Readonly(3) User Contributed Perl Documentation Readonly(3)
- -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1
NAMEReadonly - Facility for creating read-only scalars, arrays, hashes.
VERSION
This documentation describes version 1.03 of Readonly.pm, April 20,
2004.
SYNOPSIS
use Readonly;
# Read-only scalar
Readonly::Scalar $sca => $initial_value;
Readonly::Scalar my $sca => $initial_value;
# Read-only array
Readonly::Array @arr => @values;
Readonly::Array my @arr => @values;
# Read-only hash
Readonly::Hash %has => (key => value, key => value, ...);
Readonly::Hash my %has => (key => value, key => value, ...);
# or:
Readonly::Hash %has => {key => value, key => value, ...};
# You can use the read-only variables like any regular variables:
print $sca;
$something = $sca + $arr[2];
next if $has{$some_key};
# But if you try to modify a value, your program will die:
$sca = 7;
push @arr, 'seven';
delete $has{key};
# The error message is "Modification of a read-only value
attempted"
# Alternate form (Perl 5.8 and later)
Readonly $sca => $initial_value;
Readonly my $sca => $initial_value;
Readonly @arr => @values;
Readonly my @arr => @values;
Readonly %has => (key => value, key => value, ...);
Readonly my %has => (key => value, key => value, ...);
# Alternate form (for Perls earlier than v5.8)
Readonly \$sca => $initial_value;
Readonly \my $sca => $initial_value;
Readonly \@arr => @values;
Readonly \my @arr => @values;
Readonly \%has => (key => value, key => value, ...);
Readonly \my %has => (key => value, key => value, ...);
DESCRIPTION
This is a facility for creating non-modifiable variables. This is
useful for configuration files, headers, etc. It can also be useful as
a development and debugging tool, for catching updates to variables
that should not be changed.
If any of the values you pass to "Scalar", "Array", or "Hash" are
references, then those functions recurse over the data structures,
marking everything as Readonly. Usually, this is what you want: the
entire structure nonmodifiable. If you want only the top level to be
Readonly, use the alternate "Scalar1", "Array1" and "Hash1" functions.
Please note that most users of Readonly will also want to install a
companion module Readonly::XS. See the "CONS" section below for more
details.
COMPARISON WITH "use constant"
Perl provides a facility for creating constant values, via the "use
constant" pragma. There are several problems with this pragma.
· The constants created have no leading $ or @ character.
· These constants cannot be interpolated into strings.
· Syntax can get dicey sometimes. For example:
use constant CARRAY => (2, 3, 5, 7, 11, 13);
$a_prime = CARRAY[2]; # wrong!
$a_prime = (CARRAY)[2]; # right -- MUST use parentheses
· You have to be very careful in places where barewords are allowed.
For example:
use constant SOME_KEY => 'key';
%hash = (key => 'value', other_key => 'other_value');
$some_value = $hash{SOME_KEY}; # wrong!
$some_value = $hash{+SOME_KEY}; # right
(who thinks to use a unary plus when using a hash?)
· "use constant" works for scalars and arrays, not hashes.
· These constants are global ot the package in which they're declared;
cannot be lexically scoped.
· Works only at compile time.
· Can be overridden:
use constant PI => 3.14159;
...
use constant PI => 2.71828;
(this does generate a warning, however, if you have warnings
enabled).
· It is very difficult to make and use deep structures (complex data
structures) with "use constant".
COMPARISON WITH TYPEGLOB CONSTANTS
Another popular way to create read-only scalars is to modify the symbol
table entry for the variable by using a typeglob:
*a = \'value';
This works fine, but it only works for global variables ("my" variables
have no symbol table entry). Also, the following similar constructs do
not work:
*a = [1, 2, 3]; # Does NOT create a read-only array
*a = { a => 'A'}; # Does NOT create a read-only hash
PROS
Readonly.pm, on the other hand, will work with global variables and
with lexical ("my") variables. It will create scalars, arrays, or
hashes, all of which look and work like normal, read-write Perl
variables. You can use them in scalar context, in list context; you
can take references to them, pass them to functions, anything.
