NAME

       "Attribute::Storage" - declare and retrieve named attributes about CODE references

SYNOPSIS

          package My::Package;

          use Attribute::Storage;

          sub Title :ATTR(CODE)
          {
             my $package = shift;
             my ( $title ) = @_;

             return $title;
          }

          package main;

          use Attribute::Storage qw( get_subattr );
          use My::Package;

          sub myfunc :Title('The title of my function')
          {
             ...
          }

          print "Title of myfunc is: ".get_subattr(\&myfunc, 'Title')."\n";

DESCRIPTION

       This package provides a base, where a package using it can define handlers for particular
       code attributes. Other packages, using the package that defines the code attributes, can
       then use them to annotate subs.

       This is similar to "Attribute::Handlers", with the following key differences:

       •   "Attribute::Storage" will store the value returned by the attribute handling code, and
           provides convenient lookup functions to retrieve it later.  "Attribute::Handlers"
           simply invokes the handling code.

       •   "Attribute::Storage" immediately executes the attribute handling code at compile-time.
           "Attribute::Handlers" defers invocation so it can look up the symbolic name of the sub
           the attribute is attached to. "Attribute::Storage" uses B to provide the name of the
           sub at invocation time, using the name of the underlying "GV".

       •   "Attribute::Storage" works just as well on anonymous subs as named ones.

       •   "Attribute::Storage" is safe to use on code that will be reloaded, because it executes
           handlers immediately. "Attribute::Handlers" will only execute handlers at defined
           phases such as "BEGIN" or "INIT", and cannot reexecute the handlers in a file once it
           has been reloaded.

ATTRIBUTES

       Each attribute that the defining package wants to define should be done using a marked
       subroutine, in a way similar to Attribute::Handlers. When a sub in the using package is
       marked with such an attribute, the code is executed, passing in the arguments. Whatever it
       returns is stored, to be returned later when queried by "get_subattr" or "get_subattrs".
       The return value must be defined, or else the attribute will be marked as a compile error
       for perl to handle accordingly.

       Only "CODE" attributes are supported at present.

          sub AttributeName :ATTR(CODE)
          {
             my $package = shift;
             my ( $attr, $args, $here ) = @_;
             ...
             return $value;
          }

       At attachment time, the optional string that may appear within brackets following the
       attribute's name is parsed as a Perl expression in list context.  If this succeeds, the
       values are passed as a list to the handling code. If this fails, an error is returned to
       the perl compiler. If no string is present, then an empty list is passed to the handling
       code.

          package Defining;

          sub NameMap :ATTR(CODE)
          {
             my $package = shift;
             my @strings = @_;

             return { map { m/^(.*)=(.*)$/ and ( $1, $2 ) } @strings };
          }

          package Using;

          use Defining;

          sub somefunc :NameMap("foo=FOO","bar=BAR","splot=WIBBLE") { ... }

          my $map = get_subattr("somefunc", "NameMap");
          # Will yield:
          #  { foo   => "FOO",
          #    bar   => "BAR",
          #    splot => "WIBBLE" }

       Note that it is impossible to distinguish

          sub somefunc :NameMap   { ... }
          sub somefunc :NameMap() { ... }

       It is possible to create attributes that do not parse their argument as a perl list
       expression, instead they just pass the plain string as a single argument.  For this, add
       the "RAWDATA" flag to the "ATTR()" list.

          sub Title :ATTR(CODE,RAWDATA)
          {
             my $package = shift;
             my ( $text ) = @_;

             return $text;
          }

          sub thingy :Title(Here is the title for thingy) { ... }

       To obtain the name of the function to which the attribute is being applied, use the "NAME"
       flag to the "ATTR()" list.

          sub Callable :ATTR(CODE,NAME)
          {
             my $package = shift;
             my ( $subname, @args ) = @_;

             print "The Callable attribute is being applied to $package :: $subname\n";

             return;
          }

       When applied to an anonymous function ("sub { ... }"), the name will appear as "__ANON__".

