[Oberon] Protocols (interfaces) in Oberon-2

[Andreas Pirklbauer]

Correction: In Text1, it’s TextDesc = RECORD (TextProtocol.TextDesc) of course

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Protocols (sometimes called interfaces) can be added to
Oberon-2 without adding any keywords to the language.

This is one of the key differences to how it is usually defined
and implemented, e.g. in Swift [*] or in Integrated Oberon [**]

Under the new minimalistic design, what distinguishes a protocol
from an actual implementation (of the class) is that in the protocol
definition the implementations of the class methods are simply not
defined. Instead, any module that *imports* a protocol definition
can “adopt” (i.e. implement) it. See the example below.

An experimental implementation showed that if the language
is extended in *this* way, the implementation cost is minimal.

But the question is: Is it worth it? Simplicity of implementation
should of course not be a criteria for adopting a new feature.

Personally, I am rather sceptical of the usefulness of protocols.
But perhaps someone provides a good reason to adopt them.

-ap


Example:

  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;


[*] https://docs.swift.org/swift-book/LanguageGuide/Protocols.html#
[**] https://github.com/io-core/technotes/blob/main/technote014.md