Re: The "()" operator revisited.

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

On Tue, 13 Jan 2004 16:44:28 GMT, "Frank J. Lhota"
<NOSPAM.lhota.adarose@verizon.net> wrote:

>"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message
>news:dmc700tmurmei4u19nof44803bv19f6d6o@4ax.com...
>> More specifically, how would it be matched against a formal
>> generic array parameter?
>
>That is far more ambitious than this proposal.

Ada is a language with great ambitions! (:-))

> The basic idea of this
>proposal is to allow array indexing notation for collections. For example,
>assume that we have a hash table that maps employee names to employee
>records. It would be nice if we could define "()" and "():=" for this hash
>table so that we could write something like this:
>
>    declare
>       Rec : Employee_Record_Hash_Table;
>    begin
>       ...
>       Rec( "Johnson" ) := This_Rec;
>       Display( The_Rec => Rec( "Smith" ) );
>       ...
>    end;

The problem is that without notion of abstract array as a class-wide
or generic parameter type, there will be no way to write
class-wide/generic subroutines taking advantage of indexing. Let I
have all containers of different kind having () indexing. This will
not help me. Because I will still be unable as little as just to write
a universal program searching max element in *a* container.

>To get something that could match a generic array parameter would require a
>lot of restrictions on abstract arrays, in particular that the indice types
>would have to be abstract discrete types.

No. We should simply go on and introduce abstract index type as a
counterpart of the formal discrete type:

type X is (<>);

One should be consequent. We need abstract discrete, integer, index,
numeric, string, array, access types. Have I forgot something? (:-))

> This would rule out abstract
>arrays such as what we see in this example.

I am not sure that hash table is an array. But if it should be (why
not) then, we should also distinguish two different kind of index
types:

1. Ordered, dense indices. Having ".." primitive operation and so
allowing iteration of arrays built upon them.

2. Unordered indices, with no [public] order defined (or used). An
array with an unordered index would be a hash table.

--
Regards,
Dmitry A. Kazakov
www.dmitry-kazakov.de