Typo in POD

[perl/modules/Variable-Magic.git] / README

NAME
    Variable::Magic - Associate user-defined magic to variables from Perl.

VERSION
    Version 0.52

SYNOPSIS
        use Variable::Magic qw<wizard cast VMG_OP_INFO_NAME>;

        { # A variable tracer
         my $wiz = wizard(
          set  => sub { print "now set to ${$_[0]}!\n" },
          free => sub { print "destroyed!\n" },
         );

         my $a = 1;
         cast $a, $wiz;
         $a = 2;        # "now set to 2!"
        }               # "destroyed!"

        { # A hash with a default value
         my $wiz = wizard(
          data     => sub { $_[1] },
          fetch    => sub { $_[2] = $_[1] unless exists $_[0]->{$_[2]}; () },
          store    => sub { print "key $_[2] stored in $_[-1]\n" },
          copy_key => 1,
          op_info  => VMG_OP_INFO_NAME,
         );

         my %h = (_default => 0, apple => 2);
         cast %h, $wiz, '_default';
         print $h{banana}, "\n"; # "0" (there is no 'banana' key in %h)
         $h{pear} = 1;           # "key pear stored in helem"
        }

DESCRIPTION
    Magic is Perl's way of enhancing variables. This mechanism lets the user
    add extra data to any variable and hook syntactical operations (such as
    access, assignment or destruction) that can be applied to it. With this
    module, you can add your own magic to any variable without having to
    write a single line of XS.

    You'll realize that these magic variables look a lot like tied
    variables. It is not surprising, as tied variables are implemented as a
    special kind of magic, just like any 'irregular' Perl variable : scalars
    like $!, $( or $^W, the %ENV and %SIG hashes, the @ISA array, "vec()"
    and "substr()" lvalues, threads::shared variables... They all share the
    same underlying C API, and this module gives you direct access to it.

    Still, the magic made available by this module differs from tieing and
    overloading in several ways :

    *   Magic is not copied on assignment.

        You attach it to variables, not values (as for blessed references).

    *   Magic does not replace the original semantics.

        Magic callbacks usually get triggered before the original action
        takes place, and cannot prevent it from happening. This also makes
        catching individual events easier than with "tie", where you have to
        provide fallbacks methods for all actions by usually inheriting from
        the correct "Tie::Std*" class and overriding individual methods in
        your own class.

    *   Magic is multivalued.

        You can safely apply different kinds of magics to the same variable,
        and each of them will be invoked successively.

    *   Magic is type-agnostic.

        The same magic can be applied on scalars, arrays, hashes, subs or
        globs. But the same hook (see below for a list) may trigger
        differently depending on the the type of the variable.

    *   Magic is invisible at Perl level.

        Magical and non-magical variables cannot be distinguished with
        "ref", "tied" or another trick.

    *   Magic is notably faster.

        Mainly because perl's way of handling magic is lighter by nature,
        and because there is no need for any method resolution. Also, since
        you don't have to reimplement all the variable semantics, you only
        pay for what you actually use.

    The operations that can be overloaded are :

    *   *get*

        This magic is invoked when the variable is evaluated. It is never
        called for arrays and hashes.

    *   *set*

        This magic is called each time the value of the variable changes. It
        is called for array subscripts and slices, but never for hashes.

    *   *len*

        This magic only applies to arrays (though it used to also apply to
        scalars), and is triggered when the 'size' or the 'length' of the
        variable has to be known by Perl. This is typically the magic
        involved when an array is evaluated in scalar context, but also on
        array assignment and loops ("for", "map" or "grep"). The length is
        returned from the callback as an integer.

        Starting from perl 5.12, this magic is no longer called by the
        "length" keyword, and starting from perl 5.17.4 it is also no longer
        called for scalars in any situation, making this magic only
        meaningful on arrays. You can use the constants
        "VMG_COMPAT_SCALAR_LENGTH_NOLEN" and "VMG_COMPAT_SCALAR_NOLEN" to
        see if this magic is available for scalars or not.

