attributes - get/set subroutine or variable attributes
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
- use attributes __PACKAGE__, \&foo, 'method';
The second example in the synopsis does something equivalent to this:
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 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 subroutine attributes happens at compile time.
Variable attributes in
our declarations are also applied at compile time.
my variables get their attributes applied at run-time.
This means that you have to reach the run-time component of the
before those attributes will get applied. For example:
- my $x : Bent = 42 if 0;
will neither assign 42 to $x nor will it apply the
to the variable.
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'
In the description it is mentioned that
is equivalent to
- use attributes __PACKAGE__, \&foo, 'method';
As you might know this calls the
import function of
time with these parameters: 'attributes', the caller's package name, the reference
to the code and 'method'.
- attributes->import( __PACKAGE__, \&foo, 'method' );
So you want to know what
import actually does?
First of all
import gets the type of the third parameter ('CODE' in this case).
checks if there is a subroutine called
in the caller's namespace (here: 'main'). In this case a subroutine
required. Then this method is called to check if you have used a "bad attribute".
The subroutine call in this example would look like
- MODIFY_CODE_ATTRIBUTES( 'main', \&foo, 'method' );
MODIFY_<reftype>_ATTRIBUTES has to return a list of all "bad attributes".
If there are any bad attributes
(See Package-specific Attribute Handling below.)
The following are the built-in attributes for subroutines:
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.
Indicates that the referenced subroutine is a method. A subroutine so marked will not trigger the "Ambiguous call resolved as CORE::%s" warning.
The "locked" attribute has no effect in 5.10.0 and later. It was used as part of the now-removed "Perl 5.005 threads".
The following subroutines are available for general use once this module has been loaded:
This routine expects a single parameter--a reference to a
subroutine or variable. It returns a list of attributes, which may be
empty. If passed invalid arguments, it uses 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.
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.
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 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
called on a valid reference, a check is made for an appropriate attribute
'fetch' handler. See EXAMPLES to see how the "appropriate package"
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:
This method is called with two arguments: the relevant package name, and a reference to a 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.
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.
from within the scope of a null 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
q() operator. (See Quote and Quote-like Operators in perlop.)
The parameter list is passed as it was found, however, and not as per
Some examples of syntactically valid attribute lists:
- switch(10,foo(7,3)) : expensive
- Ugly('\(") :Bad
- lvalue 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
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
perl. These examples are primarily useful to see how the "appropriate
package" is found for the possible method lookups for package-defined
- package Canine;
- package Dog;
- my Canine $spot : Watchful ;
- use attributes ();
- attributes::->import(Canine => \$spot, "Watchful");
- package Felis;
- my $cat : Nervous;
- use attributes ();
- attributes::->import(Felis => \$cat, "Nervous");
- use attributes X => \&foo, "lvalue";
- use attributes Y => \&Y::x, "lvalue";
- use attributes X => \&X::foo, "lvalue";
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.
This example runs. At compile time
is called. In that
subroutine, we check if any attribute is disallowed and we return a list of
these "bad attributes".
As we return an empty list, everything is fine.
This example is aborted at compile time as we use the attribute "Test" which
returns a list that contains a single