Re: Object-relational impedence
From: Yagotta B. Kidding <ybk_at_mymail.com>
Date: Thu, 6 Mar 2008 21:30:00 +0100 (CET)
Message-ID: <Xns9A599DC474773vdghher_at_194.177.96.26>
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> No. Projection is covered by *type substitutability* .
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>
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> No, merely that the resulting entity is now deemed to be of
> another type, substitutable with the original type.
>
> type T
> {
> x, y, z
> }
>
> Set<T> ts ;
> Set< type { x, y } > ps = { e IN ts : e.x > 123 } ;
>
> The elements of ps are effectively projections of the elements in ts.
>
Date: Thu, 6 Mar 2008 21:30:00 +0100 (CET)
Message-ID: <Xns9A599DC474773vdghher_at_194.177.96.26>
S Perryman <q_at_q.com> wrote in news:fqpj0o$lke$1_at_aioe.org:
>> On Mar 6, 9:39 am, S Perryman <q..._at_q.com> wrote:
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>>>I am interested in exploring the notion of an 'ADT join' and >>>how it might be implemented by an OO prog lang.
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>> Join is indeed the most interesting operation.
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>> In the other message >> you dismissed projection as being covered by the concept of >> subclassing.
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> No. Projection is covered by *type substitutability* .
>
>
>> Can you please be more specific? If we remove some [data] >> attributes, does it mean the resulting "entity" is a subclass.
>
> No, merely that the resulting entity is now deemed to be of
> another type, substitutable with the original type.
>
> type T
> {
> x, y, z
> }
>
> Set<T> ts ;
> Set< type { x, y } > ps = { e IN ts : e.x > 123 } ;
>
> The elements of ps are effectively projections of the elements in ts.
>
Hold on, you have a value of type Set<T> assigned to a variable of type Set<type{x,y}>. How is it even syntactically correct ? Is not assignment of a value of one type to a variable of another type illegal ??? It's, like, totally basic stuff they teach you about typing and such ! Methinks you've made an inadvertent mistake, didn't you ? Received on Thu Mar 06 2008 - 21:30:00 CET