attributes(3perl)      Perl Programmers Reference Guide      attributes(3perl)

       attributes - get/set subroutine or variable attributes

         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.

       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).  "" 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.

           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.

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

           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.

           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:

           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.

           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:

           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.

   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
           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:

               package X;
               sub foo : lvalue ;


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

       4.  Code:

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


               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;

             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').

       "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.

perl v5.30.0                      2023-11-23                 attributes(3perl)
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