Re: syntaxic exploration

[Robert Eachus <rieachus@comcast.net>]

On Saturday, December 23, 2017 at 11:23:32 AM UTC-5, Jeffrey R. Carter wrote:
> On 12/22/2017 11:11 PM, Niklas Holsti wrote:
> > 
> > I think the aggregate form (null array) was suggested some time ago, in analogy 
> > with the existing (null record). It looks good to me, but as I remember, there 
> > was some objection. One problem is that if the index type has exactly one value, 
> > then a null array with that index type cannot exist (because then A'First = 
> > A'Last for any such array A).
> 
> Actually, a null array of such a type can exist if it's a string type, because 
> you can use the string literal "" for such types. AIUI, 'Last is undefined for 
> such a value. Allowing (null array) for non-string array types with an index 
> type with a single value would presumably work the same.

The rule that allows 1..0 as a null string range only has a problem if you have an array with an enumeration index type containing a single component.  I think that is one of those cases where we say, "Don't do that!" and move on.

A more complex, and potentially ugly case is for multidimensional arrays.  Defining (null array) as being empty in all dimensions with Foo'Range(n) = Bar'First..Bar'Pred(Bar'First) where Bar is the nth index SUBtype for Foo works.  If the last materialized is a (real) Numeric_Error AKA Constraint_Error?  Again, only an issue for smart alecks. In general, you should be surprised if Foo'Last(N) for a null array doesn't raise Constraint_Error.