/IronPython_Main/Runtime/Tests/LinqDlrTests/testenv/perl/lib/constant.pm

package constant;



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 $name = shift;

    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 (@_ == 1) {

	    my $scalar = $_[0];

	    *$full_name = sub () { $scalar };

	} elsif (@_) {

	    my @list = @_;

	    *$full_name = sub () { @list };

	} else {

	    *$full_name = sub () { };

	}

    }



}



1;



__END__



=head1 NAME



constant - Perl pragma to declare constants



=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($<);



    sub deg2rad { PI * $_[0] / 180 }



    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



=head1 DESCRIPTION



This will declare a symbol to be a constant with the given scalar

or list 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

of accuracy as it can use. Also, your program will be easier to

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>.



=head1 NOTES



The value or values are evaluated in a list context. You may override

this with C<scalar> as shown above.



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.)



    print "The value of PI is @{[ PI ]}.\n";



List constants are returned as lists, not as arrays.



    $homedir = USERINFO[7];		# WRONG

    $homedir = (USERINFO)[7];		# Right



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 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>.



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 ... }>).



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.



    use constant UNICORNS	=> ();

    use constant LOGFILE	=> undef;



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.



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.)



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

    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.



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

inlinable subroutines. As of version 5.004 of Perl, the appropriate

scalar constant is inserted directly in place of some subroutine

calls, thereby saving the overhead of a subroutine call. See

L<perlsub/"Constant Functions"> for details about how and when this

happens.



=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 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@redcat.com>E<gt>, with help from

many other folks.



=head1 COPYRIGHT



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