sub foo : method ;
         my ($x,@y,%z) : Bent = 1;
         my $s = sub : method { ... };

         use attributes ();    # optional, to get subroutine declarations
         my @attrlist = attributes::get(\&foo);

         use attributes 'get'; # import the attributes::get subroutine
         my @attrlist = get \&foo;

       Subroutine declarations and definitions may optionally have attribute
       lists associated with them.  (Variable "my" declarations also may, but
       see the warning below.)  Perl handles these declarations by passing
       some information about the call site and the thing being declared along
       with the attribute list to this module.  In particular, the first
       example above is equivalent to the following:

           use attributes __PACKAGE__, \&foo, 'method';

       The second example in the synopsis does something equivalent to this:

           use attributes ();
           my ($x,@y,%z);
           attributes::->import(__PACKAGE__, \$x, 'Bent');
           attributes::->import(__PACKAGE__, \@y, 'Bent');
           attributes::->import(__PACKAGE__, \%z, 'Bent');
           ($x,@y,%z) = 1;

       Yes, that's a lot of expansion.

       WARNING: attribute declarations for variables are still evolving.  The
       semantics and interfaces of such declarations could change in future
       versions.  They are present for purposes of experimentation with what
       the semantics ought to be.  Do not rely on the current implementation
       of this feature.

       There are only a few attributes currently handled by Perl itself (or
       directly by this module, depending on how you look at it.)  However,
       package-specific attributes are allowed by an extension mechanism.
       (See "Package-specific Attribute Handling" below.)

       The setting of subroutine attributes happens at compile time.  Variable
       attributes in "our" declarations are also applied at compile time.
       However, "my" variables get their attributes applied at run-time.  This
       means that you have to reach the run-time component of the "my" before
       those attributes will get applied.  For example:

           my $x : Bent = 42 if 0;

       will neither assign 42 to $x nor will it apply the "Bent" attribute to
       the variable.
       is equivalent to

         use attributes __PACKAGE__, \&foo, 'method';

       As you might know this calls the "import" function of "attributes" at
       compile time with these parameters: 'attributes', the caller's package
       name, the reference to the code and 'method'.

         attributes->import( __PACKAGE__, \&foo, 'method' );

       So you want to know what "import" actually does?

       First of all "import" gets the type of the third parameter ('CODE' in
       this case).  "" checks if there is a subroutine called
       "MODIFY_<reftype>_ATTRIBUTES" in the caller's namespace (here: 'main').
       In this case a subroutine "MODIFY_CODE_ATTRIBUTES" is required. Then
       this method is called to check if you have used a "bad attribute".  The
       subroutine call in this example would look like

         MODIFY_CODE_ATTRIBUTES( 'main', \&foo, 'method' );

       "MODIFY_<reftype>_ATTRIBUTES" has to return a list of all "bad
       attributes".  If there are any bad attributes "import" croaks.

       (See "Package-specific Attribute Handling" below.)

   Built-in Attributes
       The following are the built-in attributes for subroutines:

           Indicates that the referenced subroutine is a valid lvalue and can
           be assigned to. The subroutine must return a modifiable value such
           as a scalar variable, as described in perlsub.

           Indicates that the referenced subroutine is a method. A subroutine
           so marked will not trigger the "Ambiguous call resolved as
           CORE::%s" warning.

           The "locked" attribute has no effect in 5.10.0 and later. It was
           used as part of the now-removed "Perl 5.005 threads".

   Available Subroutines
       The following subroutines are available for general use once this
       module has been loaded:

       get This routine expects a single parameter--a reference to a
           subroutine or variable.  It returns a list of attributes, which may
           be empty.  If passed invalid arguments, it uses die() (via
           Carp::croak) to raise a fatal exception.  If it can find an
           appropriate package name for a class method lookup, it will include
           the results from a "FETCH_type_ATTRIBUTES" call in its return list,
           as described in "Package-specific Attribute Handling" below.

   Package-specific Attribute Handling
       WARNING: the mechanisms described here are still experimental.  Do not
       rely on the current implementation.  In particular, there is no
       provision for applying package attributes to 'cloned' copies of
       subroutines used as closures.  (See "Making References" in perlref for
       information on closures.)  Package-specific attribute handling may
       change incompatibly in a future release.

