From: Daniel Schierbeck <daniel.schierbeck@...>
Date: 2005-12-19T22:07:47+09:00
Subject: Re: How to know if a method is read-write or read-only

Trans wrote:
> I've now come across a second usecase for knowing if a method is
> read-write or read-only on it's object's state. Both cases boil-down to
> having meta-method behave differently depending on the behavior of
> method being delegated to, but the circumstances differ completely. To
> give an example, the current case generates method wraps for any given
> class, applying thread sycronization. If a method is read-only it's
> wraped in a Sync::SH, if read-write then Sync::Ex.
> 
> So I wonder, is there a way to determine this fact about a method?
> 
> Thanks,
> T.
> 
Do you want to know if a method changes its object's state? In that 
case, you could duplicate the object, freeze the duplicate and try to 
call the method. If an Exception is raised, then either the method has a 
bug or it is state-changing.

   class Object
     def changes_state?(method)
       self.dup.freeze.send(method)
       return false
     rescue TypeError
       return true
     end
   end

   "foo".changes_state? :upcase   => false
   "foo".changes_state? :upcase!  => true

It would be nice if a special exception was thrown when attempting to 
call a method on a frozen object, so that you could distinguish it from 
the usual TypeErrors.

Note that this method isn't very efficient, since it duplicates the 
entire object on each invocation. If you were to check many methods on a 
single object, you might want to keep the frozen duplicate in an 
instance variable.


Cheers,
Daniel