variables

Variable objects for class and instance variables

Ruby already has Method objects, why not Variable objects as well?

Some methods already exist for interacting with class and instance variables:

• Module#class_variable_defined?
• Module#class_variable_get
• Module#class_variable_set
• Object#instance_variable_defined?
• Object#instance_variable_get
• Object#instance_variable_set

But notice that these all share a common prefix - instance_variable_ or class_variable_.

This feels a little smelly, let's try to DRY it up with some Variable objects!

gem install variables

Ruby 1.8.7+

Let's experiment with a simple User class.

class User
  def initialize(name)
    @name = name
  end
end

Objects can have any number of instance variables.

user = User.new('Bob')              #=> #<User:0x007f8f6a84aa98>
user.instance_variable_get('@name') #=> "Bob"

Similar to Object#method, the instance_variable method returns a Variable object.

name = user.instance_variable(:name) #=> #<InstanceVariable: #<User>@name>

But unlike Object#method, this method does not require a variable to actually be defined.

undefined = user.instance_variable(:undefined) #=> #<InstanceVariable: #<User>@undefined>

We can check if a variable is defined by using the defined? method.

name.defined?      #=> true
undefined.defined? #=> false

Once we have a Variable object, we can get its value.

name.get      #=> "Bob"
undefined.get #=> nil

Similar to Hash#fetch, the fetch method raises an exception if the variable is undefined.

name.fetch      #=> "Bob"
undefined.fetch #=> Variables::UndefinedVariable - undefined variable "undefined"

The fetch method optionally accepts a default value to return if the variable is undefined.

name.fetch(:default)      #=> "Bob"
undefined.fetch(:default) #=> :default

Default values can also be defined with a block which is yielded with the Variable name.

name.fetch { |name| "#{name}-default" }      #=> "Bob"
undefined.fetch { |name| "#{name}-default" } #=> "@undefined-default"

The Object#instance_variable_fetch method allows us to fetch a variable's value by name.

name.fetch                          #=> "Bob"
user.instance_variable_fetch(:name) #=> "Bob"

We can update a Variable value by using the set method.

name.set('Steve')                   #=> "Steve"
user.instance_variable_get('@name') #=> "Steve"

The replace method is similar to set, but it returns the old value instead of the new value.

name.replace('Bob')                 #=> "Steve"
user.instance_variable_get('@name') #=> "Bob"

We can even temporarily replace a value for the duration of a block.

user.instance_variable_get('@name') #=> "Bob"

name.replace('Steve') do
  user.instance_variable_get('@name') #=> "Steve"
end

user.instance_variable_get('@name') #=> "Bob"

Note that when using the block form of replace, the last expression of the block is returned.

name.replace('Steve') { 1 + 1 } #=> 2

The Object#instance_variable_replace method allows us to replace a variable's value by name.

user.instance_variable_get('@name') #=> "Bob"

user.instance_variable_replace(:name, 'Steve') do
  user.instance_variable_get('@name') #=> "Steve"
end

user.instance_variable_get('@name') #=> "Bob"

The instance_variable_replace method also accepts a hash of variables to replace.

user.instance_variable_get('@name') #=> "Bob"
user.instance_variable_get('@test') #=> nil

user.instance_variable_replace(name: 'Steve', test: 'example') do
  user.instance_variable_get('@name') #=> "Steve"
  user.instance_variable_get('@test') #=> "example"
end

user.instance_variable_get('@name') #=> "Bob"
user.instance_variable_get('@test') #=> nil

Everything that we can do with instance variables can be done with class variables as well!

example = User.class_variable(:example) #=> #<ClassVariable: User@@name>

example.defined? #=> false

example.set('testing') #=> "testing"

User.class_variable_get('@@example') #=> "testing"

YARD Documentation

• Module#class_variable
• Module#class_variable_fetch
• Module#class_variable_replace
• Object#instance_variable
• Object#instance_variable_fetch
• Object#instance_variable_replace
• Variable#defined?
• Variable#fetch
• Variable#get
• Variable#name
• Variable#owner
• Variable#replace
• Variable#set

bundle exec rspec

• Fork the project.
• Make your feature addition or bug fix.
• Add tests for it. This is important so I don't break it in a future version unintentionally.
• Commit, do not mess with Rakefile, version, or history. (if you want to have your own version, that is fine but bump version in a commit by itself I can ignore when I pull)
• Send me a pull request. Bonus points for topic branches.

MIT - Copyright © 2015 Sean Huber