Re: recursive types and generics

[Robert A Duff <bobduff@shell01.TheWorld.com>]

alfonso acosta <alfonso_acosta_mail@yahoo.es> writes:

> Hi:
> 
> I need to code a recursive type: a Sequence which could contain
> sequences of the same type. Im trying to use a generic package for that
> purpouse. I provide a simplified example:
> 
> -------------------------------------------------------------------------
> type Elem_Type is (Nothing, Int, Seq);
> 
> type Seq_Acces;
> 
> type Seq_Elem (Const : Elem_Type := Nothing) is
>     record
>       case Const is
>         when Nothing => null;
>         when Int     => I : Integer;
>         when Seq     => S : Seq_Access;
>          -- "invalid use of type before its full declaration"
>       end case;
>     end record;
> 
> package Myseq is new Sequence (Element => Seq_Elem);
> 
> type Seq_Access is access Myseq.Sequence; -- provided by the generic
>                                            -- package
> --------------------------------------------------------------------------
> 
> Any ideas about fixing it?
> 
> Alfonso Acosta

How about something like this?

type Seq_Elem is tagged limited null record;

package Myseq is new Sequence (Element => Seq_Elem'Class);

type Seq_Access is access Myseq.Sequence;

type Int_Seq_Elem is new Seq_Elem with
    record
        I: Integer;
    end record;

type Seq_Seq_Elem is new Seq_Elem with
    record
        S: Seq_Access;
    end record;

- Bob