Re: On Formal IS-A definition

From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Thu, 06 May 2010 22:20:24 -0300
Message-ID: <4be36ad9$0$12458$9a566e8b_at_news.aliant.net>



David BL wrote:

> On May 6, 9:10 pm, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:
>
>

>>If one is interested specifically in subtypes of supertypes, a proper
>>subset of a type with a proper superset of operations is a proper
>>subtype of that type. Thus, circle values are a subtype of ellipse
>>values and ellipse variables are a subtype of circle variables.

>
>
> There is no subtype relationship between ellipse variables and circle
> variables (in either direction).
>
> Consider a procedure in an imperative language that is passed a
> reference to a circle variable. Most generally the variable can be
> used as an "in-out" parameter, meaning that the variable is both read
> and written by the procedure. An ellipse variable can only be
> substituted for out-parameters.

Ellipse variables are a proper subset of the variables where one might store a circle. It has a proper superset of the operations permitted for circle variables allowing one to also store a non-circular ellipse values.

Saying that one cannot apply circle value operations to ellipse variables demonstrates nothing more than a confusion between values and variables. One can apply all circle variable operations to ellipse variables. Received on Thu May 06 2010 - 20:20:24 CDT

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