From: Gennady <gfb@...>
Date: 2004-01-24T08:56:18+09:00
Subject: Re: detecting default parameters

Daniel Doel wrote:
> 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
> 
> 

Interesting. It seems like you could use SimpleDelegator here:

require 'delegate'
def default(obj)
   defparam = SimpleDelegator.new(obj)
   class << defparam
     def default?
       true
     end
   end
   defparam
end