[![Build Status](https://travis-ci.org/sanko/readonly.svg?branch=master)](https://travis-ci.org/sanko/readonly)
# NAME

ReadonlyX - Faster facility for creating read-only scalars, arrays, hashes

# Synopsis

    use strict;
    use warnings;
    use ReadonlyX;

    # Read-only scalar
    my $sca1;
    Readonly::Scalar $sca1    => 3.14;
    Readonly::Scalar my $sca2 => time;
    Readonly::Scalar my $sca3 => 'Welcome';

    # Read-only array
    my @arr1;
    Readonly::Array @arr1 => [1 .. 4];

    # or:
    Readonly::Array my @arr2 => (1, 3, 5, 7, 9);

    # Read-only hash
    my %hash1;
    Readonly::Hash %hash1    => (key => 'value', key2 => 'value');
    Readonly::Hash my %hash2 => (key => 'value', key2 => 'value');

    # or:
    Readonly::Hash my %hash3 => {key => 'value', key2 => 'value'};

    # You can use the read-only variables like any regular variables:
    print $sca1;
    my $something = $sca1 + $arr1[2];
    warn 'Blah!' if $hash1{key2};

    # But if you try to modify a value, your program will die:
    $sca2 = 7;           # "Modification of a read-only value attempted"
    push @arr1, 'seven'; # "Modification of a read-only value attempted"
    $arr1[1] = 'nine';   # "Modification of a read-only value attempted"
    delete $hash1{key};  # Attempt to delete readonly key 'key' from a restricted hash

    # Create mutable clones
    Readonly::Scalar $scalar => {qw[this that]};
    # $scalar->{'eh'} = 'foo'; # Modification of a read-only value attempted
    my $scalar_clone = Readonly::Clone $scalar;
    $scalar_clone->{'eh'} = 'foo';
    # $scalar_clone is now {this => 'that', eh => 'foo'};

# Description

This is a near-drop-in replacement for Readonly, the 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. See the section entitled
["ReadonlyX vs. Readonly"](#readonlyx-vs-readonly) for more.

# Functions

All of these functions are imported into your package by default.

## Readonly::Scalar

    Readonly::Scalar $pi      => 3.14;
    Readonly::Scalar my $aref => [qw[this that]]; # list ref
    Readonly::Scalar my $href => {qw[this that]}; # hash ref

Creates a non-modifiable scalar and assigns a value of to it. Thereafter, its
value may not be changed. Any attempt to modify the value will cause your
program to die.

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.

If $var is already a Readonly variable, the program will die with an error
about reassigning Readonly variables.

## Readonly::Array

    Readonly::Array @arr1    => [1 .. 4];
    Readonly::Array my @arr2 => (1, 3, 5, 7, 9);

Creates a non-modifiable array 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. 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.

If the array is already Readonly, the program will die with an error about
reassigning Readonly variables.

## Readonly::Hash

    Readonly::Hash %h => (key => 'value', key2 => 'value');
    Readonly::Hash %h => {key => 'value', key2 => 'value'};

Creates a non-modifiable hash 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.

If the hash is already Readonly, the program will die with an error about
reassigning Readonly variables.

## Readonly::Clone

    my $scalar_clone = Readonly::Clone $scalar;

When cloning using [Storable](https://metacpan.org/pod/Storable) or [Clone](https://metacpan.org/pod/Clone) you will notice that the value
stays readonly, which is correct. If you want to clone the value without
copying the readonly flag, use this.

    Readonly::Scalar my $scalar => {qw[this that]};
    # $scalar->{'eh'} = 'foo'; # Modification of a read-only value attempted
    my $scalar_clone = Readonly::Clone $scalar;
    $scalar_clone->{'eh'} = 'foo';
    # $scalar_clone is now {this => 'that', eh => 'foo'};

In this example, the new variable (`$scalar_clone`) is a mutable clone of the
original `$scalar`. You can change it like any other variable.

# Examples

Here are a few very simple examples again to get you started:

## 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 => ();
    # or
    Readonly::Array @a;

## Hashes

Typical usage:

    Readonly::Hash %a => (key1 => 'value1', key2 => 'value2');
    # or
    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 => ();
    # or
    Readonly::Hash %a;

If you pass an odd number of values, the program will die:

    Readonly::Hash my %a => (key1 => 'value1', "value2");
    # This dies with "Odd number of elements in hash assignment"

# ReadonlyX vs. Readonly

The original Readonly module was written for nearly twenty years ago when the
built-in capability to lock variables didn't exist in perl's core. The
original author came up with the amazingly brilliant idea to use the new (at
the time) `tie(...)` construct. It worked amazingly well! But it wasn't long
before the speed penalty of tied varibles became embarrassingly obvious. Check
any review of Readonly written before 2013; the main complaint was how slow it
was and the benchmarks proved it.

In an equally brilliant move to work around tie, Readonly::XS was released for
perl 5.8.9 and above. This bypassed `tie(...)` for basic scalars which made a
huge difference.

During all this, two very distinct APIs were also designed and supported by
Readonly. One for (then) modern perl and one written for perl 5.6. To make
this happen, time consuming eval operations were required and the codebase
grew so complex that fixing bugs were nearly impossible. Depending on what
version of perl and what other modules you had installed, Readonly was three
different modules all with different sets of quirks and bugs to fix.

Readonly was a mess. So, the original author abandoned both Readonly and
Readonly::XS. As bugs were found, they went unfixed. The combination of speed
and lack of development spawned several similar modules which usually did a
better job but none were a total drop-in replacement.

Until now.

ReadonlyX is the best of recent versions of Readonly without the old API and
without the speed penalty of `tie(...)`. It's what I'd like to do with
Readonly if resolving bugs in it wouldn't break 16 years of code out there in
Darkpan.

In short, unlike Readonly, ReadonlyX...

- ...does not use slow `tie(...)` magic or eval. There should be almost no speed penalty
- ...does not strive to work on perl versions I can't even find a working
        build of to test against
- ...has a single, clean API
- Does the right thing when it comes to deep vs. shallow structures
- Allows implicit undef values for scalars (Readonly inconsistantly allows
        this for hashes and arrays but not scalars)
- ...a lot more I can't think of right now but will add when they come to me
- is around 100 lines instead of 460ish so maintaining it will be a breeze

# Requirements

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.

ReadonlyX can be found is the branch of Readonly found here:
https://github.com/sanko/readonly/tree/ReadonlyX

# Author

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

CPAN ID: SANKO

# License and Legal

Copyright (C) 2016 by Sanko Robinson <sanko@cpan.org>

This module is free software; you can redistribute it and/or modify it under
the same terms as Perl itself.
