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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 20:37:00 +0100
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Date: 2006-03-02T20:37:00+01:00
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On Thu, 02 Mar 2006 16:34:50 +0100, Georg Bauhaus wrote:

> On Thu, 2006-03-02 at 09:34 +0100, Dmitry A. Kazakov wrote:
>> On Thu, 02 Mar 2006 05:00:06 GMT, Jeffrey R. Carter wrote:
> 
>>> 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);
> 
> If you didn't dislike generics that much,
>
> generic
>    Element_Size: NATURAL;
> package String_Tables is
> 
>    subtype SIZED_STRING is STRING(1.. Element_Size);
> 
>    type TABLE is array(POSITIVE range <>) of SIZED_STRING;
> 
> end String_Tables;

Nope:

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

function Read_From_File (File : File_Type) return Table;
function Write_To_File (File : File_Type; X : Table);
   -- Notice indefinite formal parameters and results are OK

subtype Punched_Cards is Table (80);
   -- Subtypes are OK

This is why I dislike generics. Each instance of String_Tables produces an
independent type. So there is no either named type or objects of the class
TABLE.

----
Note that this problem is known since Pascal. Strings in original Pascal
were of different types, when they had different lengths. Ada 83 introduced
the idea of unconstrained types to handle this nuisance. So different sizes
induced subtypes rather than types. Though the latter were also possible
using "type is new."

Generics is not Ada! (:-))

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