NAME

    meta - meta-programming API

SYNOPSIS

       use v5.14;
       use meta;
    
       my $metapkg = meta::get_package( "MyApp::Some::Package" );
    
       $metapkg->add_symbol(
          '&a_function' => sub { say "New function was created" }
       );
    
       MyApp::Some::Package::a_function();

DESCRIPTION

    This package provides an API for metaprogramming; that is, allowing
    code to inspect or manipulate parts of its own program structure. Parts
    of the perl interpreter itself can be accessed by means of
    "meta"-objects provided by this package. Methods on these objects allow
    inspection of details, as well as creating new items or removing
    existing ones.

    The intention of this API is to provide a nicer replacement for
    existing tricks such as no strict 'refs' and using globrefs, and also
    to be a more consistent place to add new abilities, such as more APIs
    for inspection and alteration of internal structures, metaprogramming
    around the new 'class' feature, and other such uses.

    This module should be considered experimental; no API stability
    guarantees are made at this time. Behaviour may be added, altered, or
    removed in later versions. Once a workable API shape has been found, it
    is hoped that this module will eventually become dual-life and shipped
    as part of Perl core, as the implementation for PPC 0022. See the link
    in the "SEE ALSO" section.

    Since version 0.003_002 all the entry-point functions and constructors
    in this module will provoke warnings in the meta::experimental
    category. They can be silenced by

       use meta;
       no warnings 'meta::experimental';

    Since version 0.005 the various can_...-prefixed variant accessor
    methods print deprecation warnings. They are likely to be removed soon.

FUNCTIONS

 get_package

       $metapkg = meta::get_package( $pkgname );

    Returns a metapackage reference representing the given package name,
    creating it if it did not previously exist.

    An alternative to meta::package->get in a plain function style.

 get_this_package

       $metapkg = meta::get_this_package;

    Since version 0.02.

    Returns a metapackage reference representing the package of the code
    that called the function.

    Useful for performing meta-programming on the contents of a module
    during its BEGIN or loading time. Equivalent to but more efficient than
    the following:

       meta::get_package(__PACKAGE__)

METHODS ON meta::package

 get

       $metapkg = meta::package->get( $pkgname );

    Since version 0.003_001.

    Returns a metapackage reference representing the given package name,
    creating it if it did not previously exist.

    An alternative to meta::get_package in an object constructor style.

 name

       $name = $metapkg->name;

    Returns the name of the package being represented.

 get_glob

       $metaglob = $metapkg->get_glob( $name );

    Returns a metaglob reference representing the given symbol name within
    the package, if it exists. Throws an exception if not.

 try_get_glob, can_glob

       $metaglob = $metapkg->try_get_glob( $name );
       $metaglob = $metapkg->can_glob( $name );

    Similar to "get_glob" but returns undef if the glob does not exist.

 list_globs

       @metaglobs = $metapkg->list_globs;

    Since version 0.005.

    Returns a list of all the globs in the package that may refer to
    symbols (i.e. not subpackages). They are returned in no particular
    order.

 list_subpackage_globs

 list_all_globs

       @metaglobs = $metapkg->list_subpackage_globs;
       @metaglobs = $metapkg->list_all_globs;

    Since version 0.005.

    Returns a list of all the globs in the package that refer to
    subpackages, or all globs, including subpackages. They are returned in
    no particular order.

 get_symbol

       $metasym = $metapkg->get_symbol( $name );

    Returns a metasymbol reference representing the given symbol name
    within the package. The symbol name should include the leading sigil;
    one of the characters *, $, @, % or &. Throws an exception if the
    symbol does not exist.

 try_get_symbol, can_symbol

       $metasym = $metapkg->try_get_symbol( $name );
       $metasym = $metapkg->can_symbol( $name );

    Similar to "get_symbol" but returns undef if the symbol does not exist.

 add_symbol

       $metasym = $metapkg->add_symbol( $name, $valueref );

    Creates a new symbol of the given name in the given package. The new
    symbol will refer to the item given by reference, whose type must match
    the sigil of the symbol name. Returns a metasymbol reference as per
    "get_symbol". If a symbol already existed of the given name then an
    exception is thrown.

