ClosedStruct

README.md

ClosedStruct

A stupidly simple implementation of a kind of "sorta immutable" struct that can be instantiated as conveniently as an OpenStruct but after instantiation it doesn't let me (easily) add fields or mutate them:

guy = ClosedStruct.new( name: "John", age: 23 )

# readers:
guy.name             #=> "John"
guy.age              #=> 23
guy.xxx              #=> NoMethodError: undefined method `xxx'...

# respond_to? and methods work as expected:
guy.respond_to? :age #=> true
guy.methods(false)   #=> [:name, :age]

# in-place modification is prevented, but you can create modified copies with #merge:
guy.name = "Jack"      #=> NoMethodError: undefined method `name='...
guy.merge name: "Jack" #=> a new ClosedStruct instance with name: "Jack", age: 23

closed_struct.rb

class ClosedStruct
  def initialize( attr_hash )
    @_field_names = attr_hash.keys.map(&:to_sym)
    attr_hash.each do |attr_name, attr_value|
      instance_variable_set :"@#{attr_name}", attr_value
      eigenclass.send :attr_reader, attr_name.to_sym
    end
  end

  def to_h
    field_values = @_field_names.map { |field| send field }
    Hash[ @_field_names.zip( field_values ) ]
  end

  def merge( modified_attrs = {} )
    ClosedStruct.new to_h.merge( modified_attrs )
  end

  private

  def eigenclass
    class << self
      self
    end
  end
end