[% setvar title Make constants look like variables %]

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Make constants look like variables


  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


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.


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


Since v1


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:

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


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


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.


perldoc constant

perldoc perlsub (for constant subroutines)