    Note that this does not create a copy of a variable, but stores an
    alias to the referred item itself within the symbol table.

       $metapkg->add_symbol( '@things', \my @array );
    
       push @array, "more", "values";
       # these values are now visible in the @things array

    If adding a scalar, array or hash variable, the $valueref argument is
    optional. If not provided then a new, blank variable of the correct
    type will be created.

 get_or_add_symbol

       $metasym = $metapkg->get_or_add_symbol( $name, $valueref );

    Since version 0.003_003.

    Similar to "get_symbol" but creates a new symbol if it didn't already
    exist as per "add_symbol".

    Note that if the symbol did already exist it is returned and $valueref
    will be ignored. The symbol will not be modified in that case to point
    to the value referred to instead.

 remove_symbol

       $metapkg->remove_symbol( $name );

    Removes a symbol of the given name from the given package. If the
    symbol was the last item in the glob then the glob too is removed from
    the package. If the named symbol did not previously exist then an
    exception is thrown.

    To only conditionally remove a symbol if it already exists, test for it
    first by using "try_get_symbol":

       $metapkg->try_get_symbol( '$variable' ) and
          $metapkg->remove_symbol( '$variable' );

METHODS ON METASYMBOLS

 is_glob, is_scalar, ...

       $bool = $metasym->is_glob;
       $bool = $metasym->is_scalar;
       $bool = $metasym->is_array;
       $bool = $metasym->is_hash;
       $bool = $metasym->is_subroutine;

    Returns true if the symbol being referred to is of the given type, or
    false if not.

 reference

       $ref = $metasym->reference;

    Returns a regular Perl reference to the symbol being represented.

METHODS ON meta::glob

 get

       $metaglob = meta::glob->get( $globname );

    Since version 0.003_001.

    Returns a metaglob reference representing the given symbol from the
    symbol table from a fully-qualified name, if it exists. Throws an
    exception if not.

 try_get

       $metaglob = meta::glob->try_get( $globname );

    Since version 0.003_003.

    Similar to "get" but returns undef if the given symbol does not exist.

 get_or_add

       $metaglob = meta::glob->get_or_add( $globname );

    Since version 0.003_003.

    Similar to "get" but creates the symbol if it didn't already exist.

 basename

       $name = $metaglob->basename;

    Returns the name of the glob within its package.

 get_scalar, get_array, ...

       $metasym = $metaglob->get_scalar;
       $metasym = $metaglob->get_array;
       $metasym = $metaglob->get_hash;
       $metasym = $metaglob->get_code;

    Returns a metasymbol reference representing the symbol in the given
    slot of the glob, if it exists. Throws an exception if not.

 try_get_scalar, try_get_array, ...

    Similar to "get_scalar", "get_array", etc... but returns undef if the
    given slot does not exist.

METHODS ON METAVARIABLES

 value

       $scalar = $metavar->value;
       @array  = $metavar->value;
       %hash   = $metavar->value;
    
       $count = scalar $metavar->value;

    Returns the current value of the variable, as if it appeared in regular
    Perl code.

METHODS ON METASUBROUTINES

 subname

       $name = $metasub->subname;

    Returns the (fully-qualified) name of the subroutine.

 prototype

       $proto = $metasub->prototype;

    Returns the prototype of the subroutine.

TODO

      * Setting the subname or prototype of a subroutine, inspired by the
      Sub::Util functions of the same.

      * Some kind of access around the signature of a signatured sub.

      * Access to the new parts of API introduced by Perl 5.38 to deal with
      classes, methods, fields.

SEE ALSO

    PPC 0022 "metaprogramming"
    <https://github.com/Perl/PPCs/blob/main/ppcs/ppc0022-metaprogramming.md>

AUTHOR

    Paul Evans <leonerd@leonerd.org.uk>

