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

attributes(3p) Perl Programmers Reference Guide attributes(3p) #

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

NNAAMMEE #

 attributes - get/set subroutine or variable attributes

SSYYNNOOPPSSIISS #

   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;

DDEESSCCRRIIPPTTIIOONN #

 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.

 WWAARRNNIINNGG: 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 _r_e_a_c_h 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 _n_o_r 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 --ww or
 "use warnings 'reserved'".

WWhhaatt “"iimmppoorrtt"” ddooeess 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.)

BBuuiilltt--iinn AAttttrriibbuutteess 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.

AAvvaaiillaabbllee SSuubbrroouuttiinneess 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 ddiiee(()) (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__t_y_p_e_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 _t_y_p_e value which forms part of
     the method names described in "Package-specific Attribute Handling"
     below.

 Note that these routines are _n_o_t exported by default.

PPaacckkaaggee--ssppeecciiffiicc AAttttrriibbuuttee HHaannddlliinngg WWAARRNNIINNGG: 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 _t_y_p_e, and even a blessed hash
 reference uses "HASH" as its _t_y_p_e.

 The class methods invoked for modifying and fetching are these:

 FETCH__t_y_p_e_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__t_y_p_e_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 _d_u_r_i_n_g 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.

SSyynnttaaxx ooff AAttttrriibbuuttee LLiissttss 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

EEXXPPOORRTTSS #

DDeeffaauulltt eexxppoorrttss None.

AAvvaaiillaabbllee eexxppoorrttss The routines “get” and “reftype” are exportable.

EExxppoorrtt ttaaggss ddeeffiinneedd The “:ALL” tag will get all of the above exports.

EEXXAAMMPPLLEESS #

 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.

MMOORREE EEXXAAMMPPLLEESS #

 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').

SSEEEE AALLSSOO #

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

perl v5.36.3 2023-02-15 attributes(3p)