Readonly.pm also works well with complex data structures, allowing you
to tag the whole structure as nonmodifiable, or just the top level.
Also, Readonly variables may not be reassigned. The following code
will die:
Readonly::Scalar $pi => 3.14159;
...
Readonly::Scalar $pi => 2.71828;
CONS
Readonly.pm does impose a performance penalty. It's pretty slow. How
slow? Run the "benchmark.pl" script that comes with Readonly. On my
test system, "use constant", typeglob constants, and regular read/write
Perl variables were all about the same speed, and Readonly.pm constants
were about 1/20 the speed.
However, there is relief. There is a companion module available,
Readonly::XS. If it is installed on your system, Readonly.pm uses it
to make read-only scalars much faster. With Readonly::XS, Readonly
scalars are as fast as the other types of variables. Readonly arrays
and hashes will still be relatively slow. But it's likely that most of
your Readonly variables will be scalars.
If you can't use Readonly::XS (for example, if you don't have a C
compiler, or your perl is statically linked and you don't want to re-
link it), you have to decide whether the benefits of Readonly variables
outweigh the speed issue. For most configuration variables (and other
things that Readonly is likely to be useful for), the speed issue is
probably not really a big problem. But benchmark your program if it
might be. If it turns out to be a problem, you may still want to use
Readonly.pm during development, to catch changes to variables that
should not be changed, and then remove it for production:
# For testing:
Readonly::Scalar $Foo_Directory => '/usr/local/foo';
Readonly::Scalar $Bar_Directory => '/usr/local/bar';
# $Foo_Directory = '/usr/local/foo';
# $Bar_Directory = '/usr/local/bar';
# For production:
# Readonly::Scalar $Foo_Directory => '/usr/local/foo';
# Readonly::Scalar $Bar_Directory => '/usr/local/bar';
$Foo_Directory = '/usr/local/foo';
$Bar_Directory = '/usr/local/bar';
FUNCTIONS
Readonly::Scalar $var => $value;
Creates a nonmodifiable scalar, $var, and assigns a value of $value
to it. Thereafter, its value may not be changed. Any attempt to
modify the value will cause your program to die.
A value must be supplied. If you want the variable to have "undef"
as its value, you must specify "undef".
If $value is a reference to a scalar, array, or hash, then this
function will mark the scalar, array, or hash it points to as being
Readonly as well, and it will recursively traverse the structure,
marking the whole thing as Readonly. Usually, this is what you
want. However, if you want only the $value marked as Readonly, use
"Scalar1".
If $var is already a Readonly variable, the program will die with
an error about reassigning Readonly variables.
Readonly::Array @arr => (value, value, ...);
Creates a nonmodifiable array, @arr, and assigns the specified list
of values to it. Thereafter, none of its values may be changed;
the array may not be lengthened or shortened or spliced. Any
attempt to do so will cause your program to die.
If any of the values passed is a reference to a scalar, array, or
hash, then this function will mark the scalar, array, or hash it
points to as being Readonly as well, and it will recursively
traverse the structure, marking the whole thing as Readonly.
Usually, this is what you want. However, if you want only the hash
%@arr itself marked as Readonly, use "Array1".
If @arr is already a Readonly variable, the program will die with
an error about reassigning Readonly variables.
Readonly::Hash %h => (key => value, key => value, ...);
Readonly::Hash %h => {key => value, key => value, ...};
Creates a nonmodifiable hash, %h, and assigns the specified keys
and values to it. Thereafter, its keys or values may not be
changed. Any attempt to do so will cause your program to die.
A list of keys and values may be specified (with parentheses in the
synopsis above), or a hash reference may be specified (curly braces
in the synopsis above). If a list is specified, it must have an
even number of elements, or the function will die.
If any of the values is a reference to a scalar, array, or hash,
then this function will mark the scalar, array, or hash it points
to as being Readonly as well, and it will recursively traverse the
structure, marking the whole thing as Readonly. Usually, this is
what you want. However, if you want only the hash %h itself marked
as Readonly, use "Hash1".
If %h is already a Readonly variable, the program will die with an
error about reassigning Readonly variables.
