respond_to.rb

class RespondTo
  def initialize(method_name)
    @method_name = method_name
  end

  def ===(object)
    object.respond_to?(@method_name)
  end
end

def RespondTo(method_name)
  ::RespondTo.new(method_name)
end

[
  1,
  "string",
  %w{an array},
  {a: 'hash'}
].each do |object|
  case object
  when String, Integer
    puts object
  when RespondTo(:join)
    puts object.join(' ')
  else
    puts object.inspect
  end
end

# Prints:
#   1
#   string
#   an array
#   {:a=>"hash"}