alias - declare symbolic aliases for perl data
attr - auto-declare hash attributes for convenient access
const - define compile-time scalar constants
use Alias qw(alias const attr);
alias TEN => $ten, Ten => \$ten, Ten => \&ten,
Ten => \@ten, Ten => \%ten, TeN => \*ten;
{
local @Ten;
my $ten = [1..10];
alias Ten => $ten; # local @Ten
}
const pi => 3.14, ten => 10;
package Foo;
use Alias;
sub new { bless {foo => 1, _bar => [2, 3]}, $_[0] }
sub a_method {
my $s = attr shift;
# $foo, @_bar are now local aliases for
# $_[0]{foo}, @{$_[0]{_bar}} etc.
}
sub b_method {
local $Alias::KeyFilter = "_";
local $Alias::AttrPrefix = "main::";
my $s = attr shift;
# local @::_bar is now available, ($foo, $::foo are not)
}
sub c_method {
local $Alias::KeyFilter = sub { $_ = shift; return (/^_/ ? 1 : 0) };
local $Alias::AttrPrefix = sub {
$_ = shift;
s/^_(.+)$/main::$1/;
return $_
};
my $s = attr shift;
# local @::bar is now available, ($foo, $::foo are not)
}
Provides general mechanisms for aliasing perl data for convenient access.
This module works by putting some values on the symbol table with user-supplied
names. Values that are references will get dereferenced into their base types.
This means that a value of "[1,2,3]" with a name of "foo"
will be made available as @foo, not $foo.
The exception to this rule is the default behavior of the "attr"
function, which will not dereference values which are blessed references (aka
objects). See $Alias::Deref for how to change this default behavior.
- alias
- Given a list of name => value pairs, declares aliases in
the "caller"s namespace. If the value supplied is a reference,
the alias is created for the underlying value instead of the reference
itself (there is no need to use this module to alias references--they are
automatically "aliased" on assignment). This allows the user to
alias most of the basic types.
If the value supplied is a scalar compile-time constant, the aliases become
read-only. Any attempt to write to them will fail with a run time error.
Aliases can be dynamically scoped by pre-declaring the target variable as
"local". Using "attr" for this purpose is more
convenient, and recommended.
- attr
- Given a hash reference, aliases the values of the hash to
the names that correspond to the keys. It always returns the supplied
value. The aliases are local to the enclosing block. If any of the values
are unblessed references, they are available as their dereferenced types.
Thus the action is similar to saying:
alias %{$_[0]}
but, in addition, also localizes the aliases, and does not dereference
objects. Dereferencing of objects can be forced by setting the
"Deref" option. See $Alias::Deref.
This can be used for convenient access to hash values and hash-based object
attributes.
Note that this makes available the semantics of "local"
subroutines and methods. That makes for some nifty possibilities. We could
make truly private methods by putting anonymous subs within an object.
These subs would be available within methods where we use
"attr", and will not be visible to the outside world as normal
methods. We could forbid recursion in methods by always putting an empty
sub in the object hash with the same key as the method name. This would be
useful where a method has to run code from other modules, but cannot be
certain whether that module will call it back again.
The default behavior is to create aliases for all the entries in the hash,
in the callers namespace. This can be controlled by setting a few options.
See "Configuration Variables" for details.
- const
- This is simply a function alias for "alias",
described above. Provided on demand at "use" time, since it
reads better for constant declarations. Note that hashes and arrays cannot
be so "const"rained.
The following configuration variables can be used to control the behavior of the
"attr" function. They are typically set after the "use
Alias;" statement. Another typical usage is to "local"ize them
in a block so that their values are only effective within that block.
- $Alias::KeyFilter
- Specifies the key prefix used for determining which hash
entries will be interned by "attr". Can be a CODE reference, in
which case it will be called with the key, and the boolean return value
will determine if that hash entry is a candidate attribute.
- $Alias::AttrPrefix
- Specifies a prefix to prepend to the names of localized
attributes created by "attr". Can be a CODE reference, in which
case it will be called with the key, and the result will determine the
full name of the attribute. The value can have embedded package delimiters
("::" or "'"), which cause the attributes to be
interned in that namespace instead of the "caller"s own
namespace. For example, setting it to "main::" makes "use
strict 'vars';" somewhat more palatable (since we can refer to the
attributes as $::foo, etc., without actually declaring the
attributes).