    *   *clear*

        This magic is invoked when the variable is reset, such as when an
        array is emptied. Please note that this is different from undefining
        the variable, even though the magic is called when the clearing is a
        result of the undefine (e.g. for an array, but actually a bug
        prevent it to work before perl 5.9.5 - see the history).

    *   *free*

        This magic is called when a variable is destroyed as the result of
        going out of scope (but not when it is undefined). It behaves
        roughly like Perl object destructors (i.e. "DESTROY" methods),
        except that exceptions thrown from inside a *free* callback will
        always be propagated to the surrounding code.

    *   *copy*

        This magic only applies to tied arrays and hashes, and fires when
        you try to access or change their elements.

    *   *dup*

        This magic is invoked when the variable is cloned across threads. It
        is currently not available.

    *   *local*

        When this magic is set on a variable, all subsequent localizations
        of the variable will trigger the callback. It is available on your
        perl if and only if "MGf_LOCAL" is true.

    The following actions only apply to hashes and are available if and only
    if "VMG_UVAR" is true. They are referred to as *uvar* magics.

    *   *fetch*

        This magic is invoked each time an element is fetched from the hash.

    *   *store*

        This one is called when an element is stored into the hash.

    *   *exists*

        This magic fires when a key is tested for existence in the hash.

    *   *delete*

        This magic is triggered when a key is deleted in the hash,
        regardless of whether the key actually exists in it.

    You can refer to the tests to have more insight of where the different
    magics are invoked.

FUNCTIONS
  "wizard"
        wizard(
         data     => sub { ... },
         get      => sub { my ($ref, $data [, $op]) = @_; ... },
         set      => sub { my ($ref, $data [, $op]) = @_; ... },
         len      => sub {
          my ($ref, $data, $len [, $op]) = @_; ... ; return $newlen
         },
         clear    => sub { my ($ref, $data [, $op]) = @_; ... },
         free     => sub { my ($ref, $data [, $op]) = @_, ... },
         copy     => sub { my ($ref, $data, $key, $elt [, $op]) = @_; ... },
         local    => sub { my ($ref, $data [, $op]) = @_; ... },
         fetch    => sub { my ($ref, $data, $key [, $op]) = @_; ... },
         store    => sub { my ($ref, $data, $key [, $op]) = @_; ... },
         exists   => sub { my ($ref, $data, $key [, $op]) = @_; ... },
         delete   => sub { my ($ref, $data, $key [, $op]) = @_; ... },
         copy_key => $bool,
         op_info  => [ 0 | VMG_OP_INFO_NAME | VMG_OP_INFO_OBJECT ],
        )

    This function creates a 'wizard', an opaque object that holds the magic
    information. It takes a list of keys / values as argument, whose keys
    can be :

    *   "data"

        A code (or string) reference to a private data constructor. It is
        called in scalar context each time the magic is cast onto a
        variable, with $_[0] being a reference to this variable and @_[1 ..
        @_-1] being all extra arguments that were passed to "cast". The
        scalar returned from this call is then attached to the variable and
        can be retrieved later with "getdata".

    *   "get", "set", "len", "clear", "free", "copy", "local", "fetch",
        "store", "exists" and "delete"

        Code (or string) references to the respective magic callbacks. You
        don't have to specify all of them : the magic corresponding to
        undefined entries will simply not be hooked.

        When those callbacks are executed, $_[0] is a reference to the magic
        variable and $_[1] is the associated private data (or "undef" when
        no private data constructor is supplied with the wizard). Other
        arguments depend on which kind of magic is involved :

        *       *len*

                $_[2] contains the natural, non-magical length of the
                variable (which can only be a scalar or an array as *len*
                magic is only relevant for these types). The callback is
                expected to return the new scalar or array length to use, or
                "undef" to default to the normal length.

        *       *copy*

                $_[2] is a either an alias or a copy of the current key, and
                $_[3] is an alias to the current element (i.e. the value).
                Because $_[2] might be a copy, it is useless to try to
                change it or cast magic on it.

