Re: array from static predicate on enumerated type

[Matt Borchers <mattborchers@gmail.com>]

So, I understand why holey arrays are problematic and I'm not asking for holey arrays here.  I was looking for elegant solution around the fact that Ada intentionally does not provide a reasonable answer to 'First, 'Last, etc. when used on enumerated sub-types with a Predicate.  In my cases, I have a lot of small (less than 5 rows and columns) multi-dimensional static data.  It appears instantiating a Map and populating it at program start-up is the recommended solution.  However, it sure seems to me that a static constant array of data is ideal because loading the Map cannot be guaranteed correct by the compiler whereas a table would be.

It feels like an unfinished addition or incomplete feature to actively prevent 'First, 'Last, etc. from returning something reasonable about the sub-type in question.  Return SOMETHING, even if it pertains to the parent type, and let the programmer decide how it can be useful to them.  Using my LETTERS and CURVED_LETTERS example, I still don't see why the following would be problematic:

A <= LETTERS'First
K <= LETTERS'Last
11 <= LETTERS'Length
1 <= LETTERS'Pos(B)
10 <= LETTERS'Pos(J)

B <= CURVED_LETTERS'First
J <= CURVED_LETTERS'Last
5 <= CURVED_LETTERS'Length
0 <= CURVED_LETTERS'Pos(B)
4 <= CURVED_LETTERS'Pos(J)

I didn't address 'Succ and 'Pred, but these also seem reasonable.

Again, I don't know the inner workings of the compiler, but these responses (above) for CURVED_LETTERS seem like reasonable values related to the sub-type that the compiler could handle easily allowing programmers who might want to write:

type ARR(CURVED_LETTERS) of BOOLEAN;

but for strict legality purposes have to suffice with:

type ARR(CURVED_LETTERS'Pos(CURVED_LETTERS'First)..CURVED_LETTERS'Pos(CURVED_LETTERS'Last)) of BOOLEAN;

It sure is ugly and some of you will surely yell, but we can do this now with a regular enumerated type and so it should be consistent for an enumerated sub-type.  At least creating an array from this type can be do-able with access to these attributes and it can also be verified correct by the compiler.

Matt