Re: thoughts on "holey" enumerated types

[agate!howland.reston.ans.net!noc.near.net!inmet!bobduff@ucbvax.Berkeley.E DU (Bob Duff)]

> The thread on converting integers to a sparse enumerated type has got
> me thinking.
> 
> Couldn't enumerated types always be implemented as a contiguous range, such
> that use clauses to specify member ``positions'' just affected the 'POS and
> 'VAL functions?

You mean "representation clauses" not "use clauses".  The 'Pos and 'Val
attributes are not affected by an enumeration_representation_clause.
The clause affects the internal representations, not the position
numbers.  The only effect is on the internal representation, which can
only be noticed by a program that does Unchecked_Conversion, or
interface to other languages, etc.

> That way enumerated type operations (especially loop traversal over a range,
> or array indexing) can always be efficient. The only time one needs to know a
> member's position is when converting to an integer. Using 'POS and 'VAL seems
> safer than unchecked_conversion in these cases anyway.

But 'Pos and 'Val do something different than Unchecked_Conversion.

> Ray
> blaak@csri.toronto.edu

- Bob Duff
  Ada 9X Mapping/Revision Team