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Pragmas

Perl pragma to declare constants

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NAME

constant - Perl pragma to declare constants

SYNOPSIS

  1.  use constant PI => 4 * atan2(1, 1);
    2.  
  2.  use constant DEBUG => 0;
    3.  
  3.  
  4.  print "Pi equals ", PI, "...\n" if DEBUG;
    5.  
  5.  
  6.  use constant {
    7.  
  7.  SEC   => 0,
    8.  
  8.  MIN   => 1,
    9.  
  9.  HOUR  => 2,
    10.  
  10.  MDAY  => 3,
    11.  
  11.  MON   => 4,
    12.  
  12.  YEAR  => 5,
    13.  
  13.  WDAY  => 6,
    14.  
  14.  YDAY  => 7,
    15.  
  15.  ISDST => 8,
    16.  
  16.  };
    17.  
  17.  
  18.  use constant WEEKDAYS => qw(
    19.  
  19.  Sunday Monday Tuesday Wednesday Thursday Friday Saturday
    20.  
  20.  );
    21.  
  21.  
  22.  print "Today is ", (WEEKDAYS)[ (localtime)[WDAY] ], ".\n";
    23.

DESCRIPTION

This pragma allows you to declare constants at compile-time.

When you declare a constant such as PI using the method shownabove, each machine your script runs upon can have as many digitsof accuracy as it can use. Also, your program will be easier toread, more likely to be maintained (and maintained correctly), andfar less likely to send a space probe to the wrong planet becausenobody noticed the one equation in which you wrote 3.14195.

When a constant is used in an expression, Perl replaces it with itsvalue at compile time, and may then optimize the expression further.In particular, any code in an if (CONSTANT) block will be optimizedaway if the constant is false.

NOTES

As with all use directives, defining a constant happens atcompile time. Thus, it's probably not correct to put a constantdeclaration inside of a conditional statement (like if ($foo){ use constant ... }).

Constants defined using this module cannot be interpolated intostrings like variables. However, concatenation works just fine:

  1.  print "Pi equals PI...\n"; # WRONG: does not expand "PI"
    2.  
  2.  print "Pi equals ".PI."...\n"; # right
    3.

Even though a reference may be declared as a constant, the reference maypoint to data which may be changed, as this code shows.

  1.  use constant ARRAY => [ 1,2,3,4 ];
    2.  
  2.  print ARRAY->[1];
    3.  
  3.  ARRAY->[1] = " be changed";
    4.  
  4.  print ARRAY->[1];
    5.

Dereferencing constant references incorrectly (such as using an arraysubscript on a constant hash reference, or vice versa) will be trapped atcompile time.

Constants belong to the package they are defined in. To refer to aconstant defined in another package, specify the full package name, asin Some::Package::CONSTANT. Constants may be exported by modules,and may also be called as either class or instance methods, that is,as Some::Package->CONSTANT or as $obj->CONSTANT where$obj is an instance of Some::Package. Subclasses may definetheir own constants to override those in their base class.

The use of all caps for constant names is merely a convention,although it is recommended in order to make constants stand outand to help avoid collisions with other barewords, keywords, andsubroutine names. Constant names must begin with a letter orunderscore. Names beginning with a double underscore are reserved. Somepoor choices for names will generate warnings, if warnings are enabled atcompile time.

List constants

Constants may be lists of more (or less) than one value. A constantwith no values evaluates to undef in scalar context. Note thatconstants with more than one value do not return their last value inscalar context as one might expect. They currently return the numberof values, but this may change in the future. Do not use constantswith multiple values in scalar context.

NOTE: This implies that the expression defining the value of aconstant is evaluated in list context. This may produce surprises:

  1.  use constant TIMESTAMP => localtime; # WRONG!
    2.  
  2.  use constant TIMESTAMP => scalar localtime; # right
    3.

The first line above defines TIMESTAMP as a 9-element list, asreturned by localtime() in list context. To set it to the stringreturned by localtime() in scalar context, an explicit scalarkeyword is required.

List constants are lists, not arrays. To index or slice them, theymust be placed in parentheses.

  1.  my @workdays = WEEKDAYS[1 .. 5]; # WRONG!
    2.  
  2.  my @workdays = (WEEKDAYS)[1 .. 5];  # right
    3.

