package constant;
-$VERSION = '1.00';
+use strict;
+use 5.005_64;
+use warnings::register;
+
+our($VERSION, %declared);
+$VERSION = '1.02';
+
+#=======================================================================
+
+# Some names are evil choices.
+my %keywords = map +($_, 1), qw{ BEGIN INIT CHECK END DESTROY AUTOLOAD };
+
+my %forced_into_main = map +($_, 1),
+ qw{ STDIN STDOUT STDERR ARGV ARGVOUT ENV INC SIG };
+
+my %forbidden = (%keywords, %forced_into_main);
+
+#=======================================================================
+# import() - import symbols into user's namespace
+#
+# What we actually do is define a function in the caller's namespace
+# which returns the value. The function we create will normally
+# be inlined as a constant, thereby avoiding further sub calling
+# overhead.
+#=======================================================================
+sub import {
+ my $class = shift;
+ return unless @_; # Ignore 'use constant;'
+ my %constants = ();
+ my $multiple = ref $_[0];
+
+ if ( $multiple ) {
+ if (ref $_[0] ne 'HASH') {
+ require Carp;
+ Carp::croak("Invalid reference type '".ref(shift)."' not 'HASH'");
+ }
+ %constants = %{+shift};
+ } else {
+ $constants{+shift} = undef;
+ }
+
+ foreach my $name ( keys %constants ) {
+ unless (defined $name) {
+ require Carp;
+ Carp::croak("Can't use undef as constant name");
+ }
+ my $pkg = caller;
+
+ # Normal constant name
+ if ($name =~ /^_?[^\W_0-9]\w*\z/ and !$forbidden{$name}) {
+ # Everything is okay
+
+ # Name forced into main, but we're not in main. Fatal.
+ } elsif ($forced_into_main{$name} and $pkg ne 'main') {
+ require Carp;
+ Carp::croak("Constant name '$name' is forced into main::");
+
+ # Starts with double underscore. Fatal.
+ } elsif ($name =~ /^__/) {
+ require Carp;
+ Carp::croak("Constant name '$name' begins with '__'");
+
+ # Maybe the name is tolerable
+ } elsif ($name =~ /^[A-Za-z_]\w*\z/) {
+ # Then we'll warn only if you've asked for warnings
+ if (warnings::enabled()) {
+ if ($keywords{$name}) {
+ warnings::warn("Constant name '$name' is a Perl keyword");
+ } elsif ($forced_into_main{$name}) {
+ warnings::warn("Constant name '$name' is " .
+ "forced into package main::");
+ } else {
+ # Catch-all - what did I miss? If you get this error,
+ # please let me know what your constant's name was.
+ # Write to <rootbeer@redcat.com>. Thanks!
+ warnings::warn("Constant name '$name' has unknown problems");
+ }
+ }
+
+ # Looks like a boolean
+ # use constant FRED == fred;
+ } elsif ($name =~ /^[01]?\z/) {
+ require Carp;
+ if (@_) {
+ Carp::croak("Constant name '$name' is invalid");
+ } else {
+ Carp::croak("Constant name looks like boolean value");
+ }
+
+ } else {
+ # Must have bad characters
+ require Carp;
+ Carp::croak("Constant name '$name' has invalid characters");
+ }
+
+ {
+ no strict 'refs';
+ my $full_name = "${pkg}::$name";
+ $declared{$full_name}++;
+ if ($multiple) {
+ my $scalar = $constants{$name};
+ *$full_name = sub () { $scalar };
+ } else {
+ if (@_ == 1) {
+ my $scalar = $_[0];
+ *$full_name = sub () { $scalar };
+ } elsif (@_) {
+ my @list = @_;
+ *$full_name = sub () { @list };
+ } else {
+ *$full_name = sub () { };
+ }
+ }
+ }
+ }
+}
+
+1;
+
+__END__
=head1 NAME
print "This line does nothing" unless DEBUGGING;
+ # references can be constants
+ use constant CHASH => { foo => 42 };
+ use constant CARRAY => [ 1,2,3,4 ];
+ use constant CPSEUDOHASH => [ { foo => 1}, 42 ];
+ use constant CCODE => sub { "bite $_[0]\n" };
+
+ print CHASH->{foo};
+ print CARRAY->[$i];
+ print CPSEUDOHASH->{foo};
+ print CCODE->("me");
+ print CHASH->[10]; # compile-time error
+
+ # declaring multiple constants at once
+ use constant {
+ BUFFER_SIZE => 4096,
+ ONE_YEAR => 365.2425 * 24 * 60 * 60,
+ PI => 4 * atan2( 1, 1 ),
+ DEBUGGING => 0,
+ ORACLE => 'oracle@cs.indiana.edu',
+ USERNAME => scalar getpwuid($<),
+ USERINFO => getpwuid($<),
+ };
+
=head1 DESCRIPTION
This will declare a symbol to be a constant with the given scalar
The use of all caps for constant names is merely a convention,
although it is recommended in order to make constants stand out
and to help avoid collisions with other barewords, keywords, and
-subroutine names. Constant names must begin with a letter.
+subroutine names. Constant names must begin with a letter or
+underscore. Names beginning with a double underscore are reserved. Some
+poor choices for names will generate warnings, if warnings are enabled at
+compile time.
