switch in Ruby

switch.rb

class Switch < BasicObject
  def initialize(&body)
    fail ArgumentError, 'switch body required' unless body
    
    @cases = []
    @receiver = body.binding.receiver
    instance_eval(&body)
  end

  def given(*spec, &block)
    fail ArgumentError 'block required' unless block
    
    @cases.push([spec, block])
  end

  def default(&block)
    fail ArgumentError 'block required' unless block
    fail ArgumentError 'default already specified' if @default

    @default = block
  end

  def evaluate(value)
    @cases.each do |case_list, handler|
      case_list.each do |case_item|
        if case_item === value
          return @receiver.instance_exec(case_item, value, &handler)
        end
      end
    end
    @default.call(value) if @default
  end
end

def switch(value, &body)
  Switch.new(&body).evaluate(value)
end

result = switch($stdin.gets.chomp) do
  given '0' do
    'zero'
  end
  
  given '1'..'3' do
    'one through three'
  end

  given /^4$/, /^four$/ do |m|
    "four, via #{m}"
  end

  default do
    "unknown"
  end
end

puts result