Proof of concept for an exhaustive enum micro-lib in Ruby

exhaustive_enum.rb

module ExhaustiveEnum
  def self.included(base)
    base.extend(ClassMethods)
  end

  module ClassMethods
    def switch(value, &block)
      matcher = ExhaustiveMatcher.new(self)
      matcher.match(value, &block)
    end

    def cases
      self.constants(false).map { |c| const_get(c) }
    end
  end

  class ExhaustiveMatcher
    def initialize(cls)
      @cls = cls
      @statements = {}
    end

    def match(value, &block)
      raise "No case #{value} defined" unless @cls.cases.include?(value)
      instance_eval(&block)
      missing_cases = @cls.cases - @statements.keys
      raise "Missing cases for #{missing_cases}" unless missing_cases.empty?
      @statements[value].call
    end

    def on(kase, &block)
      raise "No case #{kase} defined" unless @cls.cases.include?(kase)
      @statements[kase] = block
    end
  end
end

## Examples

module Gear
  include ExhaustiveEnum

  PARK = :park
  NEUTRAL = :neutral
  #REVERSE = :reverse
  #DRIVE = :drive
end

def example_one(val)
  Gear.switch(val) do
    on(:park) { puts 'value is park!' }
    on(:neutral) { puts 'value is neutral!' }
  end
end

example_one(:park) # => value is park!
example_one(:neutral) # => value is neutral!
begin
  example_one(:reverse) 
rescue StandardError => e
  puts e.inspect # => #<RuntimeError: No case reverse defined>
end

def not_exhaustive_example(val)
  Gear.switch(val) do
    on(:park) { puts 'value is park!' }
  end
end

begin
  not_exhaustive_example(:park)
rescue StandardError => e
  puts e.inspect # => #<RuntimeError: Missing cases for [:neutral]>
end