Re: array from static predicate on enumerated type

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On 2021-03-15 18:48, Shark8 wrote:

> So, in Ada, there's no good choice for how to actually DO an array with gaps in the index:

That applies to all containers. The problem is mapping the key to a 
dense position when the key is not dense or might be unordered.

There is no gaps in positions.

> But Array *isn't* a map, though often it is substituted as one.

Of course it is, per definition of mapping. f : key -> element.

> Array has further constraints, like the contiguous memory labeled above.

No. The constraint here is a dense index. That is not specific to a 
general map or an array. It is a fundamental property of the set of keys 
being a convex set. Convex ensures certain useful mathematical 
properties invariant to the representation of the mapping. In 
particular, it gives a way to enumerate elements. If you want to 
enumerate non-convex keys of a map you have a problem. As the OP just 
encountered.

Arbitrary predicate kills convex, so the problem. Not array. A map would 
have just same problem: a map of LETTERS is in general not substitutable 
for a map of CURVED_LETTERS.

> Ada's arrays ARE contiguous memory; so I'm not sure what you're trying to distinguish here.

No. A dense index allows contiguous representation, but it does not 
require it.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de