Re: Graph
From: mAsterdam <mAsterdam_at_vrijdag.org>
Date: Tue, 15 Jan 2008 14:06:34 +0100
Message-ID: <478cae85$0$85779$e4fe514c_at_news.xs4all.nl>
>
> Oh yes!
>
>
> Yes I agree that it depends on the context. More to the point, I
> think it's important to distinguish between value-types and non-value
> types.
>
>
> Yes. I only raised it because most programmers are exposed to OO
> concepts, and I think the LSP has coloured some people's concept of
> "is a" in a way that's at odds with the mathematician's perspective.
>
>
> Did I address your question?
Date: Tue, 15 Jan 2008 14:06:34 +0100
Message-ID: <478cae85$0$85779$e4fe514c_at_news.xs4all.nl>
David BL wrote:
> mAsterdam wrote:
>> David BL wrote: >>> mAsterdam wrote: >>>> You did not address my question. >>>> I'll rephrase it as a statement: >>>> Having a domain and a codomain is relevant >>>> to something being a function. >>>> Having a domain and a codomain is irrelevant >>>> to wether a function is a kind of relation or not. >>>> You appear to see that differently. Please explain. >>> It appears we may have a different interpretation of what "is-a" >>> means. >> Now this is a kind of can of worm-like beings!
>
> Oh yes!
>
>> I suspect it is more a matter of context: In the context of running >> programs, objects behave, and there is a concern for consistency in >> their behaviour: program correctnes, preferably provable. After the >> object dies, however, there still is data, which may later be used to >> incarnate similar objects, but also to build completely different objects. >> >> For the data sec no such behavioral consistency concern applies. >> >> A similar idea can be found athttp://alistair.cockburn.us/index.php/Constructive_deconstruction_of_..., >> search for 'envelopes'.
>
> Yes I agree that it depends on the context. More to the point, I
> think it's important to distinguish between value-types and non-value
> types.
>
>>> I am assuming that for a function to be a relation, a function >>> is not permitted to introduce additional information (not available on >>> the relation). Instead it is only allow to introduce constraints. >>> You could compare this to Date's statement that it is wrong to say >>> that a coloured rectangle is-a rectangle. This corresponds to >>> thinking of a subtype as being a subset, and BTW is not the view >>> generally held by most OO practitioners that assume is-a means LSP:- >>> http://en.wikipedia.org/wiki/Liskov_substitution_principle >> Which is, if I understood correctly, about behavioral consistency.
>
> Yes. I only raised it because most programmers are exposed to OO
> concepts, and I think the LSP has coloured some people's concept of
> "is a" in a way that's at odds with the mathematician's perspective.
>
>>> In a pure mathematical setting, the "subtype = subset" view seems more >>> appropriate, as for example when we say that a circle is-a ellipse.
>
> Did I address your question?
Yes, thank you.
Your position on this is not altogether clear yet - but that would be to much to ask in one go. I am still curious whether you found a real context where
'a function is a kind of relation'
does not hold, but you surely made it imaginable, albeit outside the database-realm.
-- What you see depends on where you stand.Received on Tue Jan 15 2008 - 14:06:34 CET