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NAME

     attributes - get/set subroutine or variable attributes



SYNOPSIS
       sub foo : method ;
       my ($x,@y,%z) : Bent ;
       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 __PACKAGE__, \$x, 'Bent';
         use attributes __PACKAGE__, \@y, 'Bent';
         use attributes __PACKAGE__, \%z, 'Bent';

     Yes, that's three invocations.

     WARNING: attribute declarations for variables are an
     experimental feature.  The semantics of such declarations
     could change or be removed 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 the section on
     "Package-specific Attribute Handling" below.)

     The setting of attributes happens at compile time.  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''.

     Built-in Attributes

     The following are the built-in attributes for subroutines:

     locked
         Setting this attribute is only meaningful when the
         subroutine or method is to be called by multiple
         threads.  When set on a method subroutine (i.e., one
         marked with the method attribute below), Perl ensures
         that any invocation of it implicitly locks its first
         argument before execution.  When set on a non-method
         subroutine, Perl ensures that a lock is taken on the
         subroutine itself before execution.  The semantics of
         the lock are exactly those of one explicitly taken with
         the `lock' operator immediately after the subroutine is
         entered.

     method
         Indicates that the referenced subroutine is a method.
         This has a meaning when taken together with the locked
         attribute, as described there.  It also means that a
         subroutine so marked will not trigger the "Ambiguous
         call resolved as CORE::%s" warning.

     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 the perlsub manpage.

     There are no built-in attributes for anything other than
     subroutines.

     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 the section on "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 the section on "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 the Making References entry in the perlref
     manpage 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 the section on "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 receives a single argument, which is a
         reference to the 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 as 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 the Quote and
     Quote-like Operators entry in the perlop manpage.)  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
         locked 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 Canine => \$spot, "Watchful";


     2.  Code:

             package Felis;
             my $cat : Nervous;

         Effect:

             use attributes Felis => \$cat, "Nervous";


     3.  Code:

             package X;
             sub foo : locked ;

         Effect:

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


     4.  Code:

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

         Effect:

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


     5.  Code:

             package X;
             sub foo { 1 }

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

             package Z;
             sub Y::bar : locked ;

         Effect:

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


     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.



SEE ALSO
     the Private Variables via my() entry in the perlsub manpage
     and the Subroutine Attributes entry in the perlsub manpage
     for details on the basic declarations; the attrs manpage for
     the obsolescent form of subroutine attribute specification
     which this module replaces; the use entry in the perlfunc
     manpage for details on the normal invocation mechanism.









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Created 1996-2024 by Maxim Chirkov

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