Re: array from static predicate on enumerated type

[Shark8 <onewingedshark@gmail.com>]

On Wednesday, March 17, 2021 at 7:04:06 PM UTC-6, Matt Borchers wrote:
> What I would expect is that P(12) would be an illegal access because 12 is not a valid value of Primes thus cannot be an index and a constraint error or out-of-range error would be thrown.
Which is reasonable.
> Wouldn't this just involve the compiler to insert "if x not in primes then raise..." if constraint checking is enabled?
Possibly, there are also dynamic-predicates to consider; imagine a subtype wherein the usage of some number excludes that number from the subtype. Maybe a "unused_room_keys" subtype for a hotel.
> Otherwise you have an erroneous program. Similarly, Primes'Succ(12) is also illegal. Primes'Pred(2) also illegal because there is no prime before 2 so presumably you have defined your Static_Predicate to enumerate the primes with 2 listed first.
Why?
Given the standard "subtype Natural is Integer range 0..Integer'Last" and "subtype Positive is Natural range Natural'Succ(Natural'First)..Natural'Last"
Positive'Pred( 1 ) = 0, and no CONSTRAINT_ERROR.
Do you want to break standard Ada usage?