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Newsgroups: comp.lang.ada
Date: Thu, 18 Mar 2021 07:25:00 -0700 (PDT)
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Subject: Re: array from static predicate on enumerated type
From: Shark8 <onewingedshark@gmail.com>
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Xref: reader02.eternal-september.org comp.lang.ada:61617
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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?