Re: Type Transcriptions

[Shark8 <OneWingedShark@gmail.com>]

On 21-Apr-14 14:33, Ludovic Brenta wrote:
> Shark8 <OneWingedShark@gmail.com> writes:
>> On 21-Apr-14 12:11, Yannick Duchêne (Hibou57) wrote:
>>> Le Mon, 21 Apr 2014 18:54:23 +0200, Brad Moore <brad.moore@shaw.ca> a
>>> écrit:
>>>
>>>> I'd suggest trying to make GenericAEADCipher a discriminated record
>>>> type, with the two length values as the discriminants, and then the
>>>> arrays are sized using the discriminants. No Generics needed.
>>>>
>>>> Brad
>>>>
>>>
>>> I guess Shark8 would already have figured it if this could be as simple.
>>> He says this, which is important: “certain class of record-type is
>>> dependent on the values in another record, yet distinctly separate”.
>>> With two separated things, a discriminant is not applicable. Or may be
>>> I've not understood.
>>
>> You have the right of it.
>> Using discriminants is the natural way to do it, but doing so would
>> (in actuality) break sticking to the spec as the record-type would
>> have fields which don't exist in the spec.
>>
>> I suppose the better way to phrase it is:
>> "Is it better to strictly stick to the spec, letting generic
>> instantiation take care of these 'extra-typal' parameters, *OR* doing
>> things the natural Ada way, and worrying about 'fixing-up' things in,
>> say, 'Input and 'Output operations."
>
> Ada 2012 has a very nice new feature: unchecked unions (clause B.3.3);
> it is a discriminated record with an aspect says that the discriminants
> are *not* in the record, but stored elsewhere instead.  I have used them
> in my tiny Ada binding to the X C binding (XCB), like so:
>
>        type Detail_T (Response_Type : Response_Type_T := Reply) is record
>           case Response_Type is
>              when Key_Press | Key_Release =>
>                 Keycode : Interfaces.Unsigned_8;
>              when Button_Press | Button_Release | Motion_Notify =>
>                 Detail  : Button_T;
>              when others =>
>                 Padding : Interfaces.Unsigned_8;
>           end case;
>        end record
>           with Unchecked_Union;
>
>        type Response_T (Sent : Boolean; Response_Type : Response_Type_T) is
>           record
>              Detail   : Detail_T (Response_Type);
>              [other components omitted]
>           end record;
>        for Response_T use record
>           Sent          at 0 range 7 .. 7;
>           Response_Type at 0 range 0 .. 6;
>        end record;
>
>
> This says that the Response_T has two discriminants, stored as the first
> two components, and that the second discriminant, Response_Type, also
> constrains the component Detail, but Detail does not contain a copy of
> this discriminant.
>
> This sounds awfully similar to the spec you're trying to implement :)
>
> In this case, Ada 2012 saves the day!
>

Thanks for the info.
Unfortunately it is of no help: the [non-]discriminant cannot be used to 
set the length of the internal 'vector' (the array).