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Readonly - Facility for creating read-only scalars, arrays, hashes.
This documentation describes version 1.03 of Readonly.pm, April 20, 2004.
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, ...);
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.
Perl provides a facility for creating constant values, via the "use constant" pragma. There are several problems with this pragma.
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.
Can be overridden:
use constant PI => 3.14159; ... use constant PI => 2.71828;
(this does generate a warning, however, if you have warnings enabled).
use constant.
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
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;
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';
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.
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.
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.
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.
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 Readonly
# 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"
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
Perl 5.000 Carp.pm (included with Perl) Exporter.pm (included with Perl) Readonly::XS is recommended but not required.
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+.
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.