ReadOnly — Wrap Any Class in a Readonly Proxy

This gem is meant as a fail-safe to guard certain data against modification, and to enforce immutability to a certain extent.

Of course nothing can be completely immutable in ruby, but by using this gem, and at the very least, the unintentional mutation of important objects can be prevented.

The gem works for any class by asking the user to define which of the methods on the original objects can be safely delegated to the object itself, and which of the methods (the mutable ones) should not be.

When wrapping a class, one must do three things in the following order:

1. include module ReadOnly::Proxy
2. call class method safe_methods and pass an array of symbols representing safe methods of the class that can be delegated
3. include module ReadOnly::DelegateSafeMethods

Gem comes with a built-in example: ReadOnly::Hash. It's an example of how any class (a Hash in this case) can be wrapped by this library to create a immutable version.

Below we demonstrate our attempts to write to this class: note that the first level key assignment raises NoMethodError because []= method was not declared as a safe method, and is not delegated.

Second level key returns a regular hash, which can be modified of course, but in this case the returned object is a deep copy of the original, and while it can be changed, this does not affect the original object.

require 'readonly/hash'
  
@writable_hash = { user: { name: 'Konstantin', phone: '555-222-5555' } }
@readonly_hash = ReadOnly::Hash.new @writable_hash

@readonly_hash[:user][:name] 
#=> Konstantin
@readonly_hash[:user][:name] = 'John'
#=> John 
@readonly_hash[:user][:name] 
#=> Konstantin

# Top level actually raises NoMethodError
@readonly_hash[:user] = [ 1,2,3 ]
#=> NoMethodError: undefined method `[]=' for {:user=>{:name=>"Konstantin", :phone=>"555-222-5555"}}:ReadOnly::Hash

Add this line to your application's Gemfile:

gem 'readonly'

And then execute:

$ bundle

Or install it yourself as:

$ gem install readonly

After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to rubygems.org.

Bug reports and pull requests are welcome on GitHub at https://github.com/kigster/readonly.

The gem is available as open source under the terms of the MIT License.