 'cloned' copies of subroutines used as
212closures. (See L<perlref/"Making References"> for information on closures.)
213Package-specific attribute handling may change incompatibly in a future
214release.
215
216When an attribute list is present in a declaration, a check is made to see
217whether an attribute 'modify' handler is present in the appropriate package
218(or its @ISA inheritance tree). Similarly, when C<attributes::get> is
219called on a valid reference, a check is made for an appropriate attribute
220'fetch' handler. See L<"EXAMPLES"> to see how the "appropriate package"
221determination works.
222
223The handler names are based on the underlying type of the variable being
224declared or of the reference passed. Because these attributes are
225associated with subroutine or variable declarations, this deliberately
226ignores any possibility of being blessed into some package. Thus, a
227subroutine declaration uses "CODE" as its I<type>, and even a blessed
228hash reference uses "HASH" as its I<type>.
229
230The class methods invoked for modifying and fetching are these:
231
232=over 4
233
234=item FETCH_I<type>_ATTRIBUTES
235
236This method receives a single argument, which is a reference to the
237variable or subroutine for which package-defined attributes are desired.
238The expected return value is a list of associated attributes.
239This list may be empty.
240
241=item MODIFY_I<type>_ATTRIBUTES
242
243This method is called with two fixed arguments, followed by the list of
244attributes from the relevant declaration. The two fixed arguments are
245the relevant package name and a reference to the declared subroutine or
246variable. The expected return value as a list of attributes which were
247not recognized by this handler. Note that this allows for a derived class
248to delegate a call to its base class, and then only examine the attributes
249which the base class didn't already handle for it.
250
251The call to this method is currently made I<during> the processing of the
252declaration. In particular, this means that a subroutine reference will
253probably be for an undefined subroutine, even if this declaration is
254actually part of the definition.
255
256=back
257
258Calling C<attributes::get()> from within the scope of a null package
259declaration C<package ;> for an unblessed variable reference will
260not provide any starting package name for the 'fetch' method lookup.
261Thus, this circumstance will not result in a method call for package-defined
262attributes. A named subroutine knows to which symbol table entry it belongs
263(or originally belonged), and it will use the corresponding package.
264An anonymous subroutine knows the package name into which it was compiled
265(unless it was also compiled with a null package declaration), and so it
266will use that package name.
267
268=head2 Syntax of Attribute Lists
269
270An attribute list is a sequence of attribute specifications, separated by
271whitespace, commas, or both. Each attribute specification is a simple
272name, optionally followed by a parenthesised parameter list.
273If such a parameter list is present, it is scanned past as for the rules
274for the C<q()> operator. (See L<perlop/"Quote and Quote-like Operators">.)
275The parameter list is passed as it was found, however, and not as per C<q()>.
276
277Some examples of syntactically valid attribute lists:
278
279 switch(10,foo(7,3)) , , expensive
280 Ugly('\(") , Bad
281 _5x5
282 locked method
283
284Some examples of syntactically invalid attribute lists (with annotation):
285
286 switch(10,foo() # ()-string not balanced
287 Ugly('(') # ()-string not balanced
288 5x5 # "5x5" not a valid identifier
289 Y2::north # "Y2::north" not a simple identifier
290 foo + bar # "+" neither a comma nor whitespace
291
292=head1 EXAMPLES
293
294Here are some samples of syntactically valid declarations, with annotation
295as to how they resolve internally into C<use attributes> invocations by
296perl. These examples are primarily useful to see how the "appropriate
297package" is found for the possible method lookups for package-defined
298attributes.
299
300=over 4
301
302=item 1.
303
304Code:
305
306 package Canine;
307 package Dog;
308 my Canine $spot : Watchful ;
309
310Effect:
311
312 use attributes Canine => \$spot, "Watchful";
313
314=item 2.
315
316Code:
317
318 package Felis;
319 my $cat : Nervous;
320
321Effect:
322
323 use attributes Felis => \$cat, "Nervous";
324
325=item 3.
326
327Code:
328
329 package X;
330 sub foo : locked ;
331
332Effect:
333
334 use attributes X => \&foo, "locked";
335
336=item 4.
337
338Code:
339
340 package X;
341 sub Y::x : locked { 1 }
342
343Effect:
344
345 use attributes Y => \&Y::x, "locked";
346
347=item 5.
348
349Code:
350
351 package X;
352 sub foo { 1 }
353
354 package Y;
355 BEGIN { *bar = \&X::foo; }
356
357 package Z;
358 sub Y::bar : locked ;
359
360Effect:
361
362 use attributes X => \&X::foo, "locked";
363
364=back
365
366This last example is purely for purposes of completeness. You should not
367be trying to mess with the attributes of something in a package that's
368not your own.
369
370=head1 SEE ALSO
371
372L<perlsub/"Private Variables via my()"> and
373L<perlsub/"Subroutine Attributes"> for details on the basic declarations;
374L<attrs> for the obsolescent form of subroutine attribute specification
375which this module replaces;
376L<perlfunc/use> for details on the normal invocation mechanism.
377
378=cut
379