Attribute::Handlers - Simpler definition of attribute handlers
This document describes version 1.02 of Attribute::Handlers.
package MyClass;
require 5.006;
use Attribute::Handlers;
no warnings 'redefine';
sub Good : ATTR(SCALAR) {
my ($package, $symbol, $referent, $attr, $data) = @_;
# Invoked for any scalar variable with a :Good attribute,
# provided the variable was declared in MyClass (or
# a derived class) or typed to MyClass.
# Do whatever to $referent here (executed in CHECK phase).
...
}
sub Bad : ATTR(SCALAR) {
# Invoked for any scalar variable with a :Bad attribute,
# provided the variable was declared in MyClass (or
# a derived class) or typed to MyClass.
...
}
sub Good : ATTR(ARRAY) {
# Invoked for any array variable with a :Good attribute,
# provided the variable was declared in MyClass (or
# a derived class) or typed to MyClass.
...
}
sub Good : ATTR(HASH) {
# Invoked for any hash variable with a :Good attribute,
# provided the variable was declared in MyClass (or
# a derived class) or typed to MyClass.
...
}
sub Ugly : ATTR(CODE) {
# Invoked for any subroutine declared in MyClass (or a
# derived class) with an :Ugly attribute.
...
}
sub Omni : ATTR {
# Invoked for any scalar, array, hash, or subroutine
# with an :Omni attribute, provided the variable or
# subroutine was declared in MyClass (or a derived class)
# or the variable was typed to MyClass.
# Use ref($_[2]) to determine what kind of referent it was.
...
}
use Attribute::Handlers autotie => { Cycle => Tie::Cycle };
my $next : Cycle(['A'..'Z']);
This module, when inherited by a package, allows that package's class to define
attribute handler subroutines for specific attributes. Variables and
subroutines subsequently defined in that package, or in packages derived from
that package may be given attributes with the same names as the attribute
handler subroutines, which will then be called in one of the compilation
phases (i.e. in a "BEGIN", "CHECK", "INIT", or
"END" block). ("UNITCHECK" blocks don't correspond to a
global compilation phase, so they can't be specified here.)
To create a handler, define it as a subroutine with the same name as the desired
attribute, and declare the subroutine itself with the attribute
":ATTR". For example:
package LoudDecl;
use Attribute::Handlers;
sub Loud :ATTR {
my ($package, $symbol, $referent, $attr, $data, $phase,
$filename, $linenum) = @_;
print STDERR
ref($referent), " ",
*{$symbol}{NAME}, " ",
"($referent) ", "was just declared ",
"and ascribed the ${attr} attribute ",
"with data ($data)\n",
"in phase $phase\n",
"in file $filename at line $linenum\n";
}
This creates a handler for the attribute ":Loud" in the class
LoudDecl. Thereafter, any subroutine declared with a ":Loud"
attribute in the class LoudDecl:
package LoudDecl;
sub foo: Loud {...}
causes the above handler to be invoked, and passed:
- [0]
- the name of the package into which it was declared;
- [1]
- a reference to the symbol table entry (typeglob) containing
the subroutine;
- [2]
- a reference to the subroutine;
- [3]
- the name of the attribute;
- [4]
- any data associated with that attribute;
- [5]
- the name of the phase in which the handler is being
invoked;
- [6]
- the filename in which the handler is being invoked;
- [7]
- the line number in this file.
Likewise, declaring any variables with the ":Loud" attribute within
the package:
package LoudDecl;
my $foo :Loud;
my @foo :Loud;
my %foo :Loud;
will cause the handler to be called with a similar argument list (except, of
course, that $_[2] will be a reference to the variable).
The package name argument will typically be the name of the class into which the
subroutine was declared, but it may also be the name of a derived class (since
handlers are inherited).
If a lexical variable is given an attribute, there is no symbol table to which
it belongs, so the symbol table argument ($_[1]) is set to the string
'LEXICAL' in that case. Likewise, ascribing an attribute to an anonymous
subroutine results in a symbol table argument of 'ANON'.