       Normally it is an error to attempt to apply the same attribute more than once to the same
       function. Sometimes however, it would make sense for an attribute to be applied many
       times. If the "ATTR()" list is given the "MULTI" flag, then applying it more than once
       will be allowed. Each invocation of the handling code will be given the previous value
       that was returned, or "undef" for the first time. It is up to the code to perform whatever
       merging logic is required.

          sub Description :ATTR(CODE,MULTI,RAWDATA)
          {
             my $package = shift;
             my ( $olddesc, $more ) = @_;

             return defined $olddesc ? "$olddesc$more\n" : "$more\n";
          }

          sub Argument :ATTR(CODE,MULTI)
          {
             my $package = shift;
             my ( $args, $argname ) = @_;

             push @$args, $argname;
             return $args;
          }

          sub Option :ATTR(CODE,MULTI)
          {
             my $package = shift;
             my ( $opts, $optname ) = @_;

             $opts and exists $opts->{$optname} and
                croak "Already have the $optname option";

             $opts->{$optname}++;
             return $opts;
          }

          ...

          sub do_copy
             :Description(Copy from SOURCE to DESTINATION)
             :Description(Optionally preserves attributes)
             :Argument("SOURCE")
             :Argument("DESTINATION")
             :Option("attrs")
             :Option("verbose")
          {
             ...
          }

FUNCTIONS

   get_subattrs
          $attrs = get_subattrs( $sub )

       Returns a HASH reference containing all the attributes defined on the given sub. The sub
       should either be passed as a CODE reference, or as a name in the caller's package. If no
       attributes are defined, a reference to an empty HASH is returned.

       The returned HASH reference is a new shallow clone, the caller may modify this hash
       arbitrarily without breaking the stored data, or other users of it.

   get_subattr
          $value = get_subattr( $sub, $attrname )

       Returns the value of a single named attribute on the given sub. The sub should either be
       passed as a CODE reference, or as a name in the caller's package. If the attribute is not
       defined, "undef" is returned.

   apply_subattrs
          $sub = apply_subattrs( @attrs_kvlist, $sub )

       A utility function to help apply attributes dynamically to the given CODE reference. The
       CODE reference is given last so that calls to the function appear similar in visual style
       to the same applied at compiletime.

          apply_subattrs
           Title => "Here is my title",
           sub { return $title };

       Is equivalent to

          sub :Title(Here is my title) { return $title }

       except that because its arguments are evaluated at runtime, they can be calculated by
       other code in ways that the compiletime version cannot.

       As the attributes are given in a key-value pair list, it is allowed to apply the same
       attribute multiple times; and the attributes are applied in the order given. The value of
       each attribute should be a plain string exactly as it would appear between the
       parentheses. Specifically, if the attribute does not use the "RAWDATA" flag, it should be
       a valid perl expression. As this is still evaluated using an "eval()" call, take care when
       handling potentially-unsafe or user-supplied data.

   apply_subattrs_for_pkg
          $sub = apply_subattrs_for_pkg( $pkg, @attrs_kvlist, $sub )

       As "apply_subattrs" but allows passing a specific package name, rather than using
       "caller".

   find_subs_with_attr
          %subs = find_subs_with_attr( $pkg, $attrname, %opts )

       A utility function to find CODE references in the given package that have the name
       attribute applied. The symbol table is checked for the given package, looking for CODE
       references that have the named attribute applied. These are returned in a key-value list,
       where the key gives the name of the function and the value is a CODE reference to it.

       $pkg can also be a reference to an array containing multiple package names, which will be
       searched in order with earlier ones taking precedence over later ones. This, for example,
       allows for subclass searching over an entire class heirarchy of packages, via the use of
       mro:

          %subs = find_subs_with_attr( mro::get_linear_isa( $class ), $attrname );

       Takes the following named options:

       matching => Regexp | CODE
               If present, gives a filter regexp or CODE reference to apply to symbol names.

                  $name =~ $matching
                  $matching->( local $_ = $name )

       filter => CODE
               If present, gives a filter CODE reference to apply to the function references
               before they are accepted as results. Note that this allows the possibility that
               the first match for a given method name to be rejected, while later ones are
               accepted.

                  $filter->( $cv, $name, $package )

AUTHOR

       Paul Evans <leonerd@leonerd.org.uk>