Re: Object-relational impedence

Yagotta B. Kidding wrote:

> S Perryman <q_at_q.com> wrote in news:fr0hft$8gf$1_at_aioe.org:

YK>Hold on, you have a value of type Set<T> assigned to a variable of YK>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)

> I think you are confused at least on two counts.

> In any OO langauage I am familiar wth, you can assign a subtype value
> to a supertype variable.

> However, you did not establish any
> subtype/supertype relationship in your example, therefore, the
> assignment is illegal.

1. You are correct, if you use the *Simula* model of type substitutability (ie Y is substitutable for X if there is an inheritance lineage from Y to
2. .
3. There are OO prog langs, strongly and weakly typed, that are not forced to use the Simula model.

> Even when you establish such relationship, no projection really takes
> place, the information is still there, it's just hidden, as can
> easily be seen by casting back to the original subtype.

Is this not an implementation aspect, as opposed to a conceptual one ?? There could be prog langs that forbid coercion back the other way etc (even if the underlying impl was using a "hidden" approach) .

Regards,
Steven Perryman Received on Sun Mar 09 2008 - 22:20:22 CET