The data argument passes in the value (if any) associated with the attribute.
For example, if &foo had been declared:
sub foo :Loud("turn it up to 11, man!") {...}
then a reference to an array containing the string "turn it up to 11,
man!" would be passed as the last argument.
Attribute::Handlers makes strenuous efforts to convert the data argument ($_[4])
to a usable form before passing it to the handler (but see
"Non-interpretive attribute handlers"). If those efforts succeed,
the interpreted data is passed in an array reference; if they fail, the raw
data is passed as a string. For example, all of these:
sub foo :Loud(till=>ears=>are=>bleeding) {...}
sub foo :Loud(qw/till ears are bleeding/) {...}
sub foo :Loud(qw/till, ears, are, bleeding/) {...}
sub foo :Loud(till,ears,are,bleeding) {...}
causes it to pass "['till','ears','are','bleeding']" as the handler's
data argument. While:
sub foo :Loud(['till','ears','are','bleeding']) {...}
causes it to pass "[ ['till','ears','are','bleeding'] ]"; the array
reference specified in the data being passed inside the standard array
reference indicating successful interpretation.
However, if the data can't be parsed as valid Perl, then it is passed as an
uninterpreted string. For example:
sub foo :Loud(my,ears,are,bleeding) {...}
sub foo :Loud(qw/my ears are bleeding) {...}
cause the strings 'my,ears,are,bleeding' and 'qw/my ears are bleeding'
respectively to be passed as the data argument.
If no value is associated with the attribute, "undef" is passed.
Regardless of the package in which it is declared, if a lexical variable is
ascribed an attribute, the handler that is invoked is the one belonging to the
package to which it is typed. For example, the following declarations:
package OtherClass;
my LoudDecl $loudobj : Loud;
my LoudDecl @loudobjs : Loud;
my LoudDecl %loudobjex : Loud;
causes the LoudDecl::Loud handler to be invoked (even if OtherClass also defines
a handler for ":Loud" attributes).
If an attribute handler is declared and the ":ATTR" specifier is given
the name of a built-in type ("SCALAR", "ARRAY",
"HASH", or "CODE"), the handler is only applied to
declarations of that type. For example, the following definition:
package LoudDecl;
sub RealLoud :ATTR(SCALAR) { print "Yeeeeow!" }
creates an attribute handler that applies only to scalars:
package Painful;
use base LoudDecl;
my $metal : RealLoud; # invokes &LoudDecl::RealLoud
my @metal : RealLoud; # error: unknown attribute
my %metal : RealLoud; # error: unknown attribute
sub metal : RealLoud {...} # error: unknown attribute
You can, of course, declare separate handlers for these types as well (but
you'll need to specify "no warnings 'redefine'" to do it quietly):
package LoudDecl;
use Attribute::Handlers;
no warnings 'redefine';
sub RealLoud :ATTR(SCALAR) { print "Yeeeeow!" }
sub RealLoud :ATTR(ARRAY) { print "Urrrrrrrrrr!" }
sub RealLoud :ATTR(HASH) { print "Arrrrrgggghhhhhh!" }
sub RealLoud :ATTR(CODE) { croak "Real loud sub torpedoed" }
You can also explicitly indicate that a single handler is meant to be used for
all types of referents like so:
package LoudDecl;
use Attribute::Handlers;
sub SeriousLoud :ATTR(ANY) { warn "Hearing loss imminent" }
(I.e. "ATTR(ANY)" is a synonym for ":ATTR").
Occasionally the strenuous efforts Attribute::Handlers makes to convert the data
argument ($_[4]) to a usable form before passing it to the handler get in the
way.