Defining multiple constants at once

Instead of writing multiple use constant statements, you may definemultiple constants in a single statement by giving, instead of theconstant name, a reference to a hash where the keys are the names ofthe constants to be defined. Obviously, all constants defined usingthis method must have a single value.

  1.  use constant {
    2.  
  2.  FOO => "A single value",
    3.  
  3.  BAR => "This", "won't", "work!", # Error!
    4.  
  4.  };
    5.

This is a fundamental limitation of the way hashes are constructed inPerl. The error messages produced when this happens will often bequite cryptic -- in the worst case there may be none at all, andyou'll only later find that something is broken.

When defining multiple constants, you cannot use the values of otherconstants defined in the same declaration. This is because thecalling package doesn't know about any constant within that groupuntil after the use statement is finished.

  1.  use constant {
    2.  
  2.  BITMASK => 0xAFBAEBA8,
    3.  
  3.  NEGMASK => ~BITMASK, # Error!
    4.  
  4.  };
    5.

Magic constants

Magical values and references can be made into constants at compiletime, allowing for way cool stuff like this. (These error numbersaren't totally portable, alas.)

  1.  use constant E2BIG => ($! = 7);
    2.  
  2.  print   E2BIG, "\n"; # something like "Arg list too long"
    3.  
  3.  print 0+E2BIG, "\n"; # "7"
    4.

You can't produce a tied constant by giving a tied scalar as thevalue. References to tied variables, however, can be used asconstants without any problems.

TECHNICAL NOTES

In the current implementation, scalar constants are actuallyinlinable subroutines. As of version 5.004 of Perl, the appropriatescalar constant is inserted directly in place of some subroutinecalls, thereby saving the overhead of a subroutine call. SeeConstant Functions in perlsub for details about how and when thishappens.

In the rare case in which you need to discover at run time whether aparticular constant has been declared via this module, you may usethis function to examine the hash %constant::declared. If the givenconstant name does not include a package name, the current package isused.

  1.  sub declared ($) {
    2.  
  2.  use constant 1.01;  # don't omit this!
    3.  
  3.  my $name = shift;
    4.  
  4.  $name =~ s/^::/main::/;
    5.  
  5.  my $pkg = caller;
    6.  
  6.  my $full_name = $name =~ /::/ ? $name : "${pkg}::$name";
    7.  
  7.  $constant::declared{$full_name};
    8.  
  8.  }
    9.

CAVEATS

In the current version of Perl, list constants are not inlinedand some symbols may be redefined without generating a warning.

It is not possible to have a subroutine or a keyword with the samename as a constant in the same package. This is probably a Good Thing.

A constant with a name in the list STDIN STDOUT STDERR ARGV ARGVOUTENV INC SIG is not allowed anywhere but in package main::, fortechnical reasons.

Unlike constants in some languages, these cannot be overriddenon the command line or via environment variables.

You can get into trouble if you use constants in a context whichautomatically quotes barewords (as is true for any subroutine call).For example, you can't say $hash{CONSTANT} because CONSTANT willbe interpreted as a string. Use $hash{CONSTANT()} or$hash{+CONSTANT} to prevent the bareword quoting mechanism fromkicking in. Similarly, since the => operator quotes a barewordimmediately to its left, you have to say CONSTANT() => 'value'(or simply use a comma in place of the big arrow) instead ofCONSTANT => 'value'.

SEE ALSO

Readonly - Facility for creating read-only scalars, arrays, hashes.

Const - Facility for creating read-only variables. Similar to Readonly,but uses SvREADONLY instead of tie.

Attribute::Constant - Make read-only variables via attribute

Scalar::Readonly - Perl extension to the SvREADONLY scalar flag

Hash::Util - A selection of general-utility hash subroutines (mostlyto lock/unlock keys and values)

BUGS

Please report any bugs or feature requests via the perlbug(1) utility.

AUTHORS

Tom Phoenix, <[email protected]>, with help frommany other folks.

Multiple constant declarations at once added by Casey West,<[email protected]>.

Documentation mostly rewritten by Ilmari Karonen,<[email protected]>.

This program is maintained by the Perl 5 Porters. The CPAN distribution is maintained by Sébastien Aperghis-Tramoni<[email protected]>.

COPYRIGHT & LICENSE

Copyright (C) 1997, 1999 Tom Phoenix

This module is free software; you can redistribute it or modify itunder the same terms as Perl itself.

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