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package constant; |
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|
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use strict; |
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use 5.006_00; |
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use warnings::register; |
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|
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our($VERSION, %declared); |
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$VERSION = '1.05'; |
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|
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#======================================================================= |
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|
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# Some names are evil choices. |
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my %keywords = map +($_, 1), qw{ BEGIN INIT CHECK END DESTROY AUTOLOAD }; |
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|
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my %forced_into_main = map +($_, 1), |
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qw{ STDIN STDOUT STDERR ARGV ARGVOUT ENV INC SIG }; |
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|
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my %forbidden = (%keywords, %forced_into_main); |
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|
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#======================================================================= |
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# import() - import symbols into user's namespace |
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# |
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# What we actually do is define a function in the caller's namespace |
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# which returns the value. The function we create will normally |
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# be inlined as a constant, thereby avoiding further sub calling |
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# overhead. |
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#======================================================================= |
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sub import { |
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my $class = shift; |
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return unless @_; # Ignore 'use constant;' |
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my %constants = (); |
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my $multiple = ref $_[0]; |
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|
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if ( $multiple ) { |
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if (ref $_[0] ne 'HASH') { |
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require Carp; |
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Carp::croak("Invalid reference type '".ref(shift)."' not 'HASH'"); |
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} |
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%constants = %{+shift}; |
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} else { |
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$constants{+shift} = undef; |
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} |
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|
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foreach my $name ( keys %constants ) { |
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unless (defined $name) { |
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require Carp; |
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Carp::croak("Can't use undef as constant name"); |
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} |
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my $pkg = caller; |
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|
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# Normal constant name |
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if ($name =~ /^_?[^\W_0-9]\w*\z/ and !$forbidden{$name}) { |
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# Everything is okay |
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|
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# Name forced into main, but we're not in main. Fatal. |
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} elsif ($forced_into_main{$name} and $pkg ne 'main') { |
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require Carp; |
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Carp::croak("Constant name '$name' is forced into main::"); |
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|
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# Starts with double underscore. Fatal. |
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} elsif ($name =~ /^__/) { |
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require Carp; |
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Carp::croak("Constant name '$name' begins with '__'"); |
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|
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# Maybe the name is tolerable |
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} elsif ($name =~ /^[A-Za-z_]\w*\z/) { |
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# Then we'll warn only if you've asked for warnings |
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if (warnings::enabled()) { |
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if ($keywords{$name}) { |
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warnings::warn("Constant name '$name' is a Perl keyword"); |
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} elsif ($forced_into_main{$name}) { |
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warnings::warn("Constant name '$name' is " . |
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"forced into package main::"); |
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} |
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} |
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|
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# Looks like a boolean |
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# use constant FRED == fred; |
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} elsif ($name =~ /^[01]?\z/) { |
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require Carp; |
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if (@_) { |
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Carp::croak("Constant name '$name' is invalid"); |
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} else { |
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Carp::croak("Constant name looks like boolean value"); |
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} |
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|
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} else { |
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# Must have bad characters |
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require Carp; |
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Carp::croak("Constant name '$name' has invalid characters"); |
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} |
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|
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{ |
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no strict 'refs'; |
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my $full_name = "${pkg}::$name"; |
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$declared{$full_name}++; |
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if ($multiple) { |
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my $scalar = $constants{$name}; |
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*$full_name = sub () { $scalar }; |
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} else { |
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if (@_ == 1) { |
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my $scalar = $_[0]; |
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*$full_name = sub () { $scalar }; |
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} elsif (@_) { |
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my @list = @_; |
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*$full_name = sub () { @list }; |
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} else { |
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*$full_name = sub () { }; |
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} |
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} |
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} |
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} |
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} |
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|
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1; |
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|
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__END__ |
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|
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=head1 NAME |
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|
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constant - Perl pragma to declare constants |
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|
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=head1 SYNOPSIS |
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|
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use constant PI => 4 * atan2(1, 1); |
