/ 
Attribute-Handlers-0.61
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/ Attribute::Handlers

NAME

Attribute::Handlers - Simpler definition of attribute handlers

VERSION

This document describes version 0.61 of Attribute::Handlers, released May 10, 2001.

SYNOPSIS

package MyClass;
require v5.6.0;
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']);

DESCRIPTION

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 at the end of the compilation phase (i.e. in a CHECK block).

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, $data) = @_;
	print STDERR
		ref($referent), " ",
		*{$symbol}{NAME}, " ",
		"($referent) ", "was just declared ",
		"and ascribed the ${attr} attribute ",
		"with data ($data)\n";
}

This creates an 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 subroutine;

[2]

a reference to the subroutine;

[3]

the name of the attribute;

[4]

any data associated with that attribute.

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

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 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 useable form before passing it to the handler (but see "Non-interpretive attribute handlers"). For example, all of these:

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

causes it to pass ['till','ears','are','bleeding'] as the handler's data argument. 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 the attribute has only a single associated scalar data value, that value is passed as a scalar. If multiple values are associated, they are passed as an array reference. 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 useable 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 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"

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) = @_;
        $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;
}

In fact, this pattern is so widely applicable that Attribute::Handlers provides a way to automate it: specifying 'autotie' in the use Attribute::Handlers statement. So, the previous example, could also be written:

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;

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.

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
};

EXAMPLES

If the class shown in SYNOPSIS were placed in the MyClass.pm 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
                          );

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

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


# 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
                        );

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


# 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
                         );

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


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

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

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.

DIAGNOSTICS

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 whe 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.

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 end of the compilation phase (when its attribute handler(s) would have been called).

AUTHOR

Damian Conway (damian@conway.org)

BUGS

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

COPYRIGHT

   Copyright (c) 2001, Damian Conway. All Rights Reserved.
 This module is free software. It may be used, redistributed
and/or modified under the terms of the Perl Artistic License
      (see http://www.perl.com/perl/misc/Artistic.html)