       When an attribute list is present in a declaration, a check is made to
       see whether an attribute 'modify' handler is present in the appropriate
       package (or its @ISA inheritance tree).  Similarly, when
       "attributes::get" is called on a valid reference, a check is made for
       an appropriate attribute 'fetch' handler.  See "EXAMPLES" to see how
       the "appropriate package" determination works.

       The handler names are based on the underlying type of the variable
       being declared or of the reference passed.  Because these attributes
       are associated with subroutine or variable declarations, this
       deliberately ignores any possibility of being blessed into some
       package.  Thus, a subroutine declaration uses "CODE" as its type, and
       even a blessed hash reference uses "HASH" as its type.

       The class methods invoked for modifying and fetching are these:

           This method is called with two arguments:  the relevant package
           name, and a reference to a variable or subroutine for which
           package-defined attributes are desired.  The expected return value
           is a list of associated attributes.  This list may be empty.

           This method is called with two fixed arguments, followed by the
           list of attributes from the relevant declaration.  The two fixed
           arguments are the relevant package name and a reference to the
           declared subroutine or variable.  The expected return value is a
           list of attributes which were not recognized by this handler.  Note
           that this allows for a derived class to delegate a call to its base
           class, and then only examine the attributes which the base class
           didn't already handle for it.

           The call to this method is currently made during the processing of
           the declaration.  In particular, this means that a subroutine
           reference will probably be for an undefined subroutine, even if
           this declaration is actually part of the definition.

       Calling "attributes::get()" from within the scope of a null package
       declaration "package ;" for an unblessed variable reference will not
       provide any starting package name for the 'fetch' method lookup.  Thus,
       this circumstance will not result in a method call for package-defined
       attributes.  A named subroutine knows to which symbol table entry it
       belongs (or originally belonged), and it will use the corresponding
       package.  An anonymous subroutine knows the package name into which it
       was compiled (unless it was also compiled with a null package
       declaration), and so it will use that package name.
           switch(10,foo(7,3))  :  expensive
           Ugly('\(") :Bad
           lvalue method

       Some examples of syntactically invalid attribute lists (with

           switch(10,foo()             # ()-string not balanced
           Ugly('(')                   # ()-string not balanced
           5x5                         # "5x5" not a valid identifier
           Y2::north                   # "Y2::north" not a simple identifier
           foo + bar                   # "+" neither a colon nor whitespace

   Default exports

   Available exports
       The routines "get" and "reftype" are exportable.

   Export tags defined
       The ":ALL" tag will get all of the above exports.

       Here are some samples of syntactically valid declarations, with
       annotation as to how they resolve internally into "use attributes"
       invocations by perl.  These examples are primarily useful to see how
       the "appropriate package" is found for the possible method lookups for
       package-defined attributes.

       1.  Code:

               package Canine;
               package Dog;
               my Canine $spot : Watchful ;


               use attributes ();
               attributes::->import(Canine => \$spot, "Watchful");

       2.  Code:

               package Felis;
               my $cat : Nervous;


               use attributes ();
               attributes::->import(Felis => \$cat, "Nervous");

       3.  Code:


               use attributes Y => \&Y::x, "lvalue";

       5.  Code:

               package X;
               sub foo { 1 }

               package Y;
               BEGIN { *bar = \&X::foo; }

               package Z;
               sub Y::bar : lvalue ;


               use attributes X => \&X::foo, "lvalue";

       This last example is purely for purposes of completeness.  You should
       not be trying to mess with the attributes of something in a package
       that's not your own.

               sub MODIFY_CODE_ATTRIBUTES {
                  my ($class,$code,@attrs) = @_;

                  my $allowed = 'MyAttribute';
                  my @bad = grep { $_ ne $allowed } @attrs;

                  return @bad;

               sub foo : MyAttribute {
                  print "foo\n";

           This example runs. At compile time "MODIFY_CODE_ATTRIBUTES" is
           called. In that subroutine, we check if any attribute is disallowed
           and we return a list of these "bad attributes".

           As we return an empty list, everything is fine.

             sub MODIFY_CODE_ATTRIBUTES {
                my ($class,$code,@attrs) = @_;

                my $allowed = 'MyAttribute';
                my @bad = grep{ $_ ne $allowed }@attrs;

                return @bad;

perl v5.14.2                      2011-09-26                 attributes(3perl)
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