attributes(3pm) 					 Perl Programmers Reference Guide					   attributes(3pm)

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 has no effect in 5.10.0 and later.  It was used as part of the now-removed "Perl 5.005 threads".

   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
";
	       }

	   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
";
	     }

	   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.

perl v5.16.2							    2012-10-25							   attributes(3pm)