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use constant DEBUG => 0; |
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|
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print "Pi equals ", PI, "...\n" if DEBUG; |
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|
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use constant { |
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SEC => 0, |
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MIN => 1, |
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HOUR => 2, |
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MDAY => 3, |
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MON => 4, |
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YEAR => 5, |
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WDAY => 6, |
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YDAY => 7, |
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ISDST => 8, |
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}; |
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|
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use constant WEEKDAYS => qw( |
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Sunday Monday Tuesday Wednesday Thursday Friday Saturday |
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); |
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|
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print "Today is ", (WEEKDAYS)[ (localtime)[WDAY] ], ".\n"; |
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|
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=head1 DESCRIPTION |
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|
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This will declare a symbol to be a constant with the given value. |
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|
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When you declare a constant such as C<PI> using the method shown |
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above, each machine your script runs upon can have as many digits |
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of accuracy as it can use. Also, your program will be easier to |
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read, more likely to be maintained (and maintained correctly), and |
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far less likely to send a space probe to the wrong planet because |
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nobody noticed the one equation in which you wrote C<3.14195>. |
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|
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When a constant is used in an expression, perl replaces it with its |
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value at compile time, and may then optimize the expression further. |
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In particular, any code in an C<if (CONSTANT)> block will be optimized |
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away if the constant is false. |
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|
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=head1 NOTES |
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|
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As with all C<use> directives, defining a constant happens at |
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compile time. Thus, it's probably not correct to put a constant |
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declaration inside of a conditional statement (like C<if ($foo) |
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{ use constant ... }>). |
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|
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Constants defined using this module cannot be interpolated into |
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strings like variables. However, concatenation works just fine: |
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|
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print "Pi equals PI...\n"; # WRONG: does not expand "PI" |
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print "Pi equals ".PI."...\n"; # right |
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|
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Even though a reference may be declared as a constant, the reference may |
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point to data which may be changed, as this code shows. |
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|
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use constant ARRAY => [ 1,2,3,4 ]; |
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print ARRAY->[1]; |
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ARRAY->[1] = " be changed"; |
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print ARRAY->[1]; |
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|
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Dereferencing constant references incorrectly (such as using an array |
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subscript on a constant hash reference, or vice versa) will be trapped at |
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compile time. |
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|
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Constants belong to the package they are defined in. To refer to a |
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constant defined in another package, specify the full package name, as |
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in C<Some::Package::CONSTANT>. Constants may be exported by modules, |
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and may also be called as either class or instance methods, that is, |
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as C<< Some::Package->CONSTANT >> or as C<< $obj->CONSTANT >> where |
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C<$obj> is an instance of C<Some::Package>. Subclasses may define |
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their own constants to override those in their base class. |
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|
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The use of all caps for constant names is merely a convention, |
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although it is recommended in order to make constants stand out |
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and to help avoid collisions with other barewords, keywords, and |
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subroutine names. Constant names must begin with a letter or |
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underscore. Names beginning with a double underscore are reserved. Some |
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poor choices for names will generate warnings, if warnings are enabled at |
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compile time. |
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|
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=head2 List constants |
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|
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Constants may be lists of more (or less) than one value. A constant |
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with no values evaluates to C<undef> in scalar context. Note that |
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constants with more than one value do I<not> return their last value in |
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scalar context as one might expect. They currently return the number |
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of values, but B<this may change in the future>. Do not use constants |
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with multiple values in scalar context. |
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|
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B<NOTE:> This implies that the expression defining the value of a |
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constant is evaluated in list context. This may produce surprises: |
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|
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use constant TIMESTAMP => localtime; # WRONG! |
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use constant TIMESTAMP => scalar localtime; # right |
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|
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The first line above defines C<TIMESTAMP> as a 9-element list, as |
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returned by localtime() in list context. To set it to the string |
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returned by localtime() in scalar context, an explicit C<scalar> |
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keyword is required. |
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|
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List constants are lists, not arrays. To index or slice them, they |
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must be placed in parentheses. |
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|
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my @workdays = WEEKDAYS[1 .. 5]; # WRONG! |
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my @workdays = (WEEKDAYS)[1 .. 5]; # right |
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|
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=head2 Defining multiple constants at once |
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|
