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

       Attribute::Handlers - Simpler definition of attribute han-

       This document describes version 0.78 of Attribute::Han-
       dlers, 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 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 ${attr} attribute ",
                               "with data ($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;

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

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           containing 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 argu-
       ment list (except, of course, that $_[2] will be a refer-
       ence 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

               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) {...}

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       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-

       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

       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-

               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

               package LoudDecl;

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

       creates an attribute handler that applies only to scalars:

               package Painful;
               use base LoudDecl;

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               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"

       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

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       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

               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;

       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

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               $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_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;

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               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.

       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 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 it.

       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":

               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:

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               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

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               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..uni-
       versal.  For example:

               package Descriptions;
               use Attribute::Handlers;

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

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               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.

       "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

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       "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

       Damian Conway (damianATconway.org)

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

                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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