MirBSD manpage: Attribute::Handlers(3p)

Attribute::HandlePerlpProgrammers ReferencAttribute::Handlers(3p)


     Attribute::Handlers - Simpler definition of attribute


     This document describes version 0.78 of Attribute::Handlers,
     released October 5, 2002.


             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.

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             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).

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

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     [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

     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

     If a lexical variable is given an attribute, there is no
     symbol table to which it belongs, so the symbol table argu-
     ment ($_[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:

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             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 lex-
     ical 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" specif-
     ier is given the name of a built-in type ("SCALAR", "ARRAY",
     "HASH", or "CODE"), the handler is only applied to declara-
     tions 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:

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             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 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"

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     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 exe-

     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 exam-

             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 'ARRAY';
                     tie $$referent, 'Tie::Cycle', $data;

             # and thereafter...

             package main;

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

             while (<>) {
                     print $next;

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     Note that, because the "Cycle" attribute receives its argu-
     ments 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: specify-
     ing '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 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:

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             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" attri-
     bute in the package that imports the
     Tie::Me::Kangaroo:Down::Sport module.

     Note that it is important to quote the __CALLER__::Roo iden-
     tifier because a 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 tied to the TIESCALAR, TIEHASH, etc. that ties

     The "autotie" mechanism supports this too. The following

             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":

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             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 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
                                         '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

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             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/);

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             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.

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     "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" attri-
         bute 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,

     "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

     "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

     "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 (damian@conway.org)


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

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              Copyright (c) 2001, 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.

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