oracular (3) attributes.3perl.gz
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.
const
This experimental attribute, introduced in Perl 5.22, only applies to anonymous subroutines. It
causes the subroutine to be called as soon as the "sub" expression is evaluated. The return value is
captured and turned into a constant subroutine.
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.
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.