=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
-
- # declaring multiple constants at once
+ use constant PI => 4 * atan2(1, 1);
+ use constant DEBUG => 0;
+
+ print "Pi equals ", PI, "...\n" if DEBUG;
+
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($<), # this works
- USERINFO => getpwuid($<), # THIS IS A BUG!
+ SEC => 0,
+ MIN => 1,
+ HOUR => 2,
+ MDAY => 3,
+ MON => 4,
+ YEAR => 5,
+ WDAY => 6,
+ YDAY => 7,
+ ISDST => 8,
};
+ use constant WEEKDAYS => qw(
+ Sunday Monday Tuesday Wednesday Thursday Friday Saturday
+ );
+
+ print "Today is ", (WEEKDAYS)[ (localtime)[WDAY] ], ".\n";
+
=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
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
-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 ... }>).
-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.)
+Constants defined using this module cannot be interpolated into
+strings like variables. However, concatenation works just fine:
- print "The value of PI is @{[ PI ]}.\n";
+ print "Pi equals PI...\n"; # WRONG: does not expand "PI"
+ print "Pi equals ".PI."...\n"; # right
-List constants are returned as lists, not as arrays.
+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 ARRAY => [ 1,2,3,4 ];
+ print ARRAY->[1];
+ ARRAY->[1] = " be changed";
+ print ARRAY->[1];
+
+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
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>. You may also use constants as either class
-or object methods, ie. C<< Other->CONST() >> or C<< $obj->CONST() >>.
-Such constant methods will be inherited as usual.
+=head2 List constants
-As with all C<use> directives, defining a constant happens at
-compile time. Thus, it's probably not correct to put a constant
-declaration inside of a conditional statement (like C<if ($foo)
-{ use constant ... }>). 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.
+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.
- use constant {
- AGE => 20,
- PERSON => { age => AGE }, # Error!
- };
- [...]
- use constant PERSON => { age => AGE }; # Right
+B<NOTE:> This implies that the expression defining the value of a
+constant is evaluated in list context. This may produce surprises:
-Giving an empty list, C<()>, as the value for a symbol makes it return
-C<undef> in scalar context and the empty list in list context.
+ use constant TIMESTAMP => localtime; # WRONG!
+ use constant TIMESTAMP => scalar localtime; # right
- use constant UNICORNS => ();
+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.
- print "Impossible!\n" if defined UNICORNS;
- my @unicorns = UNICORNS; # there are no unicorns
+List constants are lists, not arrays. To index or slice them, they
+must be placed in parentheses.
-The same effect can be achieved by omitting the value and the big
-arrow entirely, but then the symbol name must be put in quotes.
+ my @workdays = WEEKDAYS[1 .. 5]; # WRONG!
+ my @workdays = (WEEKDAYS)[1 .. 5]; # right
- use constant "UNICORNS";
+=head2 Defining multiple constants at once
-The result from evaluating a list constant with more than one element
-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.
+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!
+ };
+
+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.
+
+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.
+
+ use constant {
+ BITMASK => 0xAFBAEBA8,
+ NEGMASK => ~BITMASK, # Error!
+ };
+
+=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 => ($! = 7);
- print E2BIG, "\n"; # something like "Arg list too long"
- print 0+E2BIG, "\n"; # "7"
+ print E2BIG, "\n"; # something like "Arg list too long"
+ print 0+E2BIG, "\n"; # "7"
You can't produce a tied constant by giving a tied scalar as the
value. References to tied variables, however, can be used as
constants without any problems.
-Dereferencing constant references incorrectly (such as using an array
-subscript on a constant hash reference, or vice versa) will be trapped at
-compile time.
-
-When declaring multiple constants, all constant values B<must be
-scalars>. If you accidentally try to use a list with more (or less)
-than one value, every second value will be treated as a symbol name.
+=head1 TECHNICAL NOTES
- use constant {
- EMPTY => (), # WRONG!
- MANY => ("foo", "bar", "baz"), # WRONG!
- };
-
-This will get interpreted as below, which is probably not what you
-wanted.
-
- use constant {
- EMPTY => "MANY", # oops.
- foo => "bar", # oops!
- baz => undef, # OOPS!
- };
-
-This is a fundamental limitation of the way hashes are constructed in
-Perl. The error messages produced when this happens will often be
-quite cryptic -- in the worst case there may be none at all, and
-you'll only later find that something is broken.
+In the current implementation, scalar constants are actually
+inlinable subroutines. As of version 5.004 of Perl, the appropriate
+scalar constant is inserted directly in place of some subroutine
+calls, thereby saving the overhead of a subroutine call. See
+L<perlsub/"Constant Functions"> for details about how and when this
+happens.
In the rare case in which you need to discover at run time whether a
particular constant has been declared via this module, you may use
used.
sub declared ($) {
- 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};
+ 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
+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.
-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.
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'>
+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 =E<gt> 'value'>.
+C<< CONSTANT => 'value' >>.
=head1 AUTHOR
Multiple constant declarations at once added by Casey West,
E<lt>F<casey@geeknest.com>E<gt>.
-Assorted documentation fixes by Ilmari Karonen,
+Documentation mostly rewritten by Ilmari Karonen,
E<lt>F<perl@itz.pp.sci.fi>E<gt>.
=head1 COPYRIGHT
https://perl5.git.perl.org / perl5.git / blobdiff