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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Half Constrained Array Types and Slices
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Date: Thu, 2 Mar 2006 09:34:26 +0100
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Date: 2006-03-02T09:34:24+01:00
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On Thu, 02 Mar 2006 05:00:06 GMT, Jeffrey R. Carter wrote:

> Robert Duff present a concept from his idea for an Ada-like language (not called 
> Duff) some time ago, that there was no need for "box" ("<>") in his language:
> 
> type String (Length : Natural) is array (1 .. Length) of Character;

Not only this. Discriminants for arrays would also allow nice things like:

type Table (Element_Size : Natural) is
   array (...) of String (1..Element_Size);

or

type Container (Element_Type : Tag) is
   array (...) of Element'Class (Tag => Element_Type);

> (IIRC)
> 
> In Ada we have constrained and unconstrained array types; I call this a "half 
> constrained" array type so we have names for all of them. Since his language 
> doesn't have box, I presume an unconstrained array type would be:
> 
> type Unconstrained (Lo : Positive; Hi : Natural) is array (Lo .. Hi) of Component;

Rather:

type Unconstrained (Lo : Positive; Hi : Natural) is
    array (Integer range Lo .. Hi) of Component;

> while a constrained type would look like an Ada constrained type:
> 
> type Constrained is array (Lo .. Hi) of Component;
> 
> I was wondering how slices would work with this kind of syntax. I would expect a 
> slice of a half-constrained array to have the fixed lower bound specified in its 
> type declaration; that would differ from the others.
> 
> In Ada, we deal with slices of constrained array types by having the 1st named 
> subtype be a constrained subtype of an unconstrained base type; that wouldn't 
> seem to work here, since it would have a base type with 2 discriminants and a 
> subtype with only 1.

subtype Half_Constrained (Length : Natural) is Unconstrained (1, Length);

BTW, starting to modify Ada, one should introduce index types and allow
discriminants of any type. Then a range of indices could be just one
discriminant rather than two.

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