Constant symbols are package scoped (rather than block scoped, as
C<use strict> is). That is, you can refer to a constant from package
As with all C<use> directives, defining a constant happens at
compile time. Thus, it's probably not correct to put a constant
declaration inside of a conditional statement (like C<if ($foo)
-{ use constant ... }>).
+{ use constant ... }>). When defining multiple constants, you
+cannot use the values of other constants within the same declaration
+scope. This is because the calling package doesn't know about any
+constant within that group until I<after> the C<use> statement is
+finished.
+
+ use constant {
+ AGE => 20,
+ PERSON => { age => AGE }, # Error!
+ };
+ [...]
+ use constant PERSON => { age => AGE }; # Right
Omitting the value for a symbol gives it the value of C<undef> in
a scalar context or the empty list, C<()>, in a list context. This
isn't so nice as it may sound, though, because in this case you
must either quote the symbol name, or use a big arrow, (C<=E<gt>>),
-with nothing to point to. It is probably best to declare these
-explicitly.
+with nothing to point to. It is also illegal to do when defining
+multiple constants at once, you must declare them explicitly. It
+is probably best to declare these explicitly.
use constant UNICORNS => ();
use constant LOGFILE => undef;
print E2BIG, "\n"; # something like "Arg list too long"
print 0+E2BIG, "\n"; # "7"
+Dereferencing constant references incorrectly (such as using an array
+subscript on a constant hash reference, or vice versa) will be trapped at
+compile time.
+
+When declaring multiple constants, all constant values will be a scalar.
+This is because C<constant> can't guess the intent of the programmer
+correctly all the time since values must be expressed in scalar context
+within a hash ref.
+
+In the rare case in which you need to discover at run time whether a
+particular constant has been declared via this module, you may use
+this function to examine the hash C<%constant::declared>. If the given
+constant name does not include a package name, the current package is
+used.
+
+ sub declared ($) {
+ use constant 1.01; # don't omit this!
+ my $name = shift;
+ $name =~ s/^::/main::/;
+ my $pkg = caller;
+ my $full_name = $name =~ /::/ ? $name : "${pkg}::$name";
+ $constant::declared{$full_name};
+ }
+
=head1 TECHNICAL NOTE
In the current implementation, scalar constants are actually
and some symbols may be redefined without generating a warning.
It is not possible to have a subroutine or keyword with the same
-name as a constant. This is probably a Good Thing.
+name as a constant in the same package. This is probably a Good Thing.
+
+A constant with a name in the list C<STDIN STDOUT STDERR ARGV ARGVOUT
+ENV INC SIG> is not allowed anywhere but in package C<main::>, for
+technical reasons.
+
+Even though a reference may be declared as a constant, the reference may
+point to data which may be changed, as this code shows.
+
+ use constant CARRAY => [ 1,2,3,4 ];
+ print CARRAY->[1];
+ CARRAY->[1] = " be changed";
+ print CARRAY->[1];
Unlike constants in some languages, these cannot be overridden
on the command line or via environment variables.
+You can get into trouble if you use constants in a context which
+automatically quotes barewords (as is true for any subroutine call).
+For example, you can't say C<$hash{CONSTANT}> because C<CONSTANT> will
+be interpreted as a string. Use C<$hash{CONSTANT()}> or
+C<$hash{+CONSTANT}> to prevent the bareword quoting mechanism from
+kicking in. Similarly, since the C<=E<gt>> operator quotes a bareword
+immediately to its left, you have to say C<CONSTANT() =E<gt> 'value'>
+(or simply use a comma in place of the big arrow) instead of
+C<CONSTANT =E<gt> 'value'>.
+
=head1 AUTHOR
-Tom Phoenix, E<lt>F<rootbeer@teleport.com>E<gt>, with help from
+Tom Phoenix, E<lt>F<rootbeer@redcat.com>E<gt>, with help from
many other folks.
+Multiple constant declarations at once added by Casey West,
+E<lt>F<casey@geeknest.com>E<gt>.
+
=head1 COPYRIGHT
-Copyright (C) 1997, Tom Phoenix
+Copyright (C) 1997, 1999 Tom Phoenix
This module is free software; you can redistribute it or modify it
under the same terms as Perl itself.
=cut
-
-use strict;
-use Carp;
-use vars qw($VERSION);
-
-#=======================================================================
-
-# Some of this stuff didn't work in version 5.003, alas.
-require 5.003_96;
-
-#=======================================================================
-# import() - import symbols into user's namespace
-#
-# What we actually do is define a function in the caller's namespace
-# which returns the value. The function we create will normally
-# be inlined as a constant, thereby avoiding further sub calling
-# overhead.
-#=======================================================================
-sub import {
- my $class = shift;
- my $name = shift or return; # Ignore 'use constant;'
- croak qq{Can't define "$name" as constant} .
- qq{ (name contains invalid characters or is empty)}
- unless $name =~ /^[^\W_0-9]\w*$/;
-
- my $pkg = caller;
- {
- no strict 'refs';
- if (@_ == 1) {
- my $scalar = $_[0];
- *{"${pkg}::$name"} = sub () { $scalar };
- } elsif (@_) {
- my @list = @_;
- *{"${pkg}::$name"} = sub () { @list };
- } else {
- *{"${pkg}::$name"} = sub () { };
- }
- }
-
-}
-
-1;
projects / p5sagit/p5-mst-13.2.git / blobdiff