attributes(3p) Perl Programmers Reference Guide attributes(3p)NAMEattributes - get/set subroutine or variable attributesSYNOPSIS
sub foo : method ;
my ($x,@y,%z) : Bent = 1;
my $s = sub : method { ... };
use attributes (); # optional, to get subroutine declarations
my @attrlist = attributes::get(\&foo);
use attributes 'get'; # import the attributes::get subroutine
my @attrlist = get \&foo;
DESCRIPTION
Subroutine declarations and definitions may optionally have
attribute lists associated with them. (Variable "my"
declarations also may, but see the warning below.) Perl
handles these declarations by passing some information about
the call site and the thing being declared along with the
attribute list to this module. In particular, the first
example above is equivalent to the following:
use attributes __PACKAGE__, \&foo, 'method';
The second example in the synopsis does something equivalent
to this:
use attributes ();
my ($x,@y,%z);
attributes::->import(__PACKAGE__, \$x, 'Bent');
attributes::->import(__PACKAGE__, \@y, 'Bent');
attributes::->import(__PACKAGE__, \%z, 'Bent');
($x,@y,%z) = 1;
Yes, that's a lot of expansion.
WARNING: attribute declarations for variables are still
evolving. The semantics and interfaces of such declarations
could change in future versions. They are present for pur-
poses 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 subroutine attributes happens at compile
time. Variable attributes in "our" declarations are also
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applied at compile time. However, "my" variables get their
attributes applied at run-time. This means that you have to
reach the run-time component of the "my" before those attri-
butes will get applied. For example:
my $x : Bent = 42 if 0;
will neither assign 42 to $x nor will it apply the "Bent"
attribute to the variable.
An attempt to set an unrecognized attribute is a fatal
error. (The error is trappable, but it still stops the com-
pilation 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'".
Built-in Attributes
The following are the built-in attributes for subroutines:
locked
5.005 threads only! The use of the "locked" attribute
currently only makes sense if you are using the depre-
cated "Perl 5.005 threads" implementation of threads.
Setting this attribute is only meaningful when the sub-
routine or method is to be called by multiple threads.
When set on a method subroutine (i.e., one marked with
the method attribute below), Perl ensures that any invo-
cation of it implicitly locks its first argument before
execution. When set on a non-method subroutine, Perl
ensures that a lock is taken on the subroutine itself
before execution. The semantics of the lock are exactly
those of one explicitly taken with the "lock" operator
immediately after the subroutine is entered.
method
Indicates that the referenced subroutine is a method.
This has a meaning when taken together with the locked
attribute, as described there. It also means that a
subroutine so marked will not trigger the "Ambiguous
call resolved as CORE::%s" warning.
lvalue
Indicates that the referenced subroutine is a valid
lvalue and can be assigned to. The subroutine must
return a modifiable value such as a scalar variable, as
described in perlsub.
For global variables there is "unique" attribute: see "our"
in perlfunc.
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Available Subroutines
The following subroutines are available for general use once
this module has been loaded:
get This routine expects a single parameter--a reference to
a subroutine or variable. It returns a list of attri-
butes, which may be empty. If passed invalid arguments,
it uses die() (via Carp::croak) to raise a fatal excep-
tion. 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.
reftype
This routine expects a single parameter--a reference to
a subroutine or variable. It returns the built-in type
of the referenced variable, ignoring any package into
which it might have been blessed. This can be useful for
determining the type value which forms part of the
method names described in "Package-specific Attribute
Handling" below.
Note that these routines are not exported by default.
Package-specific Attribute Handling
WARNING: the mechanisms described here are still experimen-
tal. Do not rely on the current implementation. In partic-
ular, there is no provision for applying package attributes
to 'cloned' copies of subroutines used as closures. (See
"Making References" in perlref 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 attri-
bute '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.
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The class methods invoked for modifying and fetching are
these:
FETCH_type_ATTRIBUTES
This method receives a single argument, which is a
reference to the variable or subroutine for which
package-defined attributes are desired. The expected
return value is a list of associated attributes. This
list may be empty.
MODIFY_type_ATTRIBUTES
This method is called with two fixed arguments, followed
by the list of attributes from the relevant declaration.
The two fixed arguments are the relevant package name
and a reference to the declared subroutine or variable.
The expected return value is a list of attributes which
were not recognized by this handler. Note that this
allows for a derived class to delegate a call to its
base class, and then only examine the attributes which
the base class didn't already handle for it.
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.
Syntax of Attribute Lists
An attribute list is a sequence of attribute specifications,
separated by whitespace or a colon (with optional whi-
tespace). 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 perlop.) The parameter list is passed as
it was found, however, and not as per "q()".
Some examples of syntactically valid attribute lists:
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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
EXPORTS
Default exports
None.
Available exports
The routines "get" and "reftype" are exportable.
Export tags defined
The ":ALL" tag will get all of the above exports.
EXAMPLES
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 pri-
marily useful to see how the "appropriate package" is found
for the possible method lookups for package-defined attri-
butes.
1. Code:
package Canine;
package Dog;
my Canine $spot : Watchful ;
Effect:
use attributes ();
attributes::->import(Canine => \$spot, "Watchful");
2. Code:
package Felis;
my $cat : Nervous;
Effect:
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use attributes ();
attributes::->import(Felis => \$cat, "Nervous");
3. Code:
package X;
sub foo : locked ;
Effect:
use attributes X => \&foo, "locked";
4. Code:
package X;
sub Y::x : locked { 1 }
Effect:
use attributes Y => \&Y::x, "locked";
5. Code:
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.
SEE ALSO
"Private Variables via my()" in perlsub and "Subroutine
Attributes" in perlsub for details on the basic declara-
tions; attrs for the obsolescent form of subroutine attri-
bute specification which this module replaces; "use" in
perlfunc for details on the normal invocation mechanism.
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