ImmutableStruct¶ ↑

Creates struct-like classes (that can build value objects) that do not have setters and also have better constructors than Ruby’s built-in Struct.

This is highly useful for creating presenters, non-database-related models, or other quick and dirty classes in your application. Instead of using a Hash or OpenStruct, you can create a bit more clarity around your types by using ImmutableStruct, which is almost as convienient.

Install¶ ↑

Add to your Gemfile:

gem 'immutable-struct'

Then install:

bundle install

If not using bundler, just use RubyGems:

gem install immutable-struct

To use¶ ↑

Person = ImmutableStruct.new(:name, :age, :job, :active?, [:addresses]) do
  def minor?
    age < 18
  end
end

p = Person.new(name: "Dave",   # name will be 'Dave'
               age: 40,        # age will be 40
                               # job is omitted, so will be nil
               active: true)   # active and active? will be true
                               # addresses is omitted, but since we've selected
                               # Array coercion, it'll be []
p.name      # => "Dave"
p.age       # => 40
p.active?   # => true
p.minor?    # => false
p.addresses # => []

p2 = Person.new(name: "Dave", age: 40, active: true)

p == p2     # => true
p.eql?(p2)  # => true

SimilarPerson = ImmutableStruct.new(:name, :age, :job, :active?, [:addresses])

sp = SimilarPerson.new(name: "Dave", age: 40, active: true)

p == sp     # => false         # Different class leads to inequality

new_person = p.merge(name: "Other Dave", age: 41) # returns a new object with merged attributes
new_person.name    # => "Other Dave"
new_person.age     # => 41
new_person.active? # => true

You can coerce values into struct types by using the from method. This is similar to Ruby’s conversion functions, e.g. Integer(“1”).

dave = Person.from(p)
dave.equal?(p) # => true (object equality)

daveish = Person.from(dave.to_h)
daveish.equal?(dave) # => false
daveish == dave      # => true

You can treat the interior of the block as a normal class definition with the exception of setting constants. Use const_set to scope constants as-expected.

Point = ImmutableStruct.new(:x, :y) do
  const_set(:ZERO, 0)
  ONE_HUNDRED = 100
end
Point::ZERO # => 0
::ONE_HUNDRED # => 100
::ZERO # => NameError: uninitialized constant ZERO