class Foo
  class << self
    def make_this_method name, &block
      define_method(name, &block)
    end
  end
end

Foo.make_this_method(:bar) do |*args|
  case args.size
  when 0 ; args = ['fizz']
  when 1 ; # nothing
  else ; raise ArgumentError.new("no") if args.size > 1
  end
  arg1, = *args
  puts "bar! #{arg1}"
end

Foo.new.bar('baz') # outputs "bar! baz"
Foo.new.bar # outputs "bar! fizz"