Re: array from static predicate on enumerated type

[Egil H H <ehh.public@gmail.com>]

On Friday, March 12, 2021 at 9:49:29 PM UTC+1, Matt Borchers wrote:
> Say, for example, I define a static predicate on a sub-type of an enumerated type, like: 
> 
> type LETTERS is ( A, B, C, D, E, F, G, H, I , J, K ); 
> 
> subtype CURVED is LETTERS 
> with Static_Predicate CURVED in B | C | D | G | J; 
> 
> What I want is an array over CURVED (using CURVED as the index), but since attributes 'First and 'Last (and thus 'Range) is not allowed, this cannot be done. 
> 

You should read what the Rationale says about predicates (http://www.ada-auth.org/standards/12rat/html/Rat12-2-5.html),
especially the part about 'First_Valid and 'Last_Valid:

"These rules can also be illustrated by considering the dartboard. We might like to accumulate a count of the number of times each particular score has been achieved. So we might like to declare
type Hit_Count is array (Score) of Integer;   -- illegal
but sadly this would result in an array with holes and so is forbidden. However, we could declare an array from 1 to 60 and then initialize it with 0 for those components used for hits and –1 for the unused components. Of course, we ought not to repeat literals such as 1 and 60 because of potential maintenance problems. But, we can use new attributes First_Valid and Last_Valid thus
type Hit_Count is array (Score'First_Valid .. Score'Last_Valid) of Integer :=
       (Score => 0, others => –1);
which uses Score to indicate the used components. The attributes First_Valid and Last_Valid can be applied to any static subtype but are particularly useful with static predicates.
In detail, First_Valid returns the smallest valid value of the subtype. It takes any range and/or predicate into account whereas First only takes the range into account. Similarly Last_Valid returns the largest value. Incidentally, they are illegal on null subtypes (because null subtypes have no valid values at all)."