"Attribute::Storage" - declare and retrieve named attributes about
CODE references
package My::Package;
use Attribute::Storage;
sub Title :ATTR(CODE)
{
my $package = shift;
my ( $title ) = @_;
return $title;
}
package main;
use Attribute::Storage qw( get_subattr );
use My::Package;
sub myfunc :Title('The title of my function')
{
...
}
print "Title of myfunc is: ".get_subattr(\&myfunc, 'Title')."\n";
This package provides a base, where a package using it can define handlers for
particular code attributes. Other packages, using the package that defines the
code attributes, can then use them to annotate subs.
This is similar to "Attribute::Handlers", with the following key
differences:
- •
- "Attribute::Storage" will store the value
returned by the attribute handling code, and provides convenient lookup
functions to retrieve it later. "Attribute::Handlers" simply
invokes the handling code.
- •
- "Attribute::Storage" immediately executes the
attribute handling code at compile-time. "Attribute::Handlers"
defers invocation so it can look up the symbolic name of the sub the
attribute is attached to. "Attribute::Storage" uses B to provide
the name of the sub at invocation time, using the name of the underlying
"GV".
- •
- "Attribute::Storage" works just as well on
anonymous subs as named ones.
- •
- "Attribute::Storage" is safe to use on code that
will be reloaded, because it executes handlers immediately.
"Attribute::Handlers" will only execute handlers at defined
phases such as "BEGIN" or "INIT", and cannot reexecute
the handlers in a file once it has been reloaded.
Each attribute that the defining package wants to define should be done using a
marked subroutine, in a way similar to Attribute::Handlers. When a sub in the
using package is marked with such an attribute, the code is executed, passing
in the arguments. Whatever it returns is stored, to be returned later when
queried by "get_subattr" or "get_subattrs". The return
value must be defined, or else the attribute will be marked as a compile error
for perl to handle accordingly.
Only "CODE" attributes are supported at present.
sub AttributeName :ATTR(CODE)
{
my $package = shift;
my ( $attr, $args, $here ) = @_;
...
return $value;
}
At attachment time, the optional string that may appear within brackets
following the attribute's name is parsed as a Perl expression in list context.
If this succeeds, the values are passed as a list to the handling code. If
this fails, an error is returned to the perl compiler. If no string is
present, then an empty list is passed to the handling code.
package Defining;
sub NameMap :ATTR(CODE)
{
my $package = shift;
my @strings = @_;
return { map { m/^(.*)=(.*)$/ and ( $1, $2 ) } @strings };
}
package Using;
use Defining;
sub somefunc :NameMap("foo=FOO","bar=BAR","splot=WIBBLE") { ... }
my $map = get_subattr("somefunc", "NameMap");
# Will yield:
# { foo => "FOO",
# bar => "BAR",
# splot => "WIBBLE" }
Note that it is impossible to distinguish
sub somefunc :NameMap { ... }
sub somefunc :NameMap() { ... }
It is possible to create attributes that do not parse their argument as a perl
list expression, instead they just pass the plain string as a single argument.
For this, add the "RAWDATA" flag to the "ATTR()" list.
sub Title :ATTR(CODE,RAWDATA)
{
my $package = shift;
my ( $text ) = @_;
return $text;
}
sub thingy :Title(Here is the title for thingy) { ... }
To obtain the name of the function to which the attribute is being applied, use
the "NAME" flag to the "ATTR()" list.
sub Callable :ATTR(CODE,NAME)
{
my $package = shift;
my ( $subname, @args ) = @_;
print "The Callable attribute is being applied to $package :: $subname\n";
return;
}
When applied to an anonymous function ("sub { ... }"), the name will
appear as "__ANON__".
Normally it is an error to attempt to apply the same attribute more than once to
the same function. Sometimes however, it would make sense for an attribute to
be applied many times. If the "ATTR()" list is given the
"MULTI" flag, then applying it more than once will be allowed. Each
invocation of the handling code will be given the previous value that was
returned, or "undef" for the first time. It is up to the code to
perform whatever merging logic is required.
sub Description :ATTR(CODE,MULTI,RAWDATA)
{
my $package = shift;
my ( $olddesc, $more ) = @_;
return defined $olddesc ? "$olddesc$more\n" : "$more\n";
}
sub Argument :ATTR(CODE,MULTI)
{
my $package = shift;
my ( $args, $argname ) = @_;
push @$args, $argname;
return $args;
}
sub Option :ATTR(CODE,MULTI)
{
my $package = shift;
my ( $opts, $optname ) = @_;
$opts and exists $opts->{$optname} and
croak "Already have the $optname option";
$opts->{$optname}++;
return $opts;
}
...
sub do_copy
:Description(Copy from SOURCE to DESTINATION)
:Description(Optionally preserves attributes)
:Argument("SOURCE")
:Argument("DESTINATION")
:Option("attrs")
:Option("verbose")
{
...
}
$attrs = get_subattrs( $sub )
Returns a HASH reference containing all the attributes defined on the given sub.
The sub should either be passed as a CODE reference, or as a name in the
caller's package. If no attributes are defined, a reference to an empty HASH
is returned.
The returned HASH reference is a new shallow clone, the caller may modify this
hash arbitrarily without breaking the stored data, or other users of it.
$value = get_subattr( $sub, $attrname )
Returns the value of a single named attribute on the given sub. The sub should
either be passed as a CODE reference, or as a name in the caller's package. If
the attribute is not defined, "undef" is returned.
$sub = apply_subattrs( @attrs_kvlist, $sub )
A utility function to help apply attributes dynamically to the given CODE
reference. The CODE reference is given last so that calls to the function
appear similar in visual style to the same applied at compiletime.
apply_subattrs
Title => "Here is my title",
sub { return $title };
Is equivalent to
sub :Title(Here is my title) { return $title }
except that because its arguments are evaluated at runtime, they can be
calculated by other code in ways that the compiletime version cannot.
As the attributes are given in a key-value pair list, it is allowed to apply the
same attribute multiple times; and the attributes are applied in the order
given. The value of each attribute should be a plain string exactly as it
would appear between the parentheses. Specifically, if the attribute does not
use the "RAWDATA" flag, it should be a valid perl expression. As
this is still evaluated using an "eval()" call, take care when
handling potentially-unsafe or user-supplied data.
$sub = apply_subattrs_for_pkg( $pkg, @attrs_kvlist, $sub )
As "apply_subattrs" but allows passing a specific package name, rather
than using "caller".
%subs = find_subs_with_attr( $pkg, $attrname, %opts )
A utility function to find CODE references in the given package that have the
name attribute applied. The symbol table is checked for the given package,
looking for CODE references that have the named attribute applied. These are
returned in a key-value list, where the key gives the name of the function and
the value is a CODE reference to it.
$pkg can also be a reference to an array containing multiple package names,
which will be searched in order with earlier ones taking precedence over later
ones. This, for example, allows for subclass searching over an entire class
heirarchy of packages, via the use of mro:
%subs = find_subs_with_attr( mro::get_linear_isa( $class ), $attrname );
Takes the following named options:
- matching => Regexp | CODE
- If present, gives a filter regexp or CODE reference to
apply to symbol names.
$name =~ $matching
$matching->( local $_ = $name )
- filter => CODE
- If present, gives a filter CODE reference to apply to the
function references before they are accepted as results. Note that this
allows the possibility that the first match for a given method name to be
rejected, while later ones are accepted.
$filter->( $cv, $name, $package )
Paul Evans <
[email protected]>