attributes.3

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explicitly taken with the \f(CW\*(C`lock\*(C'\fR operator immediately after the
subroutine is entered.
.IP "method" 4
.IX Item "method"
Indicates that the referenced subroutine is a method.
This has a meaning when taken together with the \fBlocked\fR attribute,
as described there.  It also means that a subroutine so marked
will not trigger the \*(L"Ambiguous call resolved as CORE::%s\*(R" warning.
.IP "lvalue" 4
.IX Item "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.
.PP
For global variables there is \f(CW\*(C`unique\*(C'\fR attribute: see \*(L"our\*(R" in perlfunc.
.Sh "Available Subroutines"
.IX Subsection "Available Subroutines"
The following subroutines are available for general use once this module
has been loaded:
.IP "get" 4
.IX Item "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 \fIdie()\fR (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
\&\f(CW\*(C`FETCH_\f(CItype\f(CW_ATTRIBUTES\*(C'\fR call in its return list, as described in
\&\*(L"Package-specific Attribute Handling\*(R" below.
Otherwise, only built-in attributes will be returned.
.IP "reftype" 4
.IX Item "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 \fItype\fR value which forms part of
the method names described in \*(L"Package-specific Attribute Handling\*(R" below.
.PP
Note that these routines are \fInot\fR exported by default.
.Sh "Package-specific Attribute Handling"
.IX Subsection "Package-specific Attribute Handling"
\&\fB\s-1WARNING\s0\fR: 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 \*(L"Making References\*(R" in perlref for information on closures.)
Package-specific attribute handling may change incompatibly in a future
release.
.PP
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 \f(CW@ISA\fR inheritance tree).  Similarly, when \f(CW\*(C`attributes::get\*(C'\fR is
called on a valid reference, a check is made for an appropriate attribute
\&'fetch' handler.  See \*(L"\s-1EXAMPLES\s0\*(R" to see how the \*(L"appropriate package\*(R"
determination works.
.PP
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 \*(L"\s-1CODE\s0\*(R" as its \fItype\fR, and even a blessed
hash reference uses \*(L"\s-1HASH\s0\*(R" as its \fItype\fR.
.PP
The class methods invoked for modifying and fetching are these:
.IP "\s-1FETCH_\s0\fItype\fR_ATTRIBUTES" 4
.IX Item "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.
.IP "\s-1MODIFY_\s0\fItype\fR_ATTRIBUTES" 4
.IX Item "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.
.Sp
The call to this method is currently made \fIduring\fR 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.
.PP
Calling \f(CW\*(C`attributes::get()\*(C'\fR from within the scope of a null package
declaration \f(CW\*(C`package ;\*(C'\fR 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.
.Sh "Syntax of Attribute Lists"
.IX Subsection "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 \f(CW\*(C`q()\*(C'\fR operator.  (See \*(L"Quote and Quote-like Operators\*(R" in perlop.)
The parameter list is passed as it was found, however, and not as per \f(CW\*(C`q()\*(C'\fR.
.PP
Some examples of syntactically valid attribute lists:
.PP
.Vb 4
\&    switch(10,foo(7,3))  :  expensive
\&    Ugly(\*(Aq\e(") :Bad
\&    _5x5
\&    locked method
.Ve
.PP
Some examples of syntactically invalid attribute lists (with annotation):
.PP
.Vb 5
\&    switch(10,foo()             # ()\-string not balanced
\&    Ugly(\*(Aq(\*(Aq)                   # ()\-string not balanced
\&    5x5                         # "5x5" not a valid identifier
\&    Y2::north                   # "Y2::north" not a simple identifier
\&    foo + bar                   # "+" neither a colon nor whitespace
.Ve
.SH "EXPORTS"
.IX Header "EXPORTS"
.Sh "Default exports"
.IX Subsection "Default exports"
None.
.Sh "Available exports"
.IX Subsection "Available exports"
The routines \f(CW\*(C`get\*(C'\fR and \f(CW\*(C`reftype\*(C'\fR are exportable.
.Sh "Export tags defined"
.IX Subsection "Export tags defined"
The \f(CW\*(C`:ALL\*(C'\fR tag will get all of the above exports.
.SH "EXAMPLES"
.IX Header "EXAMPLES"
Here are some samples of syntactically valid declarations, with annotation
as to how they resolve internally into \f(CW\*(C`use attributes\*(C'\fR invocations by
perl.  These examples are primarily useful to see how the \*(L"appropriate
package\*(R" is found for the possible method lookups for package-defined
attributes.
.IP "1." 4
Code:
.Sp
.Vb 3
\&    package Canine;
\&    package Dog;
\&    my Canine $spot : Watchful ;
.Ve
.Sp
Effect:
.Sp
.Vb 2
\&    use attributes ();
\&    attributes::\->import(Canine => \e$spot, "Watchful");
.Ve
.IP "2." 4
Code:
.Sp
.Vb 2
\&    package Felis;
\&    my $cat : Nervous;
.Ve
.Sp
Effect:
.Sp
.Vb 2
\&    use attributes ();
\&    attributes::\->import(Felis => \e$cat, "Nervous");
.Ve
.IP "3." 4
Code:
.Sp
.Vb 2
\&    package X;
\&    sub foo : locked ;
.Ve
.Sp
Effect:
.Sp
.Vb 1
\&    use attributes X => \e&foo, "locked";
.Ve
.IP "4." 4
Code:
.Sp
.Vb 2
\&    package X;
\&    sub Y::x : locked { 1 }
.Ve
.Sp
Effect:
.Sp
.Vb 1
\&    use attributes Y => \e&Y::x, "locked";
.Ve
.IP "5." 4
Code:
.Sp
.Vb 2
\&    package X;
\&    sub foo { 1 }
\&
\&    package Y;
\&    BEGIN { *bar = \e&X::foo; }
\&
\&    package Z;
\&    sub Y::bar : locked ;
.Ve
.Sp
Effect:
.Sp
.Vb 1
\&    use attributes X => \e&X::foo, "locked";
.Ve
.PP
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.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\*(L"Private Variables via \fImy()\fR\*(R" in perlsub and
\&\*(L"Subroutine Attributes\*(R" in perlsub for details on the basic declarations;
attrs for the obsolescent form of subroutine attribute specification
which this module replaces;
\&\*(L"use\*(R" in perlfunc for details on the normal invocation mechanism.