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meta-0.004
River stage zero No dependents
10 ++
/ meta

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 0.003_003 the various undef-returning variants of can_... like functions or methods have aliases called try_get_... as this may be a more suitable naming pattern. The older can_... variants 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.

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.

name

   $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"

AUTHOR

Paul Evans <leonerd@leonerd.org.uk>