Alias(3pm)

NAME

alias - declare symbolic aliases for perl data

attr - auto-declare hash attributes for convenient access

const - define compile-time scalar constants

SYNOPSIS

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)
}

DESCRIPTION

Provides general mechanisms for aliasing perl data for convenient
access.

This module works by putting some values on the symbol table with usersupplied 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.

Functions

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 hashbased 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.
Configuration Variables
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.
Exports
alias attr

EXAMPLES

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 $@;

NOTES

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 symboltable 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 objectoriented perl code.

BUGS

"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

Version 2.32 30 Apr 1999

AUTHOR

Gurusamy Sarathy gsar@umich.edu

docs.sk

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