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Date: Thu, 31 Jul 2003 11:49:43 -0400
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Subject: Re: Non-philosophical definition of Eiffel?
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Date: 2003-07-31T11:49:43-04:00
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Dmitry A. Kazakov wrote:
> No it isn't. You forgot semantics. LSP is all about semantics.

Maybe it is to you. To me, LSP is all about programming.
I don't worry my head about pretty little semantic notions.

> This problem appears each time a derived type is
> a specialization of the base.

In OO, derived types are supposed to be able to do
whatever their base can do, and optionally more. If
you want a derived type to do less, don't make it a
(public) child of the Base. The formal expression of
this in many OO languages is that the parameter types
of overriding functions are the same as their base
function parameter types.

Now, it is true that many people use a technique of
having a wide base interface, and then implementing
derived methods which do nothing but throw exceptions.
Contravariant parameter types could be looked at in
the same way - as if the overriding function took the
wider type, but immediately did a type test and threw
an exception if the result was not of the narrower
type. I don't happen to like this way of doing things,
because I favor static polymorphism, but for some
people, this floats their boat.