attributes - get/set subroutine or variable attributes |
attributes - get/set subroutine or variable attributes
sub foo : method ; my ($x,@y,%z) : Bent ; my $s = sub : method { ... };
use attributes (); # optional, to get subroutine declarations my @attrlist = attributes::get(\&foo);
use attributes 'get'; # import the attributes::get subroutine my @attrlist = get \&foo;
Subroutine declarations and definitions may optionally have attribute lists
associated with them. (Variable my
declarations also may, but see the
warning below.) Perl handles these declarations by passing some information
about the call site and the thing being declared along with the attribute
list to this module. In particular, the first example above is equivalent to
the following:
use attributes __PACKAGE__, \&foo, 'method';
The second example in the synopsis does something equivalent to this:
use attributes __PACKAGE__, \$x, 'Bent'; use attributes __PACKAGE__, \@y, 'Bent'; use attributes __PACKAGE__, \%z, 'Bent';
Yes, that's three invocations.
WARNING: attribute declarations for variables are an experimental feature. The semantics of such declarations could change or be removed in future versions. They are present for purposes of experimentation with what the semantics ought to be. Do not rely on the current implementation of this feature.
There are only a few attributes currently handled by Perl itself (or directly by this module, depending on how you look at it.) However, package-specific attributes are allowed by an extension mechanism. (See Package-specific Attribute Handling below.)
The setting of attributes happens at compile time. An attempt to set
an unrecognized attribute is a fatal error. (The error is trappable, but
it still stops the compilation within that eval
.) Setting an attribute
with a name that's all lowercase letters that's not a built-in attribute
(such as ``foo'')
will result in a warning with -w or use warnings 'reserved'
.
The following are the built-in attributes for subroutines:
lock
operator immediately after the
subroutine is entered.
There are no built-in attributes for anything other than subroutines.
The following subroutines are available for general use once this module has been loaded:
die()
(via Carp::croak)
to raise a fatal exception. If it can find an appropriate package name
for a class method lookup, it will include the results from a
FETCH_type_ATTRIBUTES
call in its return list, as described in
Package-specific Attribute Handling below.
Otherwise, only built-in attributes will be returned.
Note that these routines are not exported by default.
WARNING: the mechanisms described here are still experimental. Do not rely on the current implementation. In particular, there is no provision for applying package attributes to 'cloned' copies of subroutines used as closures. (See Making References in the perlref manpage for information on closures.) Package-specific attribute handling may change incompatibly in a future release.
When an attribute list is present in a declaration, a check is made to see
whether an attribute 'modify' handler is present in the appropriate package
(or its @ISA inheritance tree). Similarly, when attributes::get
is
called on a valid reference, a check is made for an appropriate attribute
'fetch' handler. See EXAMPLES to see how the ``appropriate package''
determination works.
The handler names are based on the underlying type of the variable being declared or of the reference passed. Because these attributes are associated with subroutine or variable declarations, this deliberately ignores any possibility of being blessed into some package. Thus, a subroutine declaration uses ``CODE'' as its type, and even a blessed hash reference uses ``HASH'' as its type.
The class methods invoked for modifying and fetching are these:
The call to this method is currently made during the processing of the declaration. In particular, this means that a subroutine reference will probably be for an undefined subroutine, even if this declaration is actually part of the definition.
Calling attributes::get()
from within the scope of a null package
declaration package ;
for an unblessed variable reference will
not provide any starting package name for the 'fetch' method lookup.
Thus, this circumstance will not result in a method call for package-defined
attributes. A named subroutine knows to which symbol table entry it belongs
(or originally belonged), and it will use the corresponding package.
An anonymous subroutine knows the package name into which it was compiled
(unless it was also compiled with a null package declaration), and so it
will use that package name.
An attribute list is a sequence of attribute specifications, separated by
whitespace or a colon (with optional whitespace).
Each attribute specification is a simple
name, optionally followed by a parenthesised parameter list.
If such a parameter list is present, it is scanned past as for the rules
for the q()
operator. (See Quote and Quote-like Operators in the perlop manpage.)
The parameter list is passed as it was found, however, and not as per q()
.
Some examples of syntactically valid attribute lists:
switch(10,foo(7,3)) : expensive Ugly('\(") :Bad _5x5 locked method
Some examples of syntactically invalid attribute lists (with annotation):
switch(10,foo() # ()-string not balanced Ugly('(') # ()-string not balanced 5x5 # "5x5" not a valid identifier Y2::north # "Y2::north" not a simple identifier foo + bar # "+" neither a colon nor whitespace
None.
The routines get
and reftype
are exportable.
The :ALL
tag will get all of the above exports.
Here are some samples of syntactically valid declarations, with annotation
as to how they resolve internally into use attributes
invocations by
perl. These examples are primarily useful to see how the ``appropriate
package'' is found for the possible method lookups for package-defined
attributes.
package Canine; package Dog; my Canine $spot : Watchful ;
Effect:
use attributes Canine => \$spot, "Watchful";
package Felis; my $cat : Nervous;
Effect:
use attributes Felis => \$cat, "Nervous";
package X; sub foo : locked ;
Effect:
use attributes X => \&foo, "locked";
package X; sub Y::x : locked { 1 }
Effect:
use attributes Y => \&Y::x, "locked";
package X; sub foo { 1 }
package Y; BEGIN { *bar = \&X::foo; }
package Z; sub Y::bar : locked ;
Effect:
use attributes X => \&X::foo, "locked";
This last example is purely for purposes of completeness. You should not be trying to mess with the attributes of something in a package that's not your own.
Private Variables via my() in the perlsub manpage and Subroutine Attributes in the perlsub manpage for details on the basic declarations; the attrs manpage for the obsolescent form of subroutine attribute specification which this module replaces; use in the perlfunc manpage for details on the normal invocation mechanism.
attributes - get/set subroutine or variable attributes |