| attributes - get/set subroutine or variable attributes |
import does
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'.
import doesIn 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.)
The following are the built-in attributes for subroutines:
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.
prototype(..)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.
sub expression is evaluated. The return value is captured and
turned into a constant subroutine.
The following are the built-in attributes for variables:
The following subroutines are available for general use once this module has been loaded:
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.
Note that these routines are not exported by default.
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 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 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:
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.
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 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
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
None.
The routines get and reftype are exportable.
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.
package Canine;
package Dog;
my Canine $spot : Watchful ;
Effect:
use attributes ();
attributes::->import(Canine => \$spot, "Watchful");
Code:
package Felis;
my $cat : Nervous;
Effect:
use attributes ();
attributes::->import(Felis => \$cat, "Nervous");
Code:
package X;
sub foo : lvalue ;
Effect:
use attributes X => \&foo, "lvalue";
Code:
package X;
sub Y::x : lvalue { 1 }
Effect:
use attributes Y => \&Y::x, "lvalue";
Code:
package X;
sub foo { 1 }
package Y;
BEGIN { *bar = \&X::foo; }
package Z;
sub Y::bar : lvalue ;
Effect:
use attributes X => \&X::foo, "lvalue";
This last example is purely for purposes of completeness. You should not be trying to mess with the attributes of something in a package that's not your own.
sub MODIFY_CODE_ATTRIBUTES {
my ($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 the perlsub manpage and Subroutine Attributes in the perlsub manpage for details on the basic declarations; use in the perlfunc manpage for details on the normal invocation mechanism.
| attributes - get/set subroutine or variable attributes |