[Oberon] Protocols (interfaces) in Oberon-2

[Andreas Pirklbauer]

    > I have the above code successfully parsing in a fork of Andreas's
    > Extended Oberon compiler. I don't have the method table generation code
    > working yet. It turns out that the Run-time needs to have symbolic type
    > information available, which could be simply loading the smb file alongside
    > the rsc file for code that uses interfaces, or it could involve embedding a
    > hash of the name and parameters of the type-bound procedure in another
    > section of the rsc file. I haven't decided yet.

Dear Chuck,

In the proposal put forward earlier in this thread (see example below),
almost no extra code is necessary in the compiler or the runtime, because:

1. A protocol definition *is* in fact just a regular record definition - the
   only difference being that there now is an additional “executable” flag
   that indicates that its type-bound procedures cannot be executed.

2. A protocol implementation *is* is fact just a regular type extension
   (of a protocol definition - which this is why the same syntax has been
   chosen), where this flag is simply changed such that its type-bound
   procedures *can* now be executed.

That’s all there is to it. Well almost. One also needs to slightly modify
the type checking rules of the compiler such that two protocol
implementations (=type extensions) of the same protocol definition
(=the base type) are in fact assignment-compatible (unlike two
“regular” record extensions of the same base type which are not).

And of course the "non-executable” flag must be correctly propagated
across symbol and object files as well.

-ap


  MODULE TextProtocol;  (*protocol definition*)
    TYPE Text* = POINTER TO TextDesc;
      TextDesc* = RECORD data*: (*text data*) END ;
      PROCEDURE (t: Text) Insert* (string: ARRAY OF CHAR; pos: LONGINT);
      PROCEDURE (t: Text) Delete* (from, to: LONGINT);
      PROCEDURE (t: Text) Length* (): LONGINT;
  END TextProtocol;

  MODULE Text1; (*one implementation of the Text protocol*)
    IMPORT TextProtocol;
    TYPE Text* = POINTER TO TextDesc;
      TextDesc* = RECORD (TextProtocol.TextDesc) END ;  (*this means: “implements TextProtocol.TextDesc"*)
    
    PROCEDURE (t: Text) Insert* (string: ARRAY OF CHAR; pos: LONGINT);
    BEGIN (*implementation of Insert*)
    END Insert;

    PROCEDURE (t: Text) Delete* (from, to: LONGINT);
    BEGIN (*implementation of Delete*)
    END Delete;

    PROCEDURE (t: Text) Length* (): LONGINT;
    BEGIN (*implementation of Length*)
    END Insert;
  END Text1;


  MODULE Text2; (*another implementation of the Text protocol*)
    IMPORT TextProtocol;
    TYPE Text* = POINTER TO TextDesc;
      TextDesc* = RECORD (TextProtocol.TextDesc) END ;  (*this means: “implements TextProtocol.TextDesc"*)
    
    PROCEDURE (t: Text) Insert* (string: ARRAY OF CHAR; pos: LONGINT);
    BEGIN (*implementation of Insert*)
    END Insert;

    PROCEDURE (t: Text) Delete* (from, to: LONGINT);
    BEGIN (*implementation of Delete*)
    END Delete;

    PROCEDURE (t: Text) Length* (): LONGINT;
    BEGIN (*implementation of Length*)
    END Insert;
  END Text2;