TITLE

Make constants look like variables

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

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.

ABSTRACT

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

CHANGES

Since v1

Changed notation to be consistent with other value attributes
Specified behaviour of references, arrays, and lists
Added definition

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:

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.
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.
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.

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

IMPLEMENTATION

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

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.

REFERENCES

perldoc constant

perldoc perlsub (for constant subroutines)