- $Alias::Deref
- Controls the implicit dereferencing behavior of
"attr". If it is set to "" or 0, "attr" will
not dereference blessed references. If it is a true value (anything but
"", 0, or a CODE reference), all references will be made
available as their dereferenced types, including values that may be
objects. The default is "".
This option can be used as a filter if it is set to a CODE reference, in
which case it will be called with the key and the value (whenever the
value happens to be a reference), and the boolean return value will
determine if that particular reference must be dereferenced.
- alias
- attr
Run these code snippets and observe the results to become more familiar with the
features of this module.
use Alias qw(alias const attr);
$ten = 10;
alias TEN => $ten, Ten => \$ten, Ten => \&ten,
Ten => \@ten, Ten => \%ten;
alias TeN => \*ten; # same as *TeN = *ten
# aliasing basic types
$ten = 20;
print "$TEN|$Ten|$ten\n"; # should print "20|20|20"
sub ten { print "10\n"; }
@ten = (1..10);
%ten = (a..j);
&Ten; # should print "10"
print @Ten, "|", %Ten, "\n";
# this will fail at run time
const _TEN_ => 10;
eval { $_TEN_ = 20 };
print $@ if $@;
# dynamically scoped aliases
@DYNAMIC = qw(m n o);
{
my $tmp = [ qw(a b c d) ];
local @DYNAMIC;
alias DYNAMIC => $tmp, PERM => $tmp;
$DYNAMIC[2] = 'zzz';
# prints "abzzzd|abzzzd|abzzzd"
print @$tmp, "|", @DYNAMIC, "|", @PERM, "\n";
@DYNAMIC = qw(p q r);
# prints "pqr|pqr|pqr"
print @$tmp, "|", @DYNAMIC, "|", @PERM, "\n";
}
# prints "mno|pqr"
print @DYNAMIC, "|", @PERM, "\n";
# named closures
my($lex) = 'abcd';
$closure = sub { print $lex, "\n" };
alias NAMEDCLOSURE => \&$closure;
NAMEDCLOSURE(); # prints "abcd"
$lex = 'pqrs';
NAMEDCLOSURE(); # prints "pqrs"
# hash/object attributes
package Foo;
use Alias;
sub new {
bless
{ foo => 1,
bar => [2,3],
buz => { a => 4},
privmeth => sub { "private" },
easymeth => sub { die "to recurse or to die, is the question" },
}, $_[0];
}
sub easymeth {
my $s = attr shift; # localizes $foo, @bar, %buz etc with values
eval { $s->easymeth }; # should fail
print $@ if $@;
# prints "1|2|3|a|4|private|"
print join '|', $foo, @bar, %buz, $s->privmeth, "\n";
}
$foo = 6;
@bar = (7,8);
%buz = (b => 9);
Foo->new->easymeth; # this will not recurse endlessly
# prints "6|7|8|b|9|"
print join '|', $foo, @bar, %buz, "\n";
# this should fail at run-time
eval { Foo->new->privmeth };
print $@ if $@;
It is worth repeating that the aliases created by "alias" and
"const" will be created in the "caller"s namespace (we can
use the "AttrPrefix" option to specify a different namespace for
"attr"). If that namespace happens to be "local"ized, the
aliases created will be local to that block. "attr" localizes the
aliases for us.
Remember that references will be available as their dereferenced types.
Aliases cannot be lexical, since, by neccessity, they live on the symbol table.
Lexicals can be aliased. Note that this provides a means of reversing the action
of anonymous type generators "\", "[]" and "{}".
This allows us to anonymously construct data or code and give it a
symbol-table presence when we choose.
Any occurrence of "::" or "'" in names will be treated as
package qualifiers, and the value will be interned in that namespace.
Remember that aliases are very much like references, only we don't have to
dereference them as often. Which means we won't have to pound on the dollars
so much.
We can dynamically make subroutines and named closures with this scheme.
It is possible to alias packages, but that might be construed as abuse.
Using this module will dramatically reduce noise characters in object-oriented
perl code.
"use strict 'vars';" is not very usable, since we
depend so
much on the symbol table. You can declare the attributes with "use
vars" to avoid warnings. Setting $Alias::AttrPrefix to "main::"
is one way to avoid "use vars" and frustration.
Tied variables cannot be aliased properly, yet.
Version 2.32 30 Apr 1999
Gurusamy Sarathy
[email protected]
Copyright (c) 1995-99 Gurusamy Sarathy. All rights reserved. This program is
free software; you can redistribute it and/or modify it under the same terms
as Perl itself.
perl(1)