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