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NAME
attributes - get/set subroutine or variable attributes
SYNOPSIS
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
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. 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 attributes 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 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'".
What "import" does
In the description it is mentioned that
sub foo : method;
is equivalent to
use attributes __PACKAGE__, \&foo, 'method';
As you might know this calls the "import" function of "attributes" at compile 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).
"attributes.pm" checks if there is a subroutine called "MODIFY_<reftype>_ATTRIBUTES" in
the caller's namespace (here: 'main'). In this case a subroutine "MODIFY_CODE_ATTRIBUTES"
is 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 "import" croaks.
(See "Package-specific Attribute Handling" below.)
Built-in Attributes
The following are the built-in attributes for subroutines:
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.
This module allows one to set this attribute on a subroutine that is already defined.
For Perl subroutines (XSUBs are fine), it may or may not do what you want, depending
on the code inside the subroutine, with details subject to change in future Perl
versions. You may run into problems with lvalue context not being propagated properly
into the subroutine, or maybe even assertion failures. For this reason, a warning is
emitted if warnings are enabled. In other words, you should only do this if you
really know what you are doing. You have been warned.
method
Indicates that the referenced subroutine is a method. A subroutine so marked will not
trigger the "Ambiguous call resolved as CORE::%s" warning.
locked
The "locked" attribute is deprecated, and has no effect in 5.10.0 and later. It was
used as part of the now-removed "Perl 5.005 threads".
The following are the built-in attributes for variables:
shared
Indicates that the referenced variable can be shared across different threads when
used in conjunction with the threads and threads::shared modules.
unique
The "unique" attribute is deprecated, and has no effect in 5.10.0 and later. It used
to indicate that a single copy of an "our" variable was to be used by all interpreters
should the program happen to be running in a multi-interpreter environment.
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 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.
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 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 "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:
FETCH_type_ATTRIBUTES
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.
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 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 perlop.) 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
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
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
primarily useful to see how the "appropriate package" is found for the possible method
lookups for package-defined attributes.
1. Code:
package Canine;
package Dog;
my Canine $spot : Watchful ;
Effect:
use attributes ();
attributes::->import(Canine => \$spot, "Watchful");
2. Code:
package Felis;
my $cat : Nervous;
Effect:
use attributes ();
attributes::->import(Felis => \$cat, "Nervous");
3. Code:
package X;
sub foo : lvalue ;
Effect:
use attributes X => \&foo, "lvalue";
4. Code:
package X;
sub Y::x : lvalue { 1 }
Effect:
use attributes Y => \&Y::x, "lvalue";
5. Code:
package X;
sub foo { 1 }
package Y;
BEGIN { *bar = \&X::foo; }
package Z;
sub Y::bar : lvalue ;
Effect:
use attributes X => \&X::foo, "lvalue";
This last example is purely for purposes of completeness. You should not be trying to
mess with the attributes of something in a package that's not your own.
MORE EXAMPLES
1.
sub MODIFY_CODE_ATTRIBUTES {
my ($class,$code,@attrs) = @_;
my $allowed = 'MyAttribute';
my @bad = grep { $_ ne $allowed } @attrs;
return @bad;
}
sub foo : MyAttribute {
print "foo\n";
}
This example runs. At compile time "MODIFY_CODE_ATTRIBUTES" is called. In that
subroutine, we check if any attribute is disallowed and we return a list of these "bad
attributes".
As we return an empty list, everything is fine.
2.
sub MODIFY_CODE_ATTRIBUTES {
my ($class,$code,@attrs) = @_;
my $allowed = 'MyAttribute';
my @bad = grep{ $_ ne $allowed }@attrs;
return @bad;
}
sub foo : MyAttribute Test {
print "foo\n";
}
This example is aborted at compile time as we use the attribute "Test" which isn't
allowed. "MODIFY_CODE_ATTRIBUTES" returns a list that contains a single element
('Test').
SEE ALSO
"Private Variables via my()" in perlsub and "Subroutine Attributes" in perlsub for details
on the basic declarations; "use" in perlfunc for details on the normal invocation
mechanism.