Xref: utzoo comp.lang.eiffel:613 comp.object:822
Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!uunet!mcsun!sunic!liuida!isy!nicwi
From: nicwi@isy.liu.se ( Niclas Wiberg)
Newsgroups: comp.lang.eiffel,comp.object
Subject: Re: Eiffel cleanup #2: Doing away with void references
Keywords: Type system, export controls, NONE, ANY, HERE
Message-ID: <1990Jan24.130946.24107@isy.liu.se>
Date: 24 Jan 90 13:09:46 GMT
References: <227@eiffel.UUCP>
Organization: Dept of EE, University of Linkoping
Lines: 73

bertrand@eiffel.UUCP (Bertrand Meyer) writes:

>...
>
>    There is virtually no change in semantics since almost no operation
>is exported by NONE, so that
>
>    a.f
>
>will usually be incorrect if a is void.
>
>...
>
>    One of the remarkable aspects of this structure is that the
>problem of ``feature application to void references'' becomes a
>special case of type checking! Those readers who remember an earlier,
>lengthy posting on static type checking in Eiffel will see that
>the present change puts the whole discussion in a more general
>perspective.


I don't remember that earlier posting about type checking, so please
excuse me if I've got something wrong. But to me it seems that all
classes must fullfill *all* the contracts of *all* of its ancestors.
It shouldn't matter whether you export the inherited contracts or not.

For example, consider these two classes:

	class PARENT export
		some_feature
	feature
		some_feature is ... end
	end -- class PARENT


	class CHILD export

	inherit
		PARENT
	feature

	end -- class CHILD


Assume these declarations:

	p: PARENT; c: CHILD

If you create a CHILD object

	c.Create

you can of course not apply the feature some_feature to c

	c.some_feature -- This is incorrect

but if you make the assignment

	p := c

then this should be perfectly legal:

	p.some_feature


Of course you can't require the class NONE to fullfill all the
contracts of its ancestors, but then what's the point in the extension?
You still have a special case, which must be treated differently.

-- 
----------------------------------------------------------------------
Niclas Wiberg                         nicwi@isy.liu.se
Dept. of EE    Linkoping University   Sweden