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Attribute::Handlers(3) Perl Programmers	Reference Guide	Attribute::Handlers(3)

       Attribute::Handlers - Simpler definition	of attribute handlers

       This document describes version 1.01 of Attribute::Handlers.

	   package MyClass;
	   require 5.006;
	   use Attribute::Handlers;
	   no warnings 'redefine';

	   sub Good : ATTR(SCALAR) {
	       my ($package, $symbol, $referent, $attr,	$data) = @_;

	       # Invoked for any scalar	variable with a	:Good attribute,
	       # provided the variable was declared in MyClass (or
	       # a derived class) or typed to MyClass.

	       # Do whatever to	$referent here (executed in CHECK phase).

	   sub Bad : ATTR(SCALAR) {
	       # Invoked for any scalar	variable with a	:Bad attribute,
	       # provided the variable was declared in MyClass (or
	       # a derived class) or typed to MyClass.

	   sub Good : ATTR(ARRAY) {
	       # Invoked for any array variable	with a :Good attribute,
	       # provided the variable was declared in MyClass (or
	       # a derived class) or typed to MyClass.

	   sub Good : ATTR(HASH) {
	       # Invoked for any hash variable with a :Good attribute,
	       # provided the variable was declared in MyClass (or
	       # a derived class) or typed to MyClass.

	   sub Ugly : ATTR(CODE) {
	       # Invoked for any subroutine declared in	MyClass	(or a
	       # derived class)	with an	:Ugly attribute.

	   sub Omni : ATTR {
	       # Invoked for any scalar, array,	hash, or subroutine
	       # with an :Omni attribute, provided the variable	or
	       # subroutine was	declared in MyClass (or	a derived class)
	       # or the	variable was typed to MyClass.
	       # Use ref($_[2])	to determine what kind of referent it was.

	   use Attribute::Handlers autotie => {	Cycle => Tie::Cycle };

	   my $next : Cycle(['A'..'Z']);

       This module, when inherited by a	package, allows	that package's class
       to define attribute handler subroutines for specific attributes.
       Variables and subroutines subsequently defined in that package, or in
       packages	derived	from that package may be given attributes with the
       same names as the attribute handler subroutines,	which will then	be
       called in one of	the compilation	phases (i.e. in	a "BEGIN", "CHECK",
       "INIT", or "END"	block).	("UNITCHECK" blocks don't correspond to	a
       global compilation phase, so they can't be specified here.)

       To create a handler, define it as a subroutine with the same name as
       the desired attribute, and declare the subroutine itself	with the
       attribute ":ATTR". For example:

	   package LoudDecl;
	   use Attribute::Handlers;

	   sub Loud :ATTR {
	       my ($package, $symbol, $referent, $attr,	$data, $phase,
		   $filename, $linenum)	= @_;
	       print STDERR
		   ref($referent), " ",
		   *{$symbol}{NAME}, " ",
		   "($referent)	", "was	just declared ",
		   "and	ascribed the ${attr} attribute ",
		   "with data ($data)\n",
		   "in phase $phase\n",
		   "in file $filename at line $linenum\n";

       This creates a handler for the attribute	":Loud"	in the class LoudDecl.
       Thereafter, any subroutine declared with	a ":Loud" attribute in the
       class LoudDecl:

	   package LoudDecl;

	   sub foo: Loud {...}

       causes the above	handler	to be invoked, and passed:

       [0] the name of the package into	which it was declared;

       [1] a reference to the symbol table entry (typeglob) containing the

       [2] a reference to the subroutine;

       [3] the name of the attribute;

       [4] any data associated with that attribute;

       [5] the name of the phase in which the handler is being invoked;

       [6] the filename	in which the handler is	being invoked;

       [7] the line number in this file.

       Likewise, declaring any variables with the ":Loud" attribute within the

	   package LoudDecl;

	   my $foo :Loud;
	   my @foo :Loud;
	   my %foo :Loud;

       will cause the handler to be called with	a similar argument list
       (except,	of course, that	$_[2] will be a	reference to the variable).

       The package name	argument will typically	be the name of the class into
       which the subroutine was	declared, but it may also be the name of a
       derived class (since handlers are inherited).

       If a lexical variable is	given an attribute, there is no	symbol table
       to which	it belongs, so the symbol table	argument ($_[1]) is set	to the
       string 'LEXICAL'	in that	case. Likewise,	ascribing an attribute to an
       anonymous subroutine results in a symbol	table argument of 'ANON'.

