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Manual Reference Pages  -  ALIAS (3)

.ds Aq ’


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;
                                       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 callers 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 constrained.

    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 localize 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 callers 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.




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 {
        { 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 callers namespace (we can use the AttrPrefix option to specify a different namespace for attr). If that namespace happens to be localized, 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 <B>dependB> 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

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.


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