Valued

Valued is a Ruby gem that allows you to easily define value objects.

In the past, I mainly used Struct for objects of this kind. However, structs in Ruby have some drawbacks, like the fact that they implement hash access with [], which is undesirable if you want to use duck typing. Also, they require all their attributes to always be passed to their constructor, which makes it cumbersome to construct them from a JSON payload that may miss some optional attribute.

You can of course also just implement those objects yourself, but that requires a lot of boilerplate, like implementing ==, inspect, to_h and other methods you almost always need.

With Valued, you can automatically define a Struct-like class that behaves properly.

Add this line to your application's Gemfile:

gem 'valued'

And then execute:

$ bundle

Or install it yourself as:

$ gem install valued

Just include Valued in your class and then define attributes.

require 'valued'

class Quantity
  include Valued

  attributes :unit, :amount
end

After that, you can use your class similarly to a struct. You construct an instance of it with a hash:

quantity = Quantity.new(unit: 'm', amount: 2)
p quantity.amount
=> 2
p quantity.unit
=> 'm'

If you omit a parameter in the constructor, it is set to nil.

quantity = Quantity.new(amount: 2)
p quantity.unit
=> nil

Classes defined with valued automatically implement ==, eql?, hash, to_h, to_s and inspect.

quantity = Quantity.new(amount: 2, unit: 'm')
second_quantity = Quantity.new(amount: 2, unit: 'm')
quantity == second_quantity
=> true

quantity.to_h
=> {:amount=>2, :unit=>"m"}
p quantity
=> #<Quantity amount=2 unit="m">

You can create a duplicate of your object with updated attributes by using update.

quantity = Quantity.new(unit: 'm', amount: 2)
p quantity
=> #<Quantity amount=2 unit="m">
updated_quantity = quantity.update(unit: 'yard')
p updated_quantity
=> #<Quantity amount=2 unit="yard">

If you really need a mutable object, just use Valued::Mutable instead of Valued.

require 'valued'

class Quantity
  include Valued::Mutable

  attributes :unit, :amount
end

By doing that, you also get a setter for every attribute.

quantity = Quantity.new
quantity.amount = 2
quantity.unit = 'm'
p quantity
=> #<Quantity amount=2 unit="m">

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