       The data	argument passes	in the value (if any) associated with the
       attribute. For example, if &foo had been	declared:

	       sub foo :Loud("turn it up to 11,	man!") {...}

       then a reference	to an array containing the string "turn	it up to 11,
       man!" would be passed as	the last argument.

       Attribute::Handlers makes strenuous efforts to convert the data
       argument	($_[4])	to a usable form before	passing	it to the handler (but
       see "Non-interpretive attribute handlers").  If those efforts succeed,
       the interpreted data is passed in an array reference; if	they fail, the
       raw data	is passed as a string.	For example, all of these:

	   sub foo :Loud(till=>ears=>are=>bleeding) {...}
	   sub foo :Loud(qw/till ears are bleeding/) {...}
	   sub foo :Loud(qw/till, ears,	are, bleeding/)	{...}
	   sub foo :Loud(till,ears,are,bleeding) {...}

       causes it to pass "['till','ears','are','bleeding']" as the handler's
       data argument. While:

	   sub foo :Loud(['till','ears','are','bleeding']) {...}

       causes it to pass "[ ['till','ears','are','bleeding'] ]"; the array
       reference specified in the data being passed inside the standard	array
       reference indicating successful interpretation.

       However,	if the data can't be parsed as valid Perl, then	it is passed
       as an uninterpreted string. For example:

	   sub foo :Loud(my,ears,are,bleeding) {...}
	   sub foo :Loud(qw/my ears are	bleeding) {...}

       cause the strings 'my,ears,are,bleeding'	and 'qw/my ears	are bleeding'
       respectively to be passed as the	data argument.

       If no value is associated with the attribute, "undef" is	passed.

   Typed lexicals
       Regardless of the package in which it is	declared, if a lexical
       variable	is ascribed an attribute, the handler that is invoked is the
       one belonging to	the package to which it	is typed. For example, the
       following declarations:

	   package OtherClass;

	   my LoudDecl $loudobj	: Loud;
	   my LoudDecl @loudobjs : Loud;
	   my LoudDecl %loudobjex : Loud;

       causes the LoudDecl::Loud handler to be invoked (even if	OtherClass
       also defines a handler for ":Loud" attributes).

   Type-specific attribute handlers
       If an attribute handler is declared and the ":ATTR" specifier is	given
       the name	of a built-in type ("SCALAR", "ARRAY", "HASH", or "CODE"), the
       handler is only applied to declarations of that type. For example, the
       following definition:

	   package LoudDecl;

	   sub RealLoud	:ATTR(SCALAR) {	print "Yeeeeow!" }

       creates an attribute handler that applies only to scalars:

	   package Painful;
	   use base LoudDecl;

	   my $metal : RealLoud;	   # invokes &LoudDecl::RealLoud
	   my @metal : RealLoud;	   # error: unknown attribute
	   my %metal : RealLoud;	   # error: unknown attribute
	   sub metal : RealLoud	{...}	   # error: unknown attribute

       You can,	of course, declare separate handlers for these types as	well
       (but you'll need	to specify "no warnings	'redefine'" to do it quietly):

	   package LoudDecl;
	   use Attribute::Handlers;
	   no warnings 'redefine';

	   sub RealLoud	:ATTR(SCALAR) {	print "Yeeeeow!" }
	   sub RealLoud	:ATTR(ARRAY) { print "Urrrrrrrrrr!" }
	   sub RealLoud	:ATTR(HASH) { print "Arrrrrgggghhhhhh!"	}
	   sub RealLoud	:ATTR(CODE) { croak "Real loud sub torpedoed" }

       You can also explicitly indicate	that a single handler is meant to be
       used for	all types of referents like so:

	   package LoudDecl;
	   use Attribute::Handlers;

	   sub SeriousLoud :ATTR(ANY) {	warn "Hearing loss imminent" }

       (I.e. "ATTR(ANY)" is a synonym for ":ATTR").

   Non-interpretive attribute handlers
       Occasionally the	strenuous efforts Attribute::Handlers makes to convert
       the data	argument ($_[4]) to a usable form before passing it to the
       handler get in the way.