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Instead of writing multiple C<use constant> statements, you may define |
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multiple constants in a single statement by giving, instead of the |
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constant name, a reference to a hash where the keys are the names of |
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the constants to be defined. Obviously, all constants defined using |
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this method must have a single value. |
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|
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use constant { |
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FOO => "A single value", |
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BAR => "This", "won't", "work!", # Error! |
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}; |
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|
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This is a fundamental limitation of the way hashes are constructed in |
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Perl. The error messages produced when this happens will often be |
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quite cryptic -- in the worst case there may be none at all, and |
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you'll only later find that something is broken. |
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|
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When defining multiple constants, you cannot use the values of other |
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constants defined in the same declaration. This is because the |
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calling package doesn't know about any constant within that group |
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until I<after> the C<use> statement is finished. |
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|
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use constant { |
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BITMASK => 0xAFBAEBA8, |
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NEGMASK => ~BITMASK, # Error! |
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}; |
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|
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=head2 Magic constants |
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|
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Magical values and references can be made into constants at compile |
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time, allowing for way cool stuff like this. (These error numbers |
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aren't totally portable, alas.) |
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|
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use constant E2BIG => ($! = 7); |
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print E2BIG, "\n"; # something like "Arg list too long" |
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print 0+E2BIG, "\n"; # "7" |
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|
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You can't produce a tied constant by giving a tied scalar as the |
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value. References to tied variables, however, can be used as |
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constants without any problems. |
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|
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=head1 TECHNICAL NOTES |
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|
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In the current implementation, scalar constants are actually |
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inlinable subroutines. As of version 5.004 of Perl, the appropriate |
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scalar constant is inserted directly in place of some subroutine |
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calls, thereby saving the overhead of a subroutine call. See |
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L<perlsub/"Constant Functions"> for details about how and when this |
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happens. |
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|
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In the rare case in which you need to discover at run time whether a |
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particular constant has been declared via this module, you may use |
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this function to examine the hash C<%constant::declared>. If the given |
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constant name does not include a package name, the current package is |
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used. |
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|
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sub declared ($) { |
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use constant 1.01; # don't omit this! |
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my $name = shift; |
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$name =~ s/^::/main::/; |
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my $pkg = caller; |
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my $full_name = $name =~ /::/ ? $name : "${pkg}::$name"; |
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$constant::declared{$full_name}; |
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} |
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|
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=head1 BUGS |
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|
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In the current version of Perl, list constants are not inlined |
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and some symbols may be redefined without generating a warning. |
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|
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It is not possible to have a subroutine or a keyword with the same |
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name as a constant in the same package. This is probably a Good Thing. |
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|
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A constant with a name in the list C<STDIN STDOUT STDERR ARGV ARGVOUT |
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ENV INC SIG> is not allowed anywhere but in package C<main::>, for |
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technical reasons. |
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|
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Unlike constants in some languages, these cannot be overridden |
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on the command line or via environment variables. |
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|
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You can get into trouble if you use constants in a context which |
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automatically quotes barewords (as is true for any subroutine call). |
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For example, you can't say C<$hash{CONSTANT}> because C<CONSTANT> will |
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be interpreted as a string. Use C<$hash{CONSTANT()}> or |
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C<$hash{+CONSTANT}> to prevent the bareword quoting mechanism from |
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kicking in. Similarly, since the C<< => >> operator quotes a bareword |
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immediately to its left, you have to say C<< CONSTANT() => 'value' >> |
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(or simply use a comma in place of the big arrow) instead of |
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C<< CONSTANT => 'value' >>. |
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|
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=head1 AUTHOR |
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|
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Tom Phoenix, E<lt>F<rootbeer@redcat.com>E<gt>, with help from |
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many other folks. |
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|
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Multiple constant declarations at once added by Casey West, |
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E<lt>F<casey@geeknest.com>E<gt>. |
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|
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Documentation mostly rewritten by Ilmari Karonen, |
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E<lt>F<perl@itz.pp.sci.fi>E<gt>. |
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|
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=head1 COPYRIGHT |
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|
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Copyright (C) 1997, 1999 Tom Phoenix |
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|
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This module is free software; you can redistribute it or modify it |
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under the same terms as Perl itself. |
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|
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=cut |
