Path: utzoo!dciem!nrcaer!sce!cognos!alanm
From: alanm@cognos.uucp (Alan Myrvold)
Newsgroups: comp.lang.eiffel
Subject: Re: generic parameters
Message-ID: <4913@cognos.UUCP>
Date: 21 Dec 88 20:19:42 GMT
Article-I.D.: cognos.4913
References: <1826@vlot.cs.vu.nl>
Reply-To: alanm@cognos.UUCP (Alan Myrvold)
Organization: Cognos Inc., Ottawa, Canada
Lines: 49

In article <1826@vlot.cs.vu.nl> jos@cs.vu.nl () writes:
>I have some questions about the usage of generic parameters.
>
>WINDOW inherits class TREE, but it does not specify the generic
>parameter of TREE.
>
>Questions:
>
>    1. Is this correct Eiffel? (I can't find anything in the book about it)

No, the syntax error generated by the compiler is something like:

"window", 3: Wrong number of generic parameters for parent class: tree

>     If we have an object w of type WINDOW, then w.child should be
>     the same as w.child_value.  This looks suspicious, so I think
>     this is not the right solution.

I'm not an eiffel expert (yet) but ....


In eiffel, specialization can be done via inheritance and by parameterizing
generic classes. You may want to re-read sections 19.3 and 19.4 in Meyer's
book.
19.3 Simulating inheritance with genericity.
19.4 Simulating genericity with inheritance.

In the windows example, we want "a window IS A tree", so if there was
a non-parameterized version of tree we could inherit it and do our work.

Alternatively, we could have, say an object called "screen" (which may
contain a tree of windows) become a client of a parameterized version
of tree. Now "a window IS NOT A tree", but each tree node points to a 
window. 

Inheriting AND parameterizing is overkill, which is why w.child is
the same as w.child_value. It is not incorrect. 

Some times we will want the "IS A" relationship with a tree, and
other times we will simply want to become a client of tree.

So yet another example in eiffel where we are forced to ask ourselves:
"Do we buy or inherit?"

---
Alan Myrvold          3755 Riverside Dr.     uunet!mitel!sce!cognos!alanm
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