        *       *fetch*, *store*, *exists* and *delete*

                $_[2] is an alias to the current key. Note that $_[2] may
                rightfully be readonly if the key comes from a bareword, and
                as such it is unsafe to assign to it. You can ask for a copy
                instead by passing "copy_key => 1" to "wizard" which, at the
                price of a small performance hit, allows you to safely
                assign to $_[2] in order to e.g. redirect the action to
                another key.

        Finally, if "op_info => $num" is also passed to "wizard", then one
        extra element is appended to @_. Its nature depends on the value of
        $num :

        *       "VMG_OP_INFO_NAME"

                $_[-1] is the current op name.

        *       "VMG_OP_INFO_OBJECT"

                $_[-1] is the "B::OP" object for the current op.

        Both result in a small performance hit, but just getting the name is
        lighter than getting the op object.

        These callbacks are executed in scalar context and are expected to
        return an integer, which is then passed straight to the perl magic
        API. However, only the return value of the *len* magic callback
        currently holds a meaning.

    Each callback can be specified as :

    *   a code reference, which will be called as a subroutine.

    *   a string reference, where the string denotes which subroutine is to
        be called when magic is triggered. If the subroutine name is not
        fully qualified, then the current package at the time the magic is
        invoked will be used instead.

    *   a reference to "undef", in which case a no-op magic callback is
        installed instead of the default one. This may especially be helpful
        for *local* magic, where an empty callback prevents magic from being
        copied during localization.

    Note that *free* magic is never called during global destruction, as
    there is no way to ensure that the wizard object and the callback were
    not destroyed before the variable.

    Here is a simple usage example :

        # A simple scalar tracer
        my $wiz = wizard(
         get  => sub { print STDERR "got ${$_[0]}\n" },
         set  => sub { print STDERR "set to ${$_[0]}\n" },
         free => sub { print STDERR "${$_[0]} was deleted\n" },
        );

  "cast"
        cast [$@%&*]var, $wiz, @args

    This function associates $wiz magic to the supplied variable, without
    overwriting any other kind of magic. It returns true on success or when
    $wiz magic is already attached, and croaks on error. When $wiz provides
    a data constructor, it is called just before magic is cast onto the
    variable, and it receives a reference to the target variable in $_[0]
    and the content of @args in @_[1 .. @args]. Otherwise, @args is ignored.

        # Casts $wiz onto $x, passing (\$x, '1') to the data constructor.
        my $x;
        cast $x, $wiz, 1;

    The "var" argument can be an array or hash value. Magic for these
    scalars behaves like for any other, except that it is dispelled when the
    entry is deleted from the container. For example, if you want to call
    "POSIX::tzset" each time the 'TZ' environment variable is changed in
    %ENV, you can use :

        use POSIX;
        cast $ENV{TZ}, wizard set => sub { POSIX::tzset(); () };

    If you want to handle the possible deletion of the 'TZ' entry, you must
    also specify *store* magic.

  "getdata"
        getdata [$@%&*]var, $wiz

    This accessor fetches the private data associated with the magic $wiz in
    the variable. It croaks when $wiz does not represent a valid magic
    object, and returns an empty list if no such magic is attached to the
    variable or when the wizard has no data constructor.

        # Get the data attached to $wiz in $x, or undef if $wiz
        # did not attach any.
        my $data = getdata $x, $wiz;

  "dispell"
        dispell [$@%&*]variable, $wiz

    The exact opposite of "cast" : it dissociates $wiz magic from the
    variable. This function returns true on success, 0 when no magic
    represented by $wiz could be found in the variable, and croaks if the
    supplied wizard is invalid.

        # Dispell now.
        die 'no such magic in $x' unless dispell $x, $wiz;

CONSTANTS
  "MGf_COPY"
    Evaluates to true if and only if the *copy* magic is available. This is
    the case for perl 5.7.3 and greater, which is ensured by the
    requirements of this module.