Readonly $var => $value;
Readonly @arr => (value, value, ...);
Readonly %h => (key => value, ...);
Readonly %h => {key => value, ...};
The "Readonly" function is an alternate to the "Scalar", "Array",
and "Hash" functions. It has the advantage (if you consider it an
advantage) of being one function. That may make your program look
neater, if you're initializing a whole bunch of constants at once.
You may or may not prefer this uniform style.
It has the disadvantage of having a slightly different syntax for
versions of Perl prior to 5.8. For earlier versions, you must
supply a backslash, because it requires a reference as the first
parameter.
Readonly \$var => $value;
Readonly \@arr => (value, value, ...);
Readonly \%h => (key => value, ...);
Readonly \%h => {key => value, ...};
You may or may not consider this ugly.
Readonly::Scalar1 $var => $value;
Readonly::Array1 @arr => (value, value, ...);
Readonly::Hash1 %h => (key => value, key => value, ...);
Readonly::Hash1 %h => {key => value, key => value, ...};
These alternate functions create shallow Readonly variables,
instead of deep ones. For example:
Readonly::Array1 @shal => (1, 2, {perl=>'Rules', java=>'Bites'}, 4, 5);
Readonly::Array @deep => (1, 2, {perl=>'Rules', java=>'Bites'}, 4, 5);
$shal[1] = 7; # error
$shal[2]{APL}='Weird'; # Allowed! since the hash isn't Readonly
$deep[1] = 7; # error
$deep[2]{APL}='Weird'; # error, since the hash is ReadonlyEXAMPLES
# SCALARS:
# A plain old read-only value
Readonly::Scalar $a => "A string value";
# The value need not be a compile-time constant:
Readonly::Scalar $a => $computed_value;
# ARRAYS:
# A read-only array:
Readonly::Array @a => (1, 2, 3, 4);
# The parentheses are optional:
Readonly::Array @a => 1, 2, 3, 4;
# You can use Perl's built-in array quoting syntax:
Readonly::Array @a => qw/1 2 3 4/;
# You can initialize a read-only array from a variable one:
Readonly::Array @a => @computed_values;
# A read-only array can be empty, too:
Readonly::Array @a => ();
Readonly::Array @a; # equivalent
# HASHES
# Typical usage:
Readonly::Hash %a => (key1 => 'value1', key2 => 'value2');
# A read-only hash can be initialized from a variable one:
Readonly::Hash %a => %computed_values;
# A read-only hash can be empty:
Readonly::Hash %a => ();
Readonly::Hash %a; # equivalent
# If you pass an odd number of values, the program will die:
Readonly::Hash %a => (key1 => 'value1', "value2");
--> dies with "May not store an odd number of values in a hash"
EXPORTS
By default, this module exports the following symbol into the calling
program's namespace:
Readonly
The following symbols are available for import into your program, if
you like:
Scalar Scalar1
Array Array1
Hash Hash1
REQUIREMENTS
Perl 5.000
Carp.pm (included with Perl)
Exporter.pm (included with Perl)
Readonly::XS is recommended but not required.
ACKNOWLEDGEMENTS
Thanks to Slaven Rezic for the idea of one common function (Readonly)
for all three types of variables (13 April 2002).
Thanks to Ernest Lergon for the idea (and initial code) for deeply-
Readonly data structures (21 May 2002).
Thanks to Damian Conway for the idea (and code) for making the Readonly
function work a lot smoother under perl 5.8+.
AUTHOR / COPYRIGHT
Eric J. Roode, roode@cpan.org
Copyright (c) 2001-2004 by Eric J. Roode. All Rights Reserved. This
module is free software; you can redistribute it and/or modify it under
the same terms as Perl itself.
If you have suggestions for improvement, please drop me a line. If you
make improvements to this software, I ask that you please send me a
copy of your changes. Thanks.
Readonly.pm is made from 100% recycled electrons. No animals were
harmed during the development and testing of this module. Not sold in
stores! Readonly::XS sold separately. Void where prohibited.
perl v5.14.2 2004-04-20 Readonly(3)
