Re: Object-relational impedence

From: S Perryman <>
Date: Sun, 09 Mar 2008 11:25:42 +0000
Message-ID: <fr0hft$8gf$>

Yagotta B. Kidding wrote:

> S Perryman <> wrote in news:fqpj0o$lke$

TN>In the other message
TN>you dismissed projection as being covered by the concept of

SP>No. Projection is covered by *type substitutability* .

TN>Can you please be more specific? If we remove some [data] TN>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}>.

Basic type substitutability (structural equivalence) . The { x, y } type has properties in common with T (the x/y properties) . Therefore the assignment is legal.

So the assignment is a projection is of a type (x,y,z) to a type (x,y) .

Steven Perryman Received on Sun Mar 09 2008 - 12:25:42 CET

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