Re: On Formal IS-A definition

From: David BL <>
Date: Thu, 6 May 2010 16:13:52 -0700 (PDT)
Message-ID: <>

On May 6, 9:10 pm, Bob Badour <> 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. Received on Thu May 06 2010 - 18:13:52 CDT

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