Manpages - attributes.3perl

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.)

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.

The following subroutines are available for general use once this module has been loaded:

get

This routine expects a single parameterΩ-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 parameterΩ-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 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.

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

None.

The routines get and reftype are exportable.

The :ALL tag will get all of the above exports.