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

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". It	will disappear in Perl 5.28, after which its use will
	   be fatal.

       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.

       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. It will
	   disappear in	5.28, after which its use will be fatal.

   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.

perl v5.26.0			  2017-04-19			 attributes(3)

NAME | SYNOPSIS | DESCRIPTION | EXPORTS | EXAMPLES | MORE EXAMPLES | SEE ALSO

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