Re: By The Dawn's Normal Light

> The trouble comes when a relation can have an attribute whose domain
is a
> relation. Now there's an incestuous relationship between the
relational
> engine and the type engine. The type engine has to know about
relations,
> in order to be a type engine. The relational engine has to know
about
> relations in order to be a relational engine. But do they each know
that
> the other one also knows?

I don't think it's a problem in practice. A database consists of relation-typed variables, and could certainly delegate the handling of that type to the type engine, which would know about others. In fact, even in relational expressions, user-defined type-specific operators can be used, so the relational engine is going to do that anyway.

Hmmm... I'm starting to wonder what is in the relational engine then, if relations as types are handled by the type engine. Constraints are over sets of relvars, so they're outside the confines of the type engine... or are they?

I'm a little tired now to think about this, so perhaps someone can chime in. But certainly relation-valued attributes can have constraints, and so there's a recursive relationship.

Is the relation just a special type? It certainly allows more flexibility than most "type generators" (like List or even Set, which have various accessors but are parameterized primarily by the type of the element they can include).

• erk