You can turn off that eagerness-to-help by declaring an attribute handler with
the keyword "RAWDATA". For example:
sub Raw : ATTR(RAWDATA) {...}
sub Nekkid : ATTR(SCALAR,RAWDATA) {...}
sub Au::Naturale : ATTR(RAWDATA,ANY) {...}
Then the handler makes absolutely no attempt to interpret the data it receives
and simply passes it as a string:
my $power : Raw(1..100); # handlers receives "1..100"
By default, attribute handlers are called at the end of the compilation phase
(in a "CHECK" block). This seems to be optimal in most cases because
most things that can be defined are defined by that point but nothing has been
executed.
However, it is possible to set up attribute handlers that are called at other
points in the program's compilation or execution, by explicitly stating the
phase (or phases) in which you wish the attribute handler to be called. For
example:
sub Early :ATTR(SCALAR,BEGIN) {...}
sub Normal :ATTR(SCALAR,CHECK) {...}
sub Late :ATTR(SCALAR,INIT) {...}
sub Final :ATTR(SCALAR,END) {...}
sub Bookends :ATTR(SCALAR,BEGIN,END) {...}
As the last example indicates, a handler may be set up to be (re)called in two
or more phases. The phase name is passed as the handler's final argument.
Note that attribute handlers that are scheduled for the "BEGIN" phase
are handled as soon as the attribute is detected (i.e. before any subsequently
defined "BEGIN" blocks are executed).
Attributes make an excellent and intuitive interface through which to tie
variables. For example:
use Attribute::Handlers;
use Tie::Cycle;
sub UNIVERSAL::Cycle : ATTR(SCALAR) {
my ($package, $symbol, $referent, $attr, $data, $phase) = @_;
$data = [ $data ] unless ref $data eq 'ARRAY';
tie $$referent, 'Tie::Cycle', $data;
}
# and thereafter...
package main;
my $next : Cycle('A'..'Z'); # $next is now a tied variable
while (<>) {
print $next;
}
Note that, because the "Cycle" attribute receives its arguments in the
$data variable, if the attribute is given a list of arguments, $data will
consist of a single array reference; otherwise, it will consist of the single
argument directly. Since Tie::Cycle requires its cycling values to be passed
as an array reference, this means that we need to wrap non-array-reference
arguments in an array constructor:
$data = [ $data ] unless ref $data eq 'ARRAY';
Typically, however, things are the other way around: the tieable class expects
its arguments as a flattened list, so the attribute looks like:
sub UNIVERSAL::Cycle : ATTR(SCALAR) {
my ($package, $symbol, $referent, $attr, $data, $phase) = @_;
my @data = ref $data eq 'ARRAY' ? @$data : $data;
tie $$referent, 'Tie::Whatever', @data;
}
This software pattern is so widely applicable that Attribute::Handlers provides
a way to automate it: specifying 'autotie' in the "use
Attribute::Handlers" statement. So, the cycling example, could also be
written:
use Attribute::Handlers autotie => { Cycle => 'Tie::Cycle' };
# and thereafter...
package main;
my $next : Cycle(['A'..'Z']); # $next is now a tied variable
while (<>) {
print $next;
}
Note that we now have to pass the cycling values as an array reference, since
the "autotie" mechanism passes "tie" a list of arguments
as a list (as in the Tie::Whatever example),
not as an array reference
(as in the original Tie::Cycle example at the start of this section).
The argument after 'autotie' is a reference to a hash in which each key is the
name of an attribute to be created, and each value is the class to which
variables ascribed that attribute should be tied.
