[perl5.git] / dist / Attribute-Handlers / README

==============================================================================
                              Attribute::Handlers
==============================================================================


NAME
    Attribute::Handlers - Simpler definition of attribute handlers

VERSION
    This document describes version 0.79 of Attribute::Handlers, released
    November 25, 2007.

SYNOPSIS
            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']);

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 in one of
    the compilation phases (i.e. in a "BEGIN", "CHECK", "INIT", or "END"
    block). ("UNITCHECK" blocks don't correspond to a global compilation
    phase, so they can't be specified here.)

    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, $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
    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;

    [5] the name of the phase in which the handler is being invoked;

    [6] the filename in which the handler is being invoked;

    [7] the line number in this file.

    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 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 convert the data argument
    ($_[4]) to a useable form before passing it to the handler (but see
    "Non-interpretive attribute handlers"). If those efforts succeed, the
    interpreted data is passed in an array reference; 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/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. While:

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

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

    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 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 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 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 '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 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: 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 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:

            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 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::Kangaroo: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 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', \$var, @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) :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.

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.

    "Can't have two ATTR specifiers on one subroutine"
        You just can't, okay? Instead, put all the specifications 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). The maintainer of this module is now
    Rafael Garcia-Suarez (rgarciasuarez@gmail.com).

    Maintainer of the CPAN release is Steffen Mueller (smueller@cpan.org).
    Contact him with technical difficulties with respect to the packaging of
    the CPAN module.

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

COPYRIGHT AND LICENSE
             Copyright (c) 2001-2009, 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.