=head1 TITLE

Make constants look like variables

=head1 VERSION

  Maintainer: Jeremy Howard <j@howard.fm>
  Date: 10 Aug 2000
  Last Modified: 21 Sep 2000
  Mailing List: perl6-language@perl.org
  Number: 83
  Version: 3
  Status: Frozen

=head1 DISCUSSION

The syntax was widely accepted. Some posters preferred to see constants
expanded to work within complex data structures. Others preferred to keep
things simple. This RFC keeps things simple. A counter-RFC has not been
submitted.

=head1 ABSTRACT

This RFC proposes that the current constant.pm module removed, and
replaced with a syntax allowing any variable to be marked as constant.

=head1 CHANGES

=head2 Since v1

=over 4

=item *

Changed notation to be consistent with other value attributes

=item *

Specified behaviour of references, arrays, and lists

=item *

Added definition

=back

=head1 DESCRIPTION

A constant is a value that can not be changed once it is declared.
Once declared, it behaves just as if it was the actual literal value
that it contains.

Currently, constants are created in Perl using the constant.pm module:

  use constant PI => 3.1415926;

which creates an inlined subroutine:

  sub PI () {3.1415926;}

This method of creating constants has three serious drawbacks:
 
=over 8

=item Can not be interpolated in strings

Whereas variables can be interpolated into strings (e.g. "PI is $Pi"),
subroutines can not be. This makes using constants inconvenient, since
string concatenation must be used.

=item Inconsistant syntax

The sudden appearance of barewords can be quite unsettling to new users.
After becoming told that 'arrays are @name, scalars are $name, ...', the
rule suddenly stops working just because the programmer wants the value to
stay constant.

=item Redundant warnings

In persistant Perl environments such as mod_perl, inlined subroutines
often created the redundant warning 'Constant subroutine PI redefined'.
This has been a frequent source of confusion amongst new mod_perl users.

=back

It is proposed that a new syntax for declaring constants be introduced:

  my $PI : constant = 3.1415926;
  my @FIB : constant = (1,1,2,3,5,8,13,21);
  my %ENG_ERRORS : constant = (E_UNDEF=>'undefined', E_FAILED=>'failed');
  
Constants can be lexically or globally scoped (or any other new scoping
level yet to be defined).

If an array or hash is marked constant, it cannot be assigned to, and its
elements can not be assigned to:

  @FIB = (1,2,3);   # Compile time error
  @FIB[0] = 2;      # Compile time error
  %ENG_ERRORS=();   # Compile time error
  %ENG_ERRORS{E_UNDEF=>'No problem'} # Compile time error
  
To create a reference to a constant use the reference operator:

  my $ref_pi = \$PI;
  
To create a constant reference use a reference operator in the
declaration:

  my $a = 'Nothing to declare';
  my $const_ref : constant = \$a;

Note that this does not make the scalar referenced become constant:

  $$const_ref = 'Jewellery';   # No problems
  $const_ref = \4;             # Compile time error

=head1 IMPLEMENTATION

Constants should have the same behaviour as the do now. They should be
inlined, and constant expressions should be calculated at compile time.

=head1 EXTENSIONS

It may be desirable to have a way to remove constness from a value. This
will not be covered in this RFC--if it is required a separate RFC should
be written referencing this one.

=head1 REFERENCES

perldoc constant

perldoc perlsub (for constant subroutines)