Re: Discriminant within discriminants

["Jeffrey R. Carter" <spam.not.jrcarter@acm.not.spam.org>]

stuart clark wrote:
> 
> package message_types
> 
>   type id_type is range 1..8;
> 
>  type dispense_ack (size:id_type:=id_Type'first) is
>    record
>      dispense_acks:array(1..size) of acks;
>    end record;
> 
>   type message_types(dispense_acknowledgement, dispense_request);

Do you want an "is" in there before the '('?

>   type in_messages(kind :msg_types:=msg_types'first) is
>    record
>     case type is
>       when dispense_acknowledgement =>
>          acknowledgements:dispense_ack;
>       when dispense_request
>          request:integer;
>     end case
>   end record;
> 
> end message_types;
> 
> i want to declare a message that is of dispense_acknowledgment type
> with a size of 8
> 
> the only way i can get my code to compile with a message size of 8 is
> to use
> 
> declare
>   msg:message_types.in_messages;
>   dispense_ack_msg:message_types.dispense_ack(size=>8);
> begin
>   msg:=(kind=>message_types.dispense_acknowledgement,
>             acknowledgements=>dispense_ack_msg);
> 
> end;
> 
> QUESTION : can i declare a message with a type and size in the one
> statement without using the intermediate message.

Msg : Message_Types.In_Messages :=
    (Kind => Message_Types.Dispense_Acknowledgement,
     Acknowledgements =>
        (Size => 8, Dispense_Acks => (1 .. 8 => Thing) ) );

This requires that a value of the (undefined) type Acks be available 
(Thing in the example).

If you don't have such a value, then something along the lines of your 
example is required. Note that you can also do

declare
    Msg : Message_Types.In_Messages
          (Kind => Message_Types.Dispense_Acknowledgement);
    DAM : Message_Types.Dispense_Ack (Size => 8);
begin
    Msg.Acknowledgements := DAM;
end;

(Note also that most dictionaries recommend the spelling "acknowledgment".)

-- 
Jeff Carter
"I would never want to belong to any club that
would have someone like me for a member."
Annie Hall
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