Note that there is no longer any need to import the Tie::Cycle module --
Attribute::Handlers takes care of that automagically. You can even pass
arguments to the module's "import" subroutine, by appending them to
the class name. For example:
use Attribute::Handlers
autotie => { Dir => 'Tie::Dir qw(DIR_UNLINK)' };
If the attribute name is unqualified, the attribute is installed in the current
package. Otherwise it is installed in the qualifier's package:
package Here;
use Attribute::Handlers autotie => {
Other::Good => Tie::SecureHash, # tie attr installed in Other::
Bad => Tie::Taxes, # tie attr installed in Here::
UNIVERSAL::Ugly => Software::Patent # tie attr installed everywhere
};
Autoties are most commonly used in the module to which they actually tie, and
need to export their attributes to any module that calls them. To facilitate
this, Attribute::Handlers recognizes a special "pseudo-class" --
"__CALLER__", which may be specified as the qualifier of an
attribute:
package Tie::Me::Kangaroo::Down::Sport;
use Attribute::Handlers autotie =>
{ '__CALLER__::Roo' => __PACKAGE__ };
This causes Attribute::Handlers to define the "Roo" attribute in the
package that imports the Tie::Me::Kangaroo::Down::Sport module.
Note that it is important to quote the __CALLER__::Roo identifier because a bug
in perl 5.8 will refuse to parse it and cause an unknown error.
Passing the tied object to "tie"
Occasionally it is important to pass a reference to the object being tied to the
TIESCALAR, TIEHASH, etc. that ties it.
The "autotie" mechanism supports this too. The following code:
use Attribute::Handlers autotieref => { Selfish => Tie::Selfish };
my $var : Selfish(@args);
has the same effect as:
tie my $var, 'Tie::Selfish', @args;
But when "autotieref" is used instead of "autotie":
use Attribute::Handlers autotieref => { Selfish => Tie::Selfish };
my $var : Selfish(@args);
the effect is to pass the "tie" call an extra reference to the
variable being tied:
tie my $var, 'Tie::Selfish', \$var, @args;
If the class shown in "SYNOPSIS" were placed in the MyClass.pm module,
then the following code:
package main;
use MyClass;
my MyClass $slr :Good :Bad(1**1-1) :Omni(-vorous);
package SomeOtherClass;
use base MyClass;
sub tent { 'acle' }
sub fn :Ugly(sister) :Omni('po',tent()) {...}
my @arr :Good :Omni(s/cie/nt/);
my %hsh :Good(q/bye/) :Omni(q/bus/);
would cause the following handlers to be invoked:
# my MyClass $slr :Good :Bad(1**1-1) :Omni(-vorous);
MyClass::Good:ATTR(SCALAR)( 'MyClass', # class
'LEXICAL', # no typeglob
\$slr, # referent
'Good', # attr name
undef # no attr data
'CHECK', # compiler phase
);
MyClass::Bad:ATTR(SCALAR)( 'MyClass', # class
'LEXICAL', # no typeglob
\$slr, # referent
'Bad', # attr name
0 # eval'd attr data
'CHECK', # compiler phase
);
MyClass::Omni:ATTR(SCALAR)( 'MyClass', # class
'LEXICAL', # no typeglob
\$slr, # referent
'Omni', # attr name
'-vorous' # eval'd attr data
'CHECK', # compiler phase
);
# sub fn :Ugly(sister) :Omni('po',tent()) {...}
MyClass::UGLY:ATTR(CODE)( 'SomeOtherClass', # class
\*SomeOtherClass::fn, # typeglob
\&SomeOtherClass::fn, # referent
'Ugly', # attr name
'sister' # eval'd attr data
'CHECK', # compiler phase
);
MyClass::Omni:ATTR(CODE)( 'SomeOtherClass', # class
\*SomeOtherClass::fn, # typeglob
\&SomeOtherClass::fn, # referent
'Omni', # attr name
['po','acle'] # eval'd attr data
'CHECK', # compiler phase
);
# my @arr :Good :Omni(s/cie/nt/);
MyClass::Good:ATTR(ARRAY)( 'SomeOtherClass', # class
'LEXICAL', # no typeglob
\@arr, # referent
'Good', # attr name
undef # no attr data
'CHECK', # compiler phase
);
MyClass::Omni:ATTR(ARRAY)( 'SomeOtherClass', # class
'LEXICAL', # no typeglob
\@arr, # referent
'Omni', # attr name
"" # eval'd attr data
'CHECK', # compiler phase
