From: "Weirich, James" <James.Weirich@...>
Date: 2003-11-22T04:06:01+09:00
Subject: Re: "stereotyping" (was: Re: Strong Typing (Re: Managing metadata	 about attribute types) )

> This is a wonderful idea.  Let me restate it to make sure I 
> understand it.  Just as you can do the following:
> 
> class Person
>   def greet(other_person)
>     other_person.to do 
>       say "hello"
>     end
>   end
> end
> 
> class Person
>   def some_other_method
>   end
> end
> 
> 
> ...(I've defined class Person somewhere, but I can always open up the 
> class again and define something else in it):  it would be 
> wonderful if you can *either* inline it *or* make it external in 
> a similar fashion.  [...]
> I think the ability to do this would make the whole idea a lot more 
> palatable to a lot of us.

I've been thinking about DbC stuff for some time and something in Matz Ruby
2 talk last Saturday sparked an idea along these lines ...

   class Person
     def greet(other_person)
       marry(other_person)
     end
   end

Then specify the contract in a separate file ...

   class Person
     def greet:pre(other_person)
       assert ! self.married?
       assert ! other_person.married?
     end

     def greet:post(other_person)
       assert other_person.married?
       assert self.married?
       assert_equal self, other_person.spouse
       assert_equal other_person, self.spouse
     end
   end

An aside: Unfortunately, the post method doesn't have access to the result
returned from the function.  In this case it doesn't matter, but many post
conditions assert something interesting about the result.  I guess I would
have to use a wrap method for that.

-- 
-- Jim Weirich / Compuware
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