       You can turn off	that eagerness-to-help by declaring an attribute
       handler with the	keyword	"RAWDATA". For example:

	   sub Raw	    : ATTR(RAWDATA) {...}
	   sub Nekkid	    : ATTR(SCALAR,RAWDATA) {...}
	   sub Au::Naturale : ATTR(RAWDATA,ANY)	{...}

       Then the	handler	makes absolutely no attempt to interpret the data it
       receives	and simply passes it as	a string:

	   my $power : Raw(1..100);	   # handlers receives "1..100"

   Phase-specific attribute handlers
       By default, attribute handlers are called at the	end of the compilation
       phase (in a "CHECK" block). This	seems to be optimal in most cases
       because most things that	can be defined are defined by that point but
       nothing has been	executed.

       However,	it is possible to set up attribute handlers that are called at
       other points in the program's compilation or execution, by explicitly
       stating the phase (or phases) in	which you wish the attribute handler
       to be called. For example:

	   sub Early	:ATTR(SCALAR,BEGIN) {...}
	   sub Normal	:ATTR(SCALAR,CHECK) {...}
	   sub Late	:ATTR(SCALAR,INIT) {...}
	   sub Final	:ATTR(SCALAR,END) {...}
	   sub Bookends	:ATTR(SCALAR,BEGIN,END)	{...}

       As the last example indicates, a	handler	may be set up to be (re)called
       in two or more phases. The phase	name is	passed as the handler's	final

       Note that attribute handlers that are scheduled for the "BEGIN" phase
       are handled as soon as the attribute is detected	(i.e. before any
       subsequently defined "BEGIN" blocks are executed).

   Attributes as "tie" interfaces
       Attributes make an excellent and	intuitive interface through which to
       tie variables. For example:

	   use Attribute::Handlers;
	   use Tie::Cycle;

	   sub UNIVERSAL::Cycle	: ATTR(SCALAR) {
	       my ($package, $symbol, $referent, $attr,	$data, $phase) = @_;
	       $data = [ $data ] unless	ref $data eq 'ARRAY';
	       tie $$referent, 'Tie::Cycle', $data;

	   # and thereafter...

	   package main;

	   my $next : Cycle('A'..'Z');	   # $next is now a tied variable

	   while (<>) {
	       print $next;

       Note that, because the "Cycle" attribute	receives its arguments in the
       $data variable, if the attribute	is given a list	of arguments, $data
       will consist of a single	array reference; otherwise, it will consist of
       the single argument directly. Since Tie::Cycle requires its cycling
       values to be passed as an array reference, this means that we need to
       wrap non-array-reference	arguments in an	array constructor:

	   $data = [ $data ] unless ref	$data eq 'ARRAY';

       Typically, however, things are the other	way around: the	tieable	class
       expects its arguments as	a flattened list, so the attribute looks like:

	   sub UNIVERSAL::Cycle	: ATTR(SCALAR) {
	       my ($package, $symbol, $referent, $attr,	$data, $phase) = @_;
	       my @data	= ref $data eq 'ARRAY' ? @$data	: $data;
	       tie $$referent, 'Tie::Whatever',	@data;

       This software pattern is	so widely applicable that Attribute::Handlers
       provides	a way to automate it: specifying 'autotie' in the "use
       Attribute::Handlers" statement. So, the cycling example,	could also be

	   use Attribute::Handlers autotie => {	Cycle => 'Tie::Cycle' };

	   # and thereafter...

	   package main;

	   my $next : Cycle(['A'..'Z']);     # $next is	now a tied variable

	   while (<>) {
	       print $next;

       Note that we now	have to	pass the cycling values	as an array reference,
       since the "autotie" mechanism passes "tie" a list of arguments as a
       list (as	in the Tie::Whatever example), not as an array reference (as
       in the original Tie::Cycle example at the start of this section).

       The argument after 'autotie' is a reference to a	hash in	which each key
       is the name of an attribute to be created, and each value is the	class
       to which	variables ascribed that	attribute should be tied.

       Note that there is no longer any	need to	import the Tie::Cycle module
       -- Attribute::Handlers takes care of that automagically.	You can	even
       pass arguments to the module's "import" subroutine, by appending	them
       to the class name. For example:

	   use Attribute::Handlers
		autotie	=> { Dir => 'Tie::Dir qw(DIR_UNLINK)' };

       If the attribute	name is	unqualified, the attribute is installed	in the
       current package.	Otherwise it is	installed in the qualifier's package:

	   package Here;

	   use Attribute::Handlers autotie => {
		Other::Good => Tie::SecureHash,	# tie attr installed in	Other::
			Bad => Tie::Taxes,	# tie attr installed in	Here::
	    UNIVERSAL::Ugly => Software::Patent	# tie attr installed everywhere

