In article <s2ofr1$1cg3$1@gioia.aioe.org>,
"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote:

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.

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.

i don't understand,
since Ada.Containers.Ordered_Maps.Key_Type is private,
and so new Ada.Containers.Ordered_Maps (CURVED_LETTERS, Integer) is
allowed.

--
RAPID maintainer
http://savannah.nongnu.org/projects/rapid/

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