  "MGf_DUP"
    Evaluates to true if and only if the *dup* magic is available. This is
    the case for perl 5.7.3 and greater, which is ensured by the
    requirements of this module.

  "MGf_LOCAL"
    Evaluates to true if and only if the *local* magic is available. This is
    the case for perl 5.9.3 and greater.

  "VMG_UVAR"
    When this constant is true, you can use the *fetch*, *store*, *exists*
    and *delete* magics on hashes. Initial "VMG_UVAR" capability was
    introduced in perl 5.9.5, with a fully functional implementation shipped
    with perl 5.10.0.

  "VMG_COMPAT_SCALAR_LENGTH_NOLEN"
    True for perls that don't call *len* magic when taking the "length" of a
    magical scalar.

  "VMG_COMPAT_SCALAR_NOLEN"
    True for perls that don't call *len* magic on scalars. Implies
    "VMG_COMPAT_SCALAR_LENGTH_NOLEN".

  "VMG_COMPAT_ARRAY_PUSH_NOLEN"
    True for perls that don't call *len* magic when you push an element in a
    magical array. Starting from perl 5.11.0, this only refers to pushes in
    non-void context and hence is false.

  "VMG_COMPAT_ARRAY_PUSH_NOLEN_VOID"
    True for perls that don't call *len* magic when you push in void context
    an element in a magical array.

  "VMG_COMPAT_ARRAY_UNSHIFT_NOLEN_VOID"
    True for perls that don't call *len* magic when you unshift in void
    context an element in a magical array.

  "VMG_COMPAT_ARRAY_UNDEF_CLEAR"
    True for perls that call *clear* magic when undefining magical arrays.

  "VMG_COMPAT_HASH_DELETE_NOUVAR_VOID"
    True for perls that don't call *delete* magic when you delete an element
    from a hash in void context.

  "VMG_COMPAT_GLOB_GET"
    True for perls that call *get* magic for operations on globs.

  "VMG_PERL_PATCHLEVEL"
    The perl patchlevel this module was built with, or 0 for non-debugging
    perls.

  "VMG_THREADSAFE"
    True if and only if this module could have been built with thread-safety
    features enabled.

  "VMG_FORKSAFE"
    True if and only if this module could have been built with fork-safety
    features enabled. This is always true except on Windows where it is
    false for perl 5.10.0 and below.

  "VMG_OP_INFO_NAME"
    Value to pass with "op_info" to get the current op name in the magic
    callbacks.

  "VMG_OP_INFO_OBJECT"
    Value to pass with "op_info" to get a "B::OP" object representing the
    current op in the magic callbacks.

COOKBOOK
  Associate an object to any perl variable
    This technique can be useful for passing user data through limited APIs.
    It is similar to using inside-out objects, but without the drawback of
    having to implement a complex destructor.

        {
         package Magical::UserData;

         use Variable::Magic qw<wizard cast getdata>;

         my $wiz = wizard data => sub { \$_[1] };

         sub ud (\[$@%*&]) : lvalue {
          my ($var) = @_;
          my $data = &getdata($var, $wiz);
          unless (defined $data) {
           $data = \(my $slot);
           &cast($var, $wiz, $slot)
                     or die "Couldn't cast UserData magic onto the variable";
          }
          $$data;
         }
        }

        {
         BEGIN { *ud = \&Magical::UserData::ud }

         my $cb;
         $cb = sub { print 'Hello, ', ud(&$cb), "!\n" };

         ud(&$cb) = 'world';
         $cb->(); # Hello, world!
        }

  Recursively cast magic on datastructures
    "cast" can be called from any magical callback, and in particular from
    "data". This allows you to recursively cast magic on datastructures :

        my $wiz;
        $wiz = wizard data => sub {
         my ($var, $depth) = @_;
         $depth ||= 0;
         my $r = ref $var;
         if ($r eq 'ARRAY') {
          &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for @$var;
         } elsif ($r eq 'HASH') {
          &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for values %$var;
         }
         return $depth;
        },
        free => sub {
         my ($var, $depth) = @_;
         my $r = ref $var;
         print "free $r at depth $depth\n";
         ();
        };

        {
         my %h = (
          a => [ 1, 2 ],
          b => { c => 3 }
         );
         cast %h, $wiz;
        }

    When %h goes out of scope, this prints something among the lines of :

        free HASH at depth 0
        free HASH at depth 1
        free SCALAR at depth 2
        free ARRAY at depth 1
        free SCALAR at depth 3
        free SCALAR at depth 3

    Of course, this example does nothing with the values that are added
    after the "cast".

