Re: Non-philosophical definition of Eiffel?

[Dmitry A. Kazakov <mailbox@dmitry-kazakov.de>]

On Thu, 31 Jul 2003 11:49:43 -0400, Hyman Rosen <hyrosen@mail.com>
wrote:

>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.

Barbara's (+ rarely mentioned Wing) paper was called "A Behavioral
Notion of Subtyping". Now tell me that behavior is not about
semantics.

>> 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.

This is a vague narration of LSP-subtyping, not OO. There could be
other notions as well. For instance, both C++, and Ada implement
subtypings of other kinds. This was probably the reason why OO-people
invented the term "subclassing", to stress a nasty truth that almost
everything we are doing is not LSP conform. (:-))

>If you want a derived type to do less, don't make it a
>(public) child of the Base.

"to do" less vs. more vs. something other is again semantics. And
inspect your sentence carefully, you will find that any polymorphic
behaviour contradicts LSP. In fact, an ability for a callee to detect
the actual type is enough to break substitutability.

> 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.

There are many ways. 

>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.

You contradict yourself. If (according to LSP) a derived type is
always substitutable for the base, why should it throw an exception in
a contravariant parameter? It is not so, either in C++ where all
arguments instead of the hidden one are contravariant, or in Ada
(class-wide arguments).

---
Regards,
Dmitry Kazakov
www.dmitry-kazakov.de