From: Logan Capaldo <logancapaldo@...>
Date: 2006-02-17T04:29:55+09:00
Subject: Re: OT: Is this worth a try?


On Feb 16, 2006, at 12:35 PM, Edgardo Hames wrote:

> On 2/16/06, Jens Auer <jens.auer-news@betaversion.net> wrote:
>>
>> Design by contract is a little bit more than placing asserts at the
>> beginning and end of a method. I would be realy glad if someone  
>> points
>> me to some library wich implements DbC completely in Ruby, eg the  
>> fact
>> that a method can only widen the supermethod's contracts which is
>> automatically checked.
>
> I assume that "automatically checked" means checking the contracts at
> compile time.
> That's an undecidible problem.
>

No he means that a override of a method implemented in a subclass can  
only widen the contract, and that that is automatically checked. Not  
that the contract is fulfilled at compile time.

e.g.

class A
   def meth(n)
    precondition: n > 1
   end
end

class B < A
    def meth(n)
       precondition: n > 0 #Ok, because any inputs that were valid  
for A#meth are valid for B#meth, this is checked at compile time
    end
end

class C < A
   def meth(n)
    precondition: n > 2 #Will not compile, C#meth accepts a narrower  
range of inputs than A, does not follow Liskov Substitution Principle
   end
end

Note this is not  the same thing as checking whether a call will  
fulfill conditions at compile time, just that the set of values  
accepted by a subclasses overridden version of  a  method is  a  
superset of the set of values accepted by the parent's version of the  
method.


> Cheers,
> Ed
> --
> Encontr� a "Tu psic�pata favorito" http://tuxmaniac.blogspot.com
> 	
> Thou shalt study thy libraries and strive not to reinvent them  
> without cause,
> that thy code may be short and readable and thy days pleasant and  
> productive.
> -- Seventh commandment for C programmers
>