Ruby switch/case that complains if you forget to cover a case. Inspired by types in Elm. Just a for-fun experiment; not intended for real use.

ruby_exhaustive_switch_case.rb

# By Henrik Nyh <https://henrik.nyh.se> under the MIT license.

class Enum
  class Switch
    def initialize(value_to_match_against, *values)
      @checked_values = values.map { |v| [ v, false ] }.to_h

      values.each do |value|
        define_singleton_method(value) { |&block|
          @checked_values[value] = true
          @block_to_call = block if value == value_to_match_against
        }
      end
    end

    def verify_we_covered_all_cases_and_call_block_for_matching_value
      unchecked_values = @checked_values.reject { |_k, v| v }.keys

      if unchecked_values.any?
        raise "Did not cover these cases: #{unchecked_values.inspect}"
      end

      @block_to_call.call if @block_to_call
    end
  end

  def initialize(*values)
    @values = values
  end

  def switch(value)
    s = Switch.new(value, *@values)
    yield s
    s.verify_we_covered_all_cases_and_call_block_for_matching_value
  end
end

def Enum(*values)
  Enum.new(*values)
end


states = Enum(:hungry, :thirsty)

# This works:

states.switch(:thirsty) do |s|
  s.hungry { puts "hungry!" }
  s.thirsty { puts "thirsty!" }
end

# This raises, catching the fact that you didn't define the "hungry" case:

states.switch(:thirsty) do |s|
  #s.hungry { puts "hungry!" }
  s.thirsty { puts "thirsty!" }
end

👏