attributes - get/set subroutine or variable attributes

- NAME
- SYNOPSIS
- DESCRIPTION

- Built-in Attributes
- Available Subroutines
- Package-specific Attribute Handling
- Syntax of Attribute Lists

- EXPORTS

- Default exports
- Available exports
- Export tags defined

- EXAMPLES
- SEE ALSO

# 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'`.

## Built-in Attributes

The following are the built-in attributes for subroutines:

locked
  5.005 threads only! The use of the ``locked'' attribute currently only makes sense if you are using the deprecated ``Perl 5.005 threads'' implementation of threads.

  Setting this attribute is only meaningful when the subroutine or method is to be called by multiple threads. When set on a method subroutine (i.e., one marked with the method attribute below), Perl ensures that any invocation of it implicitly locks its first argument before execution. When set on a non-method subroutine, Perl ensures that a lock is taken on the subroutine itself before execution. The semantics of the lock are exactly those of one explicitly taken with the `lock` operator immediately after the subroutine is entered.

method
  Indicates that the referenced subroutine is a method. This has a meaning when taken together with the locked attribute, as described there. It also means that a subroutine so marked will not trigger the ``Ambiguous call resolved as CORE::%s'' warning.

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 the perlsub manpage.

For global variables there is `unique` attribute: see our in the perlfunc manpage.

## 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 the perlref manpage 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 receives a single argument, which is a reference to the 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 the perlop manpage.) 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
        locked 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.

Code:

        package Canine;
        package Dog;
        my Canine $spot : Watchful ;

Effect:

        use attributes ();
        attributes::->import(Canine => \$spot, "Watchful");

Code:

        package Felis;
        my $cat : Nervous;

Effect:

        use attributes ();
        attributes::->import(Felis => \$cat, "Nervous");

Code:

        package X;
        sub foo : locked ;

Effect:

        use attributes X => \&foo, "locked";

Code:

        package X;
        sub Y::x : locked { 1 }

Effect:

        use attributes Y => \&Y::x, "locked";

Code:

        package X;
        sub foo { 1 }

        package Y;
        BEGIN { *bar = \&X::foo; }

        package Z;
        sub Y::bar : locked ;

Effect:

        use attributes X => \&X::foo, "locked";

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.

# SEE ALSO

Private Variables via my() in the perlsub manpage and Subroutine Attributes in the perlsub manpage for details on the basic declarations; the attrs manpage for the obsolescent form of subroutine attribute specification which this module replaces; use in the perlfunc manpage for details on the normal invocation mechanism.

attributes - get/set subroutine or variable attributes