       Autoties	are most commonly used in the module to	which they actually
       tie, and	need to	export their attributes	to any module that calls them.
       To facilitate this, Attribute::Handlers recognizes a special "pseudo-
       class" -- "__CALLER__", which may be specified as the qualifier of an

	   package Tie::Me::Kangaroo:Down::Sport;

	   use Attribute::Handlers autotie =>
		{ '__CALLER__::Roo' => __PACKAGE__ };

       This causes Attribute::Handlers to define the "Roo" attribute in	the
       package that imports the	Tie::Me::Kangaroo:Down::Sport module.

       Note that it is important to quote the __CALLER__::Roo identifier
       because a bug in	perl 5.8 will refuse to	parse it and cause an unknown

       Passing the tied	object to "tie"

       Occasionally it is important to pass a reference	to the object being
       tied to the TIESCALAR, TIEHASH, etc. that ties it.

       The "autotie" mechanism supports	this too. The following	code:

	   use Attribute::Handlers autotieref => { Selfish => Tie::Selfish };
	   my $var : Selfish(@args);

       has the same effect as:

	   tie my $var,	'Tie::Selfish',	@args;

       But when	"autotieref" is	used instead of	"autotie":

	   use Attribute::Handlers autotieref => { Selfish => Tie::Selfish };
	   my $var : Selfish(@args);

       the effect is to	pass the "tie" call an extra reference to the variable
       being tied:

	   tie my $var,	'Tie::Selfish',	\$var, @args;

       If the class shown in "SYNOPSIS"	were placed in the module,
       then the	following code:

	   package main;
	   use MyClass;

	   my MyClass $slr :Good :Bad(1**1-1) :Omni(-vorous);

	   package SomeOtherClass;
	   use base MyClass;

	   sub tent { 'acle' }

	   sub fn :Ugly(sister)	:Omni('po',tent()) {...}
	   my @arr :Good :Omni(s/cie/nt/);
	   my %hsh :Good(q/bye/) :Omni(q/bus/);

       would cause the following handlers to be	invoked:

	   # my	MyClass	$slr :Good :Bad(1**1-1)	:Omni(-vorous);

	   MyClass::Good:ATTR(SCALAR)( 'MyClass',	   # class
				       'LEXICAL',	   # no	typeglob
				       \$slr,		   # referent
				       'Good',		   # attr name
				       undef		   # no	attr data
				       'CHECK',		   # compiler phase

	   MyClass::Bad:ATTR(SCALAR)( 'MyClass',	   # class
				      'LEXICAL',	   # no	typeglob
				      \$slr,		   # referent
				      'Bad',		   # attr name
				      0			   # eval'd attr data
				      'CHECK',		   # compiler phase

	   MyClass::Omni:ATTR(SCALAR)( 'MyClass',	   # class
				       'LEXICAL',	   # no	typeglob
				       \$slr,		   # referent
				       'Omni',		   # attr name
				       '-vorous'	   # eval'd attr data
				       'CHECK',		   # compiler phase

	   # sub fn :Ugly(sister) :Omni('po',tent()) {...}

	   MyClass::UGLY:ATTR(CODE)( 'SomeOtherClass',	   # class
				     \*SomeOtherClass::fn, # typeglob
				     \&SomeOtherClass::fn, # referent
				     'Ugly',		   # attr name
				     'sister'		   # eval'd attr data
				     'CHECK',		   # compiler phase

	   MyClass::Omni:ATTR(CODE)( 'SomeOtherClass',	   # class
				     \*SomeOtherClass::fn, # typeglob
				     \&SomeOtherClass::fn, # referent
				     'Omni',		   # attr name
				     ['po','acle']	   # eval'd attr data
				     'CHECK',		   # compiler phase

	   # my	@arr :Good :Omni(s/cie/nt/);

	   MyClass::Good:ATTR(ARRAY)( 'SomeOtherClass',	   # class
				      'LEXICAL',	   # no	typeglob
				      \@arr,		   # referent
				      'Good',		   # attr name
				      undef		   # no	attr data
				      'CHECK',		   # compiler phase

	   MyClass::Omni:ATTR(ARRAY)( 'SomeOtherClass',	   # class
				      'LEXICAL',	   # no	typeglob
				      \@arr,		   # referent
				      'Omni',		   # attr name
				      ""		   # eval'd attr data
				      'CHECK',		   # compiler phase

