MAN page from Fedora 30 perl-Attribute-Handlers-1.01-451.module_f30+8492+fa3b7961.noarch.rpm


Section: Perl Programmers Reference Guide (3pm)
Updated: 2019-10-24


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 todefine attribute handler subroutines for specific attributes. Variablesand subroutines subsequently defined in that package, or in packagesderived from that package may be given attributes with the same names asthe attribute handler subroutines, which will then be called in one ofthe compilation phases (i.e. in a "BEGIN", "CHECK", "INIT", or "END"block). ("UNITCHECK" blocks don't correspond to a global compilationphase, so they can't be specified here.)

To create a handler, define it as a subroutine with the same name asthe 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 classLoudDecl:

    package LoudDecl;    sub foo: Loud {...}

causes the above handler to be invoked, and passed:

the name of the package into which it was declared;
a reference to the symbol table entry (typeglob) containing the subroutine;
a reference to the subroutine;
the name of the attribute;
any data associated with that attribute;
the name of the phase in which the handler is being invoked;
the filename in which the handler is being invoked;
the line number in this file.

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

    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 intowhich the subroutine was declared, but it may also be the name of a derivedclass (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 thestring 'LEXICAL' in that case. Likewise, ascribing an attribute toan anonymous subroutine results in a symbol table argument of 'ANON'.

The data argument passes in the value (if any) associated with theattribute. 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 convertthe data argument ($_[4]) to a usable form before passing it tothe handler (but see ``Non-interpretive attribute handlers'').If those efforts succeed, the interpreted data is passed in an arrayreference; 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'sdata argument. While:

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

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

However, if the data can't be parsed as valid Perl, thenit 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 thedata 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 isascribed an attribute, the handler that is invoked is the one belonging tothe 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 alsodefines a handler for ":Loud" attributes). 

Type-specific attribute handlers

If an attribute handler is declared and the ":ATTR" specifier isgiven 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 beused 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 convertthe data argument ($_[4]) to a usable form before passing it tothe handler get in the way.

You can turn off that eagerness-to-help by declaringan 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 itreceives 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 compilationphase (in a "CHECK" block). This seems to be optimal in most cases becausemost things that can be defined are defined by that point but nothing hasbeen executed.

However, it is possible to set up attribute handlers that are called atother points in the program's compilation or execution, by explicitlystating the phase (or phases) in which you wish the attribute handler tobe 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 intwo or more phases. The phase name is passed as the handler's final argument.

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

Attributes as tie interfaces

Attributes make an excellent and intuitive interface through which to tievariables. 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, $datawill consist of a single array reference; otherwise, it will consist of thesingle argument directly. Since Tie::Cycle requires its cycling values tobe passed as an array reference, this means that we need to wrapnon-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 expectsits 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::Handlersprovides a way to automate it: specifying 'autotie' in the"use Attribute::Handlers" statement. 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::Whatever example), not as an array reference (as inthe original Tie::Cycle example at the start of this section).

The argument after 'autotie' is a reference to a hash in which each key isthe name of an attribute to be created, and each value is the class to whichvariables 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 passarguments to the module's "import" subroutine, by appending them to theclass name. For example:

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

If the attribute name is unqualified, the attribute is installed in thecurrent 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. Tofacilitate 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 packagethat imports the Tie::Me::Kangaroo:Down::Sport module.

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

Passing the tied object to "tie"

Occasionally it is important to pass a reference to the object being tiedto 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 variablebeing tied:

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


If the class shown in ``SYNOPSIS'' were placed in the MyClass.pmmodule, 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 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.


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 returnsundef. Note that "findsym" memoizes the typeglobs it has previouslysuccessfully found, so subsequent calls with the same arguments should bemuch faster.



Bad attribute type: ATTR(%s)
An attribute handler was specified with an ":ATTR(ref_type)", but thetype 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 notfor the specified type of declaration. Typically encountered when tryingto 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. Anattribute with an all-lowercase name might have a meaning to Perlitself some day, even though most don't yet. Use a mixed-case attributename, instead.
Can't have two ATTR specifiers on one subroutine
You just can't, okay?Instead, put all the specifications together with commas between themin 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 arenot declarable) that Perl can tie.
Internal error: %s symbol went missing
Something is rotten in the state of the program. An attributedsubroutine ceased to exist between the point it was declared and the pointat 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 appliedto a lexical variable. Since the variable may not be available during ENDthis won't happen.


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

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



Typed lexicals
Type-specific attribute handlers
Non-interpretive attribute handlers
Phase-specific attribute handlers
Attributes as tie interfaces

This document was created byman2html,using the manual pages.