PERL MAGIC HISTORY
    The places where magic is invoked have changed a bit through perl
    history. Here is a little list of the most recent ones.

    *   5.6.x

        *p14416* : *copy* and *dup* magic.

    *   5.8.9

        *p28160* : Integration of *p25854* (see below).

        *p32542* : Integration of *p31473* (see below).

    *   5.9.3

        *p25854* : *len* magic is no longer called when pushing an element
        into a magic array.

        *p26569* : *local* magic.

    *   5.9.5

        *p31064* : Meaningful *uvar* magic.

        *p31473* : *clear* magic was not invoked when undefining an array.
        The bug is fixed as of this version.

    *   5.10.0

        Since "PERL_MAGIC_uvar" is uppercased, "hv_magic_check()" triggers
        *copy* magic on hash stores for (non-tied) hashes that also have
        *uvar* magic.

    *   5.11.x

        *p32969* : *len* magic is no longer invoked when calling "length"
        with a magical scalar.

        *p34908* : *len* magic is no longer called when pushing / unshifting
        an element into a magical array in void context. The "push" part was
        already covered by *p25854*.

        *g9cdcb38b* : *len* magic is called again when pushing into a
        magical array in non-void context.

EXPORT
    The functions "wizard", "cast", "getdata" and "dispell" are only
    exported on request. All of them are exported by the tags ':funcs' and
    ':all'.

    All the constants are also only exported on request, either individually
    or by the tags ':consts' and ':all'.

CAVEATS
    In order to hook hash operations with magic, you need at least perl
    5.10.0 (see "VMG_UVAR").

    If you want to store a magic object in the private data slot, you will
    not be able to recover the magic with "getdata", since magic is not
    copied by assignment. You can work around this gotcha by storing a
    reference to the magic object instead.

    If you define a wizard with *free* magic and cast it on itself, it
    results in a memory cycle, so this destructor will not be called when
    the wizard is freed.

DEPENDENCIES
    perl 5.8.

    A C compiler. This module may happen to build with a C++ compiler as
    well, but don't rely on it, as no guarantee is made in this regard.

    Carp (core since perl 5), XSLoader (since 5.006).

    Copy tests need Tie::Array (core since perl 5.005) and Tie::Hash (since
    5.002). Some uvar tests need Hash::Util::FieldHash (since 5.009004).
    Glob tests need Symbol (since 5.002). Threads tests need threads and
    threads::shared (both since 5.007003).

SEE ALSO
    perlguts and perlapi for internal information about magic.

    perltie and overload for other ways of enhancing objects.

AUTHOR
    Vincent Pit, "<perl at profvince.com>", <http://www.profvince.com>.

    You can contact me by mail or on "irc.perl.org" (vincent).

BUGS
    Please report any bugs or feature requests to "bug-variable-magic at
    rt.cpan.org", or through the web interface at
    <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Variable-Magic>. I will
    be notified, and then you'll automatically be notified of progress on
    your bug as I make changes.

SUPPORT
    You can find documentation for this module with the perldoc command.

        perldoc Variable::Magic

    Tests code coverage report is available at
    <http://www.profvince.com/perl/cover/Variable-Magic>.

COPYRIGHT & LICENSE
    Copyright 2007,2008,2009,2010,2011,2012 Vincent Pit, all rights
    reserved.

    This program is free software; you can redistribute it and/or modify it
    under the same terms as Perl itself.