	   # my	%hsh :Good(q/bye) :Omni(q/bus/);

	   MyClass::Good:ATTR(HASH)( 'SomeOtherClass',	   # class
				     'LEXICAL',		   # no	typeglob
				     \%hsh,		   # referent
				     'Good',		   # attr name
				     'q/bye'		   # raw attr data
				     'CHECK',		   # compiler phase

	   MyClass::Omni:ATTR(HASH)( 'SomeOtherClass',	   # class
				     'LEXICAL',		   # no	typeglob
				     \%hsh,		   # referent
				     'Omni',		   # attr name
				     'bus'		   # eval'd attr data
				     'CHECK',		   # compiler phase

       Installing handlers into	UNIVERSAL, makes them...err..universal.	 For

	   package Descriptions;
	   use Attribute::Handlers;

	   my %name;
	   sub name { return $name{$_[2]}||*{$_[1]}{NAME} }

	   sub UNIVERSAL::Name :ATTR {
	       $name{$_[2]} = $_[4];

	   sub UNIVERSAL::Purpose :ATTR	{
	       print STDERR "Purpose of	", &name, " is $_[4]\n";

	   sub UNIVERSAL::Unit :ATTR {
	       print STDERR &name, " measured in $_[4]\n";

       Let's you write:

	   use Descriptions;

	   my $capacity	: Name(capacity)
			: Purpose(to store max storage capacity	for files)
			: Unit(Gb);

	   package Other;

	   sub foo : Purpose(to	foo all	data before barring it)	{ }

	   # etc.

       This module offers a single utility function, "findsym()".

	       my $symbol = Attribute::Handlers::findsym($package, $referent);

	   The function	looks in the symbol table of $package for the typeglob
	   for $referent, which	is a reference to a variable or	subroutine
	   (SCALAR, ARRAY, HASH, or CODE). If it finds the typeglob, it
	   returns it. Otherwise, it returns undef. Note that "findsym"
	   memoizes the	typeglobs it has previously successfully found,	so
	   subsequent calls with the same arguments should be much faster.

       "Bad attribute type: ATTR(%s)"
	   An attribute	handler	was specified with an ":ATTR(ref_type)", but
	   the type of referent	it was defined to handle wasn't	one of the
	   five	permitted: "SCALAR", "ARRAY", "HASH", "CODE", or "ANY".

       "Attribute handler %s doesn't handle %s attributes"
	   A handler for attributes of the specified name was defined, but not
	   for the specified type of declaration. Typically encountered	when
	   trying to apply a "VAR" attribute handler to	a subroutine, or a
	   "SCALAR" attribute handler to some other type of variable.

       "Declaration of %s attribute in package %s may clash with future
       reserved	word"
	   A handler for an attributes with an all-lowercase name was
	   declared. An	attribute with an all-lowercase	name might have	a
	   meaning to Perl itself some day, even though	most don't yet.	Use a
	   mixed-case attribute	name, instead.

       "Can't have two ATTR specifiers on one subroutine"
	   You just can't, okay?  Instead, put all the specifications together
	   with	commas between them in a single	"ATTR(specification)".

       "Can't autotie a	%s"
	   You can only	declare	autoties for types "SCALAR", "ARRAY", and
	   "HASH". They're the only things (apart from typeglobs -- which are
	   not declarable) that	Perl can tie.

       "Internal error:	%s symbol went missing"
	   Something is	rotten in the state of the program. An attributed
	   subroutine ceased to	exist between the point	it was declared	and
	   the point at	which its attribute handler(s) would have been called.

       "Won't be able to apply END handler"
	   You have defined an END handler for an attribute that is being
	   applied to a	lexical	variable.  Since the variable may not be
	   available during END	this won't happen.

       Damian Conway ( The maintainer of this module	is now
       Rafael Garcia-Suarez (

       Maintainer of the CPAN release is Steffen Mueller (
       Contact him with	technical difficulties with respect to the packaging
       of the CPAN module.

       There are undoubtedly serious bugs lurking somewhere in code this funky
       :-) Bug reports and other feedback are most welcome.

		Copyright (c) 2001-2014, Damian	Conway.	All Rights Reserved.
	      This module is free software. It may be used, redistributed
		  and/or modified under	the same terms as Perl itself.

perl v5.32.0			  2020-06-14		Attribute::Handlers(3)


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