Comply with the 0x80th commandment

[perl5.git] / lib / constant.pm

diff --git a/lib/constant.pm b/lib/constant.pm
index 464e20c..0b8efb3 100644 (file)
--- a/lib/constant.pm
+++ b/lib/constant.pm
@@ -1,6 +1,134 @@
 package constant;
 
 package constant;
 

-$VERSION = '1.00';
+use strict;
+use 5.006_00;
+use warnings::register;
+
+our($VERSION, %declared);
+$VERSION = '1.07';
+
+#=======================================================================
+
+# Some names are evil choices.
+my %keywords = map +($_, 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];
+    my $pkg = caller;
+    my $symtab;
+
+    if ($] > 5.009002) {
+       no strict 'refs';
+       $symtab = \%{$pkg . '::'};
+    };
+
+    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 $name) {
+           require Carp;
+           Carp::croak("Can't use undef as constant name");
+       }
+
+       # Normal constant name
+       if ($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 '$name' is " .
+                       "forced into package main::");
+               }
+           }
+
+       # Looks like a boolean
+       # use constant FRED == fred;
+       } elsif ($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 || @_ == 1) {
+               my $scalar = $multiple ? $constants->{$name} : $_[0];
+               if ($symtab && !exists $symtab->{$name}) {
+                   # No typeglob yet, so we can use a reference as space-
+                   # efficient proxy for a constant subroutine
+                   # The check in Perl_ck_rvconst knows that inlinable
+                   # constants from cv_const_sv are read only. So we have to:
+                   Internals::SvREADONLY($scalar, 1);
+                   $symtab->{$name} = \$scalar;
+                   Internals::inc_sub_generation;
+               } else {
+                   if(!exists $symtab->{$name}) {
+                       print STDERR "$name $scalar\n";
+                   }
+                   *$full_name = sub () { $scalar };
+               }
+           } elsif (@_) {
+               my @list = @_;
+               *$full_name = sub () { @list };
+           } else {
+               *$full_name = sub () { };
+           }
+       }
+    }
+}
+
+1;
+
+__END__

 
 =head1 NAME
 
 
 =head1 NAME
 


@@ -8,22 +136,32 @@ constant - Perl pragma to declare constants
 
 =head1 SYNOPSIS
 
 
 =head1 SYNOPSIS
 

-    use constant BUFFER_SIZE   => 4096;
-    use constant ONE_YEAR      => 365.2425 * 24 * 60 * 60;
-    use constant PI            => 4 * atan2 1, 1;
-    use constant DEBUGGING     => 0;
-    use constant ORACLE                => 'oracle@cs.indiana.edu';
-    use constant USERNAME      => scalar getpwuid($<);
-    use constant USERINFO      => getpwuid($<);
+    use constant PI    => 4 * atan2(1, 1);
+    use constant DEBUG => 0;
+
+    print "Pi equals ", PI, "...\n" if DEBUG;

 
 

-    sub deg2rad { PI * $_[0] / 180 }
+    use constant {
+        SEC   => 0,
+        MIN   => 1,
+        HOUR  => 2,
+        MDAY  => 3,
+        MON   => 4,
+        YEAR  => 5,
+        WDAY  => 6,
+        YDAY  => 7,
+        ISDST => 8,
+    };

 
 

-    print "This line does nothing"             unless DEBUGGING;
+    use constant WEEKDAYS => qw(
+        Sunday Monday Tuesday Wednesday Thursday Friday Saturday
+    );
+
+    print "Today is ", (WEEKDAYS)[ (localtime)[WDAY] ], ".\n";

 
 =head1 DESCRIPTION
 
 
 =head1 DESCRIPTION
 

-This will declare a symbol to be a constant with the given scalar
-or list value.
+This will declare a symbol to be a constant with the given value.

 
 When you declare a constant such as C<PI> using the method shown
 above, each machine your script runs upon can have as many digits
 
 When you declare a constant such as C<PI> using the method shown
 above, each machine your script runs upon can have as many digits


@@ -32,61 +170,121 @@ read, more likely to be maintained (and maintained correctly), and
 far less likely to send a space probe to the wrong planet because
 nobody noticed the one equation in which you wrote C<3.14195>.
 
 far less likely to send a space probe to the wrong planet because
 nobody noticed the one equation in which you wrote C<3.14195>.
 

+When a constant is used in an expression, perl replaces it with its
+value at compile time, and may then optimize the expression further.
+In particular, any code in an C<if (CONSTANT)> block will be optimized
+away if the constant is false.
+

 =head1 NOTES
 
 =head1 NOTES
 

-The value or values are evaluated in a list context. You may override
-this with C<scalar> as shown above.
+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 ... }>).
+
+Constants defined using this module cannot be interpolated into
+strings like variables.  However, concatenation works just fine:
+
+    print "Pi equals PI...\n";        # WRONG: does not expand "PI"
+    print "Pi equals ".PI."...\n";    # right

 
 

-These constants do not directly interpolate into double-quotish
-strings, although you may do so indirectly. (See L<perlref> for
-details about how this works.)
+Even though a reference may be declared as a constant, the reference may
+point to data which may be changed, as this code shows.

 
 

-    print "The value of PI is @{[ PI ]}.\n";
+    use constant ARRAY => [ 1,2,3,4 ];
+    print ARRAY->[1];
+    ARRAY->[1] = " be changed";
+    print ARRAY->[1];

 
 

-List constants are returned as lists, not as arrays.
+Dereferencing constant references incorrectly (such as using an array
+subscript on a constant hash reference, or vice versa) will be trapped at
+compile time.

