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.
- prototype(..)
- The "prototype" attribute is an alternate means of specifying a
prototype on a sub. The desired prototype is within the parens.
The prototype from the attribute is assigned to the sub immediately after
the prototype from the sub, which means that if both are declared at the
same time, the traditionally defined prototype is ignored. In other words,
"sub foo($$) : prototype(@) {}" is indistinguishable from
"sub foo(@){}".
If illegalproto warnings are enabled, the prototype declared inside this
attribute will be sanity checked at compile time.
- 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.
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.