variant.rb

class Variant
  class Caser
    attr_reader :value
    def initialize(variant)
      @variant = variant
      @invoked = false
    end

    def try_case(tag, &b)
      if @variant.tag == tag
        @invoked = true
        @value = yield(*@variant.values)
      end
    end

    def invoked?
      @invoked
    end
  end

  def self.spec
    @spec ||= {}
  end

  def self.caser
    @caser ||= Class.new(Caser)
  end

  attr_reader :values
  def initialize(*values)
    @values = values
  end

  def self.variant(tag, *names)
    parent_class = self
    klass = spec[tag] = Class.new(self)
    klass.class_eval do
      names.each_with_index do |name, i|
        define_method(name) { @values[i] }
      end

      define_method(:type) { parent_class }
      define_method(:tag) { tag }
    end

    caser.class_eval do
      define_method(tag) { |&b| try_case(tag, &b) }
    end

    (class << self; self; end).class_eval do
      define_method(tag) { |*values| klass.new(*values) }
    end
  end

  def inspect
    "#<#{type}.#{tag}(#{values.map(&:inspect).join(', ')})>"
  end

  def cases(cases={}, &b)
    if block_given?
      caser = self.class.caser.new(self)
      yield caser
      if caser.invoked?
        caser.value
      else
        non_exhaustive_cases!
      end
    else
      selection = cases[tag] || cases[:else] || non_exhaustive_cases!
      selection.call(*values)
    end
  end
end

class Order < Variant
  variant :delivery, :address
  variant :digital, :email
  variant :pickup, :store_id
end

# n constructors
# Order.delivery('123 Main St')
# Order.digital('customer@example.com')
# Order.pickup(456)
# 
# order.cases(
#   delivery: ->(address) { "delivering to #{address}" },
#   digital: ->(email) { "emailing to #{email}" },
#   pickup: ->(store_id) {
#     "picking up from #{Store.find(store_id).address}"
#   },
# )
# 
# order.cases do |c|
#   c.delivery { |address| "delivering to #{address}" }
#   c.digital { |email| "emailing to #{email}" }
#   c.pickup do |store_id|
#     "picking up from #{Store.find(store_id).address}"
#   end
# end