);
# my %hsh :Good(q/bye) :Omni(q/bus/);
MyClass::Good:ATTR(HASH)( 'SomeOtherClass', # class
'LEXICAL', # no typeglob
\%hsh, # referent
'Good', # attr name
'q/bye' # raw attr data
'CHECK', # compiler phase
);
MyClass::Omni:ATTR(HASH)( 'SomeOtherClass', # class
'LEXICAL', # no typeglob
\%hsh, # referent
'Omni', # attr name
'bus' # eval'd attr data
'CHECK', # compiler phase
);
Installing handlers into UNIVERSAL, makes them...err..universal. For example:
package Descriptions;
use Attribute::Handlers;
my %name;
sub name { return $name{$_[2]}||*{$_[1]}{NAME} }
sub UNIVERSAL::Name :ATTR {
$name{$_[2]} = $_[4];
}
sub UNIVERSAL::Purpose :ATTR {
print STDERR "Purpose of ", &name, " is $_[4]\n";
}
sub UNIVERSAL::Unit :ATTR {
print STDERR &name, " measured in $_[4]\n";
}
Let's you write:
use Descriptions;
my $capacity : Name(capacity)
: Purpose(to store max storage capacity for files)
: Unit(Gb);
package Other;
sub foo : Purpose(to foo all data before barring it) { }
# etc.
This module offers a single utility function, "findsym()".
- findsym
-
my $symbol = Attribute::Handlers::findsym($package, $referent);
The function looks in the symbol table of $package for the typeglob for
$referent, which is a reference to a variable or subroutine (SCALAR,
ARRAY, HASH, or CODE). If it finds the typeglob, it returns it. Otherwise,
it returns undef. Note that "findsym" memoizes the typeglobs it
has previously successfully found, so subsequent calls with the same
arguments should be much faster.
- "Bad attribute type: ATTR(%s)"
- An attribute handler was specified with an
":ATTR(ref_type)", but the type of referent it was
defined to handle wasn't one of the five permitted: "SCALAR",
"ARRAY", "HASH", "CODE", or
"ANY".
- "Attribute handler %s doesn't handle %s
attributes"
- A handler for attributes of the specified name was
defined, but not for the specified type of declaration. Typically
encountered when trying to apply a "VAR" attribute handler to a
subroutine, or a "SCALAR" attribute handler to some other type
of variable.
- "Declaration of %s attribute in package %s may clash
with future reserved word"
- A handler for an attributes with an all-lowercase name was
declared. An attribute with an all-lowercase name might have a meaning to
Perl itself some day, even though most don't yet. Use a mixed-case
attribute name, instead.
- "Can't have two ATTR specifiers on one
subroutine"
- You just can't, okay? Instead, put all the specifications
together with commas between them in a single "ATTR(
specification)".
- "Can't autotie a %s"
- You can only declare autoties for types "SCALAR",
"ARRAY", and "HASH". They're the only things (apart
from typeglobs -- which are not declarable) that Perl can tie.
- "Internal error: %s symbol went missing"
- Something is rotten in the state of the program. An
attributed subroutine ceased to exist between the point it was declared
and the point at which its attribute handler(s) would have been
called.
- "Won't be able to apply END handler"
- You have defined an END handler for an attribute that is
being applied to a lexical variable. Since the variable may not be
available during END this won't happen.
Damian Conway (
[email protected]). The maintainer of this module is now Rafael
Garcia-Suarez (
[email protected]).
Maintainer of the CPAN release is Steffen Mueller (
[email protected]). Contact
him with technical difficulties with respect to the packaging of the CPAN
module.
There are undoubtedly serious bugs lurking somewhere in code this funky :-) Bug
reports and other feedback are most welcome.
Copyright (c) 2001-2014, Damian Conway. All Rights Reserved.
This module is free software. It may be used, redistributed
and/or modified under the same terms as Perl itself.