Detecting unspecified arguments in Ruby, inspired by http://www.harukizaemon.com/2011/04/detecting-unspecified-method-arguments-in-ruby.html

default_arguments.rb

class Object

  def default!
    @__is_default = true
    self
  end
  
  def default?
    @__is_default
  end
  
end

def my_method(param = nil.default!)
  puts "Parameter = #{ param.inspect }, is default? #{ !!(param.default?) }"
end

# With a real argument:
my_method 123 # => Parameter = 123, is default? false

# With a real nil argument:
my_method nil # => Parameter = nil, is default? false

# With no argument:
my_method # => Parameter = nil, is default? true

# PROBLEM WITH THIS IMPLEMENTATION:

# Call with a real nil argument again:
my_method nil # => Parameter = nil, is_default? true

nil is a singleton, so once you sent :default! to it, it will always return true to :default? message.

Try to change order of calls and you'll see.

Yeah, you're totally right... I had forgotten about that until one of my coworkers pointed it out to me. In that case, I'd probably keep the api (which I like), and instead have default! return a proxy around an object, that defines default?.

Of course, using a proxy runs into problems with passing that proxied object into other methods that check for default-ness.