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NAME

       attributes - get/set subroutine or variable attributes

SYNOPSIS

         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;

DESCRIPTION

       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.

       An attempt to set an unrecognized attribute is a fatal error.  (The error is trappable,
       but it still stops the compilation within that "eval".)  Setting an attribute with a name
       that's all lowercase letters that's not a built-in attribute (such as "foo") will result
       in a warning with -w or "use warnings 'reserved'".

   What "import" does
       In the description it is mentioned that

         sub foo : method;

       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).
       "attributes.pm" 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:

       lvalue
           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.

           This module allows one to set this attribute on a subroutine that is already defined.
           For Perl subroutines (XSUBs are fine), it may or may not do what you want, depending
           on the code inside the subroutine, with details subject to change in future Perl
           versions.  You may run into problems with lvalue context not being propagated properly
           into the subroutine, or maybe even assertion failures.  For this reason, a warning is
           emitted if warnings are enabled.  In other words, you should only do this if you
           really know what you are doing.  You have been warned.

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

       prototype(..)
           The "prototype" attribute is an alternate means of specifying a prototype on a sub.
           The desired prototype is within the parens.

           The prototype from the attribute is assigned to the sub immediately after the
           prototype from the sub, which means that if both are declared at the same time, the
           traditionally defined prototype is ignored.  In other words, "sub foo($$) :
           prototype(@) {}" is indistinguishable from "sub foo(@){}".

           If illegalproto warnings are enabled, the prototype declared inside this attribute
           will be sanity checked at compile time.

       const
           This experimental attribute, introduced in Perl 5.22, only applies to anonymous
           subroutines.  It causes the subroutine to be called as soon as the "sub" expression is
           evaluated.  The return value is captured and turned into a constant subroutine.

       The following are the built-in attributes for variables:

       shared
           Indicates that the referenced variable can be shared across different threads when
           used in conjunction with the threads and threads::shared modules.

   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.  Otherwise, only built-in attributes will be returned.

       reftype
           This routine expects a single parameter--a reference to a subroutine or variable.  It
           returns the built-in type of the referenced variable, ignoring any package into which
           it might have been blessed.  This can be useful for determining the type value which
           forms part of the method names described in "Package-specific Attribute Handling"
           below.

       Note that these routines are not exported by default.

   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:

       FETCH_type_ATTRIBUTES
           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.

       MODIFY_type_ATTRIBUTES
           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.

   Syntax of Attribute Lists
       An attribute list is a sequence of attribute specifications, separated by whitespace or a
       colon (with optional whitespace).  Each attribute specification is a simple name,
       optionally followed by a parenthesised parameter list.  If such a parameter list is
       present, it is scanned past as for the rules for the "q()" operator.  (See "Quote and
       Quote-like Operators" in perlop.)  The parameter list is passed as it was found, however,
       and not as per "q()".

       Some examples of syntactically valid attribute lists:

           switch(10,foo(7,3))  :  expensive
           Ugly('\(") :Bad
           _5x5
           lvalue method

       Some examples of syntactically invalid attribute lists (with annotation):

           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

EXPORTS

   Default exports
       None.

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

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

EXAMPLES

       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 ;

           Effect:

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

       2.  Code:

               package Felis;
               my $cat : Nervous;

           Effect:

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

       3.  Code:

               package X;
               sub foo : lvalue ;

           Effect:

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

       4.  Code:

               package X;
               sub Y::x : lvalue { 1 }

           Effect:

               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 ;

           Effect:

               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.

MORE EXAMPLES

       1.
               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.

       2.
             sub MODIFY_CODE_ATTRIBUTES {
                my ($class,$code,@attrs) = @_;

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

                return @bad;
             }

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

           This example is aborted at compile time as we use the attribute "Test" which isn't
           allowed.  "MODIFY_CODE_ATTRIBUTES" returns a list that contains a single element
           ('Test').

SEE ALSO

       "Private Variables via my()" in perlsub and "Subroutine Attributes" in perlsub for details
       on the basic declarations; "use" in perlfunc for details on the normal invocation
       mechanism.