attribute::handlers(3)

NAME

Attribute::Handlers - Simpler definition of attribute han
dlers

VERSION

This document describes version 0.77 of Attribute::Han
dlers, released June 8, 2002.

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 subse
quently 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).

To create a handler, define it as a subroutine with the same name as the desired attribute, and declare the sub routine itself with the attribute ":ATTR". For example:: package LoudDecl;
use Attribute::Handlers;; sub Loud :ATTR {
my ($package, $symbol, $referent, $attr,

$data, $phase) = @_;
print STDERR
ref($referent), " ",
*{$symbol}{NAME}, " ",
"($referent) ", "was just declared

",
"and ascribed the ${attr} at

tribute ",
"with data ($data)0,
"in phase $phase0;

}
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) con: taining the subroutine;
[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.
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 argu ment 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 han dlers"). 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 uninter preted 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 argu ment.
If the attribute has only a single associated scalar data value, that value is passed as a scalar. If multiple val ues 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 declara tions:: 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" speci fier 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 $metal : RealLoud; # invokes &LoudDe; cl::RealLoud
my @metal : RealLoud; # error: unknown; attribute
my %metal : RealLoud; # error: unknown; attribute
sub metal : RealLoud {...} # error: unknown; attribute
You can, of course, declare separate handlers for these types as well (but you'll need to specify "no warnings 'redefine'" to do it quietly):: package LoudDecl;
use Attribute::Handlers;
no warnings 'redefine';; sub RealLoud :ATTR(SCALAR) { print "Yeeeeow!" }
sub RealLoud :ATTR(ARRAY) { print "Urrrrrrrrrr!" }
sub RealLoud :ATTR(HASH) { print "Arrrrrgggghhhh; hh!" }
sub RealLoud :ATTR(CODE) { croak "Real loud sub; torpedoed" }
You can also explicitly indicate that a single handler is meant to be used for all types of referents like so:: package LoudDecl;
use Attribute::Handlers;; sub SeriousLoud :ATTR(ANY) { warn "Hearing loss; imminent" }
(I.e. "ATTR(ANY)" is a synonym for ":ATTR").
Non-interpretive attribute handlers
Occasionally the strenuous efforts Attribute::Handlers makes to convert the data argument ($_[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 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 $power : Raw(1..100); # handlers re; ceives "1..100"
Phase-specific attribute handlers
By default, attribute handlers are called at the end of the compilation phase (in a "CHECK" block). This seems to be optimal in most cases because most things that can be defined are defined by that point but nothing has been executed.
However, it is possible to set up attribute handlers that are called at other points in the program's compilation or execution, by explicitly stating the phase (or phases) in which you wish the attribute handler to be called. For example:: sub Early :ATTR(SCALAR,BEGIN) {...}
sub Normal :ATTR(SCALAR,CHECK) {...}
sub Late :ATTR(SCALAR,INIT) {...}
sub Final :ATTR(SCALAR,END) {...}
sub Bookends :ATTR(SCALAR,BEGIN,END) {...}
As the last example indicates, a handler may be set up to be (re)called in two or more phases. The phase name is passed as the handler's final argument.
Note that attribute handlers that are scheduled for the "BEGIN" phase are handled as soon as the attribute is detected (i.e. before any subsequently defined "BEGIN" blocks are executed).
Attributes as "tie" interfaces
Attributes make an excellent and intuitive interface through which to tie variables. For example:: use Attribute::Handlers;
use Tie::Cycle;; sub UNIVERSAL::Cycle : ATTR(SCALAR) {
my ($package, $symbol, $referent, $attr,

$data, $phase) = @_;
$data = [ $data ] unless ref $data eq 'AR

RAY';
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 argu ment directly. Since Tie::Cycle requires its cycling val ues 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: speci fying 'autotie' in the "use Attribute::Handlers" state ment. So, the cycling 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;
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::What ever example), not as an array reference (as in the origi nal 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_UN

LINK)' };
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 facilitiate this, Attribute::Handlers recognizes a special "pseudo-class" -"__CALLER__", which may be specified as the qualifier of an attribute:: 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::Kanga roo:Down::Sport module.
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', ar, @args;

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) :Om; ni(-vorous);; MyClass::Good:ATTR(SCALAR)( 'MyClass', #; class
'LEXICAL', #

no typeglob
lr, #

referent
'Good', #

attr name
undef #

no attr data
'CHECK', #

compiler phase; );
MyClass::Bad:ATTR(SCALAR)( 'MyClass', #
class: 'LEXICAL', #; no typeglob
lr, #; referent
'Bad', #; attr name
0 #; eval'd attr data
'CHECK', #; compiler phase
);
MyClass::Omni:ATTR(SCALAR)( 'MyClass', #
class: 'LEXICAL', #; no typeglob
lr, #; 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: omeOtherClass::fn, #; typeglob
SomeOtherClass::fn, #; referent
'Ugly', #; attr name
'sister' #; eval'd attr data
'CHECK', #; compiler phase
);
MyClass::Omni:ATTR(CODE)( 'SomeOtherClass', #
class: omeOtherClass::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..uni versal. 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]0;; }; sub UNIVERSAL::Unit :ATTR {
print STDERR &name, " measured in $_[4]0;; }
Let's you write:: use Descriptions;; my $capacity : Name(capacity)
: Purpose(to store max storage capac

ity 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 declara
tion. 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.
"Can't have two ATTR specifiers on one subroutine": You just can't, okay? Instead, put all the specifica
tions 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.

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

served.
This module is free software. It may be used, re
distributed: and/or modified under the same terms as Perl; itself.

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