[Oberon] Protocols (interfaces) in Oberon-2

[Jörg]

Chuck

 

Okay. My proposal in standard Oberon-07 for your new requirement is this:

 

MODULE Data;

TYPE

  Any* = POINTER TO Empty;

  Empty* = RECORD END;

END Data.

 

MODULE Jsonify;
IMPORT Data;
TYPE
  Methods* = POINTER TO MDesc;
  MDesc* = RECORD
    toJSON: PROCEDURE(this: Data.Any);
  END;
END Jsonify.


MODULE myData;

IMPORT Data;

TYPE

  Tree* = POINTER TO TreeDesc;

  TreeDesc* = RECORD (Data.Empty)

    left, right: Tree;

    j: Jsonify;

  END;

VAR t: Tree;

PROCEDURE J(t: Data.Any);

  BEGIN

    CASE t OF Tree: (* implement here your Tree-specific JSON routine *) 

    END

  END J;

PROCEDURE New*(VAR t: Tree); BEGIN NEW(t); Jsonify.New(t.j); t.j.toJSON := J END New


BEGIN

  New(t); t.j.toJSON(t)

END myData.

 

br

Jörg

 

Von: Oberon <oberon-bounces at lists.inf.ethz.ch> im Auftrag von Charles Perkins <chuck at kuracali.com>
Antworten an: ETH Oberon and related systems <oberon at lists.inf.ethz.ch>
Datum: Montag, 26. Oktober 2020 um 17:03
An: ETH Oberon and related systems <oberon at lists.inf.ethz.ch>
Betreff: Re: [Oberon] Protocols (interfaces) in Oberon-2

 

Hi Jörg and other interested people,

 

There is just one piece missing I think for the usage of Interfaces that I am looking for. I would like to be able to write a routine that doesn't know what Data is (in the provided example) but can still hold a pointer to it or to anything else that implements the JSONify interface (and not anything that doesn't implement the JSONify interface) and can call it's ToJSON procedure.

 

In my previous reply I incorrectly thought that a record in Oberon can be an extension of two base records, which is not right, and not what Jörg is presenting. 

 

Apologies for my confusion,

Chuck

 

 

On Mon, Oct 26, 2020 at 7:37 AM Charles Perkins <chuck at kuracali.com> wrote:

Hi Jörg,

 

With this example I think I understand how Oberon can in fact already do what I want in composing Interfaces. In Wirthian fashion the mechanism is explicit rather than implicit. I had not made the mental jump to how a record may be based on more than one base record... with that, everything else falls into place.

 

I observe that this means that interfaces are 'opt in' rather than automatic based on just matching a subset of method names, but that's typical and arguably more safe anyway.

 

I'm going to think on this some more.

 

Thank you very much for taking the time to illustrate this to us.

 

Best,

Chuck

 

 

On Sun, Oct 25, 2020 at 11:55 PM Jörg <joerg.straube at iaeth.ch> wrote:

Chuck

        > Andreas, in your scheme can you create separate protocols, for example
        > "Jsonify" with the method ToJSON and a different protocol "Persistify" with
        > the methods "Store" and "Load", and have other records implement one or the
        > other or neither or both?

Here my proposal for doing this in standard Oberon-07.
In the previous mail, I separated interface/protocol and implementation for clarity. The same should be done here. For brevity, I combined the two here.

MODULE Data;
(* Definition of whatever your internal data structure looks like. Here just an example *)
TYPE
  Tree* = POINTER TO TreeDesc;
  TreeDesc* = RECORD val*: ARRAY 15 OF CHAR; left*, right*: Tree END
END Data.

MODULE Jsonify;
IMPORT Data;
TYPE
  Methods* = POINTER TO MDesc;
  MDesc* = RECORD
    toJSON: PROCEDURE(this: Data.Tree);
  END;
  (* empty or default implementation *)
  PROCEDURE J(this: Data.Tree); END J;
  PROCEDURE New*(VAR m: Method); BEGIN NEW(m); m.toJSON := J END;
END Jsonify.

MODULE Persistify;
IMPORT Data;
TYPE
  Methods* = POINTER TO MDesc;
  MDesc* = RECORD
    Load: PROCEDURE(VAR this: Data.Tree);
    Store: PROCEDURE(this: Data.Tree)
  END;
  (* empty or default implementation *)
  PROCEDURE L(VAR this: Data.Tree); BEGIN this := NIL END L;
  PROCEDURE S(this: Data.Tree); END S;
  PROCEDURE New*(VAR m: Methods); BEGIN NEW(m); m.Load := L; m.Store := S END Init;
END Persistify.

Here now a module using both interfaces/protocols and overwrite even one persist procedure with an own version, if wanted. 
MODULE Usage;
IMPORT Data, Jsonify, Persistify;
TYPE
  User = RECORD (Data.Tree)
    j: Jsonify;
    p: Persistify
  END;
VAR u: User;
PROCEDURE myLoad(this: Data.Tree); (* implement your version of persist Load *) END myLoad;
PROCEDURE New*(VAR u: User);
  BEGIN NEW(u); Jsonify.New(u.j); Persistify.New(u.p); u.p.Load := myLoad END New;

BEGIN
  New(u); u.j.toJSON(u); u.p.Load(u) (* this calls my version *)
END Usage.

Adding the qualifiers "j" and "p" circumvents the ambiguity in case the two interfaces defined methods with the same name.

br
Jörg



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