Re: Why should the value for a discriminant be static?

[Jacob Sparre Andersen <sparre@nbi.dk>]

Randy Brukardt <randy@rrsoftware.com> writes:
> Jacob Sparre Andersen <sparre@nbi.dk> wrote in message
> news:rlszmyd956d.fsf@jacob.crs4.it...

> > Occasionally I run into the limitation that the value for a
> > discriminant be static.  I can design my way around the limitation
> > (when I remember it), but I would like to know the reason.  (also
> > because it might make it easier for me to remember the rule)
> 
> The only place I know of off-hand where there is such a limitation
> is with aggregates.

Yes.  That was also what I ran into today.

> There, the limitation is necessary so that compiler can know the set
> of components for the aggregate.

That makes sense.

> Otherwise, it would be impossible to check that components are
> given, which is *the* major benefit of using an aggregate over a
> series of component assignments.

Yes.

> I suppose such a check could be done at runtime, but that would be
> substantially less safe, as errors could exist in a program for a
> long time and occur only in unusual conditions after deployment.

True.  And although I think I also had the record components correct
the first time around (except for the non-static discriminant), the
changed code is probably safer.

> Anyway, does that answer your question, or was this in some other
> context?

It answers my question.  And I think it is enough to make me remember
this rule in the future.  Thanks.

Jacob
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