Re: Why can't you create a out of order subtype?

[Robert A Duff <bobduff@shell01.TheWorld.com>]

Martin Dowie <martin.dowie@btopenworld.com> writes:

> Marius Amado Alves wrote:
> > For run-time I find the "Value of Image" idiom a better one. It's
> > readily available. No Unchecked_Conversion required.
> > declare
> >    M : Msg := Whatever;
> >    V : Vowelmsg;
> > begin
> >    V := Vowelmsg'Value (Msg'Image (M));
> >    -- if control reaches here then M is a vowel
> > exception
> >    when Constraint_Error => -- M is not a vowel
> > end;

If there are no further uses of V, then I think 11.6 allows the compiler
to eliminate the raising of Constraint_Error.  I don't much like
11.6...

> That's very, very, very slow...

True (if it works at all).  But you could use the "Value of Image" trick
to fill in a table that maps from one type to the other.  That would
work, and uses of the table would be pretty fast.

- Bob