From: Daniel Doel <daniel.doel@...>
Date: 2004-01-24T08:39:22+09:00
Subject: Re: detecting default parameters

Ian Macdonald wrote:
> Hello,
> 
> Does anyone know of a way to detect whether default parameters are 
> beingused in a method? I'm looking for something like the pseudo-
> method'default?' in the following example:
> 
> def my_method(foo='bar')
>  if foo.default?
>    puts "warning: default value #{foo} being used for 'foo'"
>  end
> end
> 
> I suspect Ruby no longer holds any state at this point to reflect the
> fact that the caller did not supply a parameter, but I'm hoping 
> there'ssome way to do it.

Try this:

#!/usr/bin/ruby

class Object
	def default?
		false
	end
end

def default(obj)
	defparam = obj.clone

	class << defparam
		def default?
			true
		end
	end

	defparam
end

def foo(bar=(default 'foo'))
	if bar.default?
		puts "default"
	else
		puts "custom"
	end

	puts bar
end

foo 'baz'
foo

This might be as close as you'll get. It doesn't work for everything, though, since you can't clone everything, and I don't want to redefine #default? on things you can't clone (like Fixnums) since that'd make things wrong.

- Dan