# NAME

Readonly - Facility for creating read-only scalars, arrays, hashes

# 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"](#cons) section below for more details.

# Comparison with "use constant"

Perl provides a facility for creating constant values, via the [constant](https://metacpan.org/pod/constant)
pragma.  There are several problems with this pragma.

- The constants created have no leading sigils.
- 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 `eg/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 Readonly

# Examples

These are a few very simple examples:

## 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/Lists

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");
    # This dies with "May not store an odd number of values in a hash"

# Exports

Historically, this module exports the `Readonly` symbol into the calling
program's namespace by default. The following symbols are also available for
import into your program, if you like: `Scalar`, `Scalar1`, `Array`,
`Array1`, `Hash`, and `Hash1`.

# Requirements

[Readonly::XS](https://metacpan.org/pod/Readonly::XS) is recommended but not required. There are no non-core
requirements.

# Bug Reports

If email is better for you, [my address is mentioned below](#author) but I
would rather have bugs sent through the issue tracker found at
http://github.com/sanko/readonly/issues.

Please check the TODO file included with this distribution in case your bug
is already known (...I probably won't file bug reports to myself).

# Acknowladgements

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

Sanko Robinson <sanko@cpan.org> - http://sankorobinson.com/

CPAN ID: SANKO

Original author: Eric J. Roode, roode@cpan.org

# License and Legal

Copyright (C) 2013 by Sanko Robinson <sanko@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.
