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398 lines
13 KiB
Perl
398 lines
13 KiB
Perl
package constant;
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use 5.005;
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use strict;
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use warnings::register;
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use vars qw($VERSION %declared);
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$VERSION = '1.21';
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#=======================================================================
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# Some names are evil choices.
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my %keywords = map +($_, 1), qw{ BEGIN INIT CHECK END DESTROY AUTOLOAD };
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$keywords{UNITCHECK}++ if $] > 5.009;
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my %forced_into_main = map +($_, 1),
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qw{ STDIN STDOUT STDERR ARGV ARGVOUT ENV INC SIG };
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my %forbidden = (%keywords, %forced_into_main);
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my $str_end = $] >= 5.006 ? "\\z" : "\\Z";
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my $normal_constant_name = qr/^_?[^\W_0-9]\w*$str_end/;
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my $tolerable = qr/^[A-Za-z_]\w*$str_end/;
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my $boolean = qr/^[01]?$str_end/;
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BEGIN {
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# We'd like to do use constant _CAN_PCS => $] > 5.009002
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# but that's a bit tricky before we load the constant module :-)
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# By doing this, we save 1 run time check for *every* call to import.
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no strict 'refs';
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my $const = $] > 5.009002;
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*_CAN_PCS = sub () {$const};
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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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my $pkg = caller;
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my $flush_mro;
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my $symtab;
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if (_CAN_PCS) {
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no strict 'refs';
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$symtab = \%{$pkg . '::'};
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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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unless (defined $_[0]) {
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require Carp;
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Carp::croak("Can't use undef as constant name");
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}
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$constants->{+shift} = undef;
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}
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foreach my $name ( keys %$constants ) {
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# Normal constant name
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if ($name =~ $normal_constant_name and !$forbidden{$name}) {
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# Everything is okay
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# Name forced into main, but we're not in main. Fatal.
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} elsif ($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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# 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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# Maybe the name is tolerable
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} elsif ($name =~ $tolerable) {
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# Then we'll warn only if you've asked for warnings
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if (warnings::enabled()) {
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if ($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 '$name' is " .
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"forced into package main::");
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}
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}
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# Looks like a boolean
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# use constant FRED == fred;
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} elsif ($name =~ $boolean) {
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require Carp;
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if (@_) {
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Carp::croak("Constant name '$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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} 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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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 || @_ == 1) {
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my $scalar = $multiple ? $constants->{$name} : $_[0];
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# Work around perl bug #xxxxx: Sub names (actually glob
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# names in general) ignore the UTF8 flag. So we have to
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# turn it off to get the "right" symbol table entry.
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utf8::is_utf8 $name and utf8::encode $name;
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# The constant serves to optimise this entire block out on
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# 5.8 and earlier.
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if (_CAN_PCS && $symtab && !exists $symtab->{$name}) {
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# No typeglob yet, so we can use a reference as space-
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# efficient proxy for a constant subroutine
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# The check in Perl_ck_rvconst knows that inlinable
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# constants from cv_const_sv are read only. So we have to:
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Internals::SvREADONLY($scalar, 1);
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$symtab->{$name} = \$scalar;
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++$flush_mro;
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} else {
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*$full_name = sub () { $scalar };
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}
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} elsif (@_) {
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my @list = @_;
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*$full_name = sub () { @list };
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} else {
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*$full_name = sub () { };
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}
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}
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}
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# Flush the cache exactly once if we make any direct symbol table changes.
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mro::method_changed_in($pkg) if _CAN_PCS && $flush_mro;
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}
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1;
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__END__
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=head1 NAME
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constant - Perl pragma to declare constants
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=head1 SYNOPSIS
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use constant PI => 4 * atan2(1, 1);
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use constant DEBUG => 0;
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print "Pi equals ", PI, "...\n" if DEBUG;
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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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use constant WEEKDAYS => qw(
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Sunday Monday Tuesday Wednesday Thursday Friday Saturday
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);
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print "Today is ", (WEEKDAYS)[ (localtime)[WDAY] ], ".\n";
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=head1 DESCRIPTION
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This pragma allows you to declare constants at compile-time.
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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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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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=head1 NOTES
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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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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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print "Pi equals PI...\n"; # WRONG: does not expand "PI"
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print "Pi equals ".PI."...\n"; # right
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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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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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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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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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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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=head2 List constants
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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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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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use constant TIMESTAMP => localtime; # WRONG!
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use constant TIMESTAMP => scalar localtime; # right
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The first line above defines C<TIMESTAMP> as a 9-element list, as
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returned by C<localtime()> in list context. To set it to the string
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returned by C<localtime()> in scalar context, an explicit C<scalar>
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keyword is required.
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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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my @workdays = WEEKDAYS[1 .. 5]; # WRONG!
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my @workdays = (WEEKDAYS)[1 .. 5]; # right
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=head2 Defining multiple constants at once
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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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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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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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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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use constant {
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BITMASK => 0xAFBAEBA8,
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NEGMASK => ~BITMASK, # Error!
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};
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=head2 Magic constants
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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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use constant E2BIG => ($! = 7);
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print E2BIG, "\n"; # something like "Arg list too long"
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print 0+E2BIG, "\n"; # "7"
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You can't produce a tied constant by giving a tied scalar as the
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value. References to tied variables, however, can be used as
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constants without any problems.
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=head1 TECHNICAL NOTES
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In the current implementation, scalar constants are actually
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inlinable subroutines. As of version 5.004 of Perl, the appropriate
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scalar constant is inserted directly in place of some subroutine
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calls, thereby saving the overhead of a subroutine call. See
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L<perlsub/"Constant Functions"> for details about how and when this
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happens.
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In the rare case in which you need to discover at run time whether a
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particular constant has been declared via this module, you may use
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this function to examine the hash C<%constant::declared>. If the given
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constant name does not include a package name, the current package is
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used.
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sub declared ($) {
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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{$full_name};
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}
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=head1 CAVEATS
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In the current version of Perl, list constants are not inlined
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and some symbols may be redefined without generating a warning.
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It is not possible to have a subroutine or a keyword with the same
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name as a constant in the same package. This is probably a Good Thing.
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A constant with a name in the list C<STDIN STDOUT STDERR ARGV ARGVOUT
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ENV INC SIG> is not allowed anywhere but in package C<main::>, for
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technical reasons.
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Unlike constants in some languages, these cannot be overridden
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on the command line or via environment variables.
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You can get into trouble if you use constants in a context which
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automatically quotes barewords (as is true for any subroutine call).
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For example, you can't say C<$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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=head1 SEE ALSO
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L<Readonly> - Facility for creating read-only scalars, arrays, hashes.
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L<Const> - Facility for creating read-only variables. Similar to C<Readonly>,
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but uses C<SvREADONLY> instead of C<tie>.
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L<Attribute::Constant> - Make read-only variables via attribute
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L<Scalar::Readonly> - Perl extension to the C<SvREADONLY> scalar flag
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L<Hash::Util> - A selection of general-utility hash subroutines (mostly
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to lock/unlock keys and values)
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=head1 BUGS
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Please report any bugs or feature requests via the perlbug(1) utility.
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=head1 AUTHORS
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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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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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Documentation mostly rewritten by Ilmari Karonen,
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E<lt>F<perl@itz.pp.sci.fi>E<gt>.
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This program is maintained by the Perl 5 Porters.
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The CPAN distribution is maintained by SE<eacute>bastien Aperghis-Tramoni
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E<lt>F<sebastien@aperghis.net>E<gt>.
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=head1 COPYRIGHT & LICENSE
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Copyright (C) 1997, 1999 Tom Phoenix
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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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=cut
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