From: Jim Weirich <jim@...>
Date: 2006-02-17T05:36:02+09:00
Subject: Re: OT: Is this worth a try?

Logan Capaldo wrote:
> 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

Actually, in Eiffel, this wouldn't be a compile error.  Preconditions in 
a derived class is logically ORed with the those in the parent class. 
So that the full precondition for C#meth will be

    precondition: (n > 2) || (n > 1)

Since the preconditions are ORed, they can only be widened in derivied 
classes.  If you were using full Eiffel syntax, you would state derived 
class preconditions with "require else" rather than the plain "require" 
to help you remember that you can only widen the preconditions.

Likewise postconditions in derived classes are ANDed with those in 
parent classes, so they can only be narrowed (and "ensure and" is used 
rather than the vanilla "ensure" [1]).

--
-- Jim Weirich

[1] Ensure in the Eiffel sense (defining preconditions) rather than 
ensure in the Ruby sense (finally blocks).

-- 
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