 
 

-    $homedir = USERINFO[7];            # WRONG
-    $homedir = (USERINFO)[7];          # Right
+Constants belong to the package they are defined in.  To refer to a
+constant defined in another package, specify the full package name, as
+in C<Some::Package::CONSTANT>.  Constants may be exported by modules,
+and may also be called as either class or instance methods, that is,
+as C<< Some::Package->CONSTANT >> or as C<< $obj->CONSTANT >> where
+C<$obj> is an instance of C<Some::Package>.  Subclasses may define
+their 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 out
 and to help avoid collisions with other barewords, keywords, and
 
 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
-Other as C<Other::CONST>.
+=head2 List constants

 
 

-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 ... }>).
+Constants may be lists of more (or less) than one value.  A constant
+with no values evaluates to C<undef> in scalar context.  Note that
+constants with more than one value do I<not> return their last value in
+scalar context as one might expect.  They currently return the number
+of values, but B<this may change in the future>.  Do not use constants
+with multiple values in scalar context.
+
+B<NOTE:> This implies that the expression defining the value of a
+constant is evaluated in list context.  This may produce surprises:
+
+    use constant TIMESTAMP => localtime;                # WRONG!
+    use constant TIMESTAMP => scalar localtime;         # right
+
+The first line above defines C<TIMESTAMP> as a 9-element list, as
+returned by localtime() in list context.  To set it to the string
+returned by localtime() in scalar context, an explicit C<scalar>
+keyword is required.
+
+List constants are lists, not arrays.  To index or slice them, they
+must be placed in parentheses.
+
+    my @workdays = WEEKDAYS[1 .. 5];            # WRONG!
+    my @workdays = (WEEKDAYS)[1 .. 5];          # right
+
+=head2 Defining multiple constants at once
+
+Instead of writing multiple C<use constant> statements, you may define
+multiple constants in a single statement by giving, instead of the
+constant name, a reference to a hash where the keys are the names of
+the constants to be defined.  Obviously, all constants defined using
+this method must have a single value.
+
+    use constant {
+        FOO => "A single value",
+        BAR => "This", "won't", "work!",        # Error!
+    };

 
 

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

 
 

-    use constant UNICORNS      => ();
-    use constant LOGFILE       => undef;
+When defining multiple constants, you cannot use the values of other
+constants defined in the same declaration.  This is because the
+calling package doesn't know about any constant within that group
+until I<after> the C<use> statement is finished.

 
 

-The result from evaluating a list constant in a scalar context is
-not documented, and is B<not> guaranteed to be any particular value
-in the future. In particular, you should not rely upon it being
-the number of elements in the list, especially since it is not
-B<necessarily> that value in the current implementation.
+    use constant {
+        BITMASK => 0xAFBAEBA8,
+        NEGMASK => ~BITMASK,                    # Error!
+    };

 
 

-Magical values, tied values, and references can be made into
-constants at compile time, allowing for way cool stuff like this.
-(These error numbers aren't totally portable, alas.)
+=head2 Magic constants
+
+Magical values and references can be made into constants at compile
+time, allowing for way cool stuff like this.  (These error numbers
+aren't totally portable, alas.)

 
     use constant E2BIG => ($! = 7);
 
     use constant E2BIG => ($! = 7);

-    print   E2BIG, "\n";       # something like "Arg list too long"
-    print 0+E2BIG, "\n";       # "7"
+    print   E2BIG, "\n";        # something like "Arg list too long"
+    print 0+E2BIG, "\n";        # "7"
+
+You can't produce a tied constant by giving a tied scalar as the
+value.  References to tied variables, however, can be used as
+constants without any problems.

 
 

-=head1 TECHNICAL NOTE
+=head1 TECHNICAL NOTES

 
 In the current implementation, scalar constants are actually
 inlinable subroutines. As of version 5.004 of Perl, the appropriate
 
 In the current implementation, scalar constants are actually
 inlinable subroutines. As of version 5.004 of Perl, the appropriate


@@ -95,13 +293,32 @@ calls, thereby saving the overhead of a subroutine call. See
 L<perlsub/"Constant Functions"> for details about how and when this
 happens.
 
 L<perlsub/"Constant Functions"> for details about how and when this
 happens.
 

+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 BUGS
 
 In the current version of Perl, list constants are not inlined
 and some symbols may be redefined without generating a warning.
 
 =head1 BUGS
 
 In the current version of Perl, list constants are not inlined
 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.
+It is not possible to have a subroutine or a keyword with the same
+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. 

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


@@ -111,62 +328,27 @@ 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<$hash{+CONSTANT}> to prevent the bareword quoting mechanism from
 For example, you can't say C<$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'>
-instead of C<CONSTANT =E<gt> 'value'>.
+kicking in.  Similarly, since the C<< => >> operator quotes a bareword
+immediately to its left, you have to say C<< CONSTANT() => 'value' >>
+(or simply use a comma in place of the big arrow) instead of
+C<< CONSTANT => 'value' >>.

 
 =head1 AUTHOR
 
 
 =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.
 
 many other folks.
 

+Multiple constant declarations at once added by Casey West,
+E<lt>F<casey@geeknest.com>E<gt>.
+
+Documentation mostly rewritten by Ilmari Karonen,
+E<lt>F<perl@itz.pp.sci.fi>E<gt>.
+

 =head1 COPYRIGHT
 
 =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
 
 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;