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Oberon-2

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This article refers to the newer Oberon-2 programming language. For the older original Oberon language, see Oberon-1.

Oberon-2 is an extension of the original Oberon programming language that adds limited reflection and object-oriented programming facilities, open arrays as pointer base types, read-only field export and reintroduces the FOR loop from Modula-2.

Oberon-2
Paradigmimperative, structured, modular, object-oriented
Designed byNiklaus Wirth, Hanspeter Mössenböck
First appeared1991
Typing disciplinestrong, static
Influenced by
Oberon programming language

It was developed in 1991 at ETH Zurich by Niklaus Wirth and Hanspeter Mössenböck, who is now at Institut für Systemsoftware (SSW) of the University of Linz, Austria. Oberon-2 is a superset of Oberon, and is fully compatible with it. Oberon-2 was a redesign of Object Oberon.

Oberon-2 inherited limited reflection and single inheritance ("type extension") without interfaces or mixins from Oberon, but added efficient virtual methods ("type bound procedures"). Method calls were resolved at run-time using C++-style virtual method tables.

Compared to fully object-oriented programming languages like Smalltalk, in Oberon-2 basic types are not objects, classes are not objects, many operations are not methods, there is no message passing (to a certain extent it can be emulated by reflection and through message extension, as demonstrated in Oberon System 3), and polymorphism is limited to subclasses of a common class (no duck typing like in Smalltalk/Ruby, and it's not possible to define interfaces like in Java). Oberon-2 does not support encapsulation at object/class level, but modules can be used for this purpose.

Reflection in Oberon-2 does not use meta-objects, but simply reads from type descriptors compiled into the executable binaries, and exposed in the modules that define the types and/or procedures. If the format of these structures are exposed at the language level (as is the case for Oberon System 3, for example), reflection could be implemented at the library level. It could therefore be implemented almost entirely at library level, without changing the language code. Indeed, Oberon System 3 makes use of language-level and library-level reflection capabilities extensively.

History of the Oberon languages

The original Oberon was designed to be a 'safe' language; it employs array bounds checking, garbage collection and strong type checking. These features, particularly ones which enable logic errors to be detected as early as possible (i.e. at compile-time), can significantly reduce the number of bugs occurring in a program at runtime. However, some features included in other languages in an attempt to reduce bugs (e.g. enumerations and programmer-defined ranges on integers), were omitted. Consequently, more care should be taken by the programmer, when working with numeric expressions, to avoid logic errors.

Oberon was intended to make mistakes harder in part by making code less opaque, and in part because features not included cannot be misused. This approach can be taken even further, as in APL, which is both exceptionally terse and renowned for being less than easy to understand, but Oberon was deliberately constructed to not oversimplify.

As this is an intent whose success cannot be easily quantified, there remains some disagreement that Oberon has achieved its intended goals in this respect. One objection to its strategy of language design simplification was expressed by Jean Ichbiah, the architect of Ada when Wirth criticized Ada for being too big; he responded "There are times when Wirth believes in small solutions for big problems. I don't believe in that sort of miracle. Big problems need big solutions!" Oberon developers have even felt that Oberon went too far in this respect – Oberon-2 returned the 'FOR' statement to that version of the language.

It can be argued that failure to include a feature may force the programmer to reimplement the feature in his code, leading to multiple 'wheel reinvention' and consequent problems. Libraries can mitigate this – more or less –effectively depending on the feature and a language's graceful use of such libraries. Java is an example of a relatively simple language (though far less so than Oberon) embedded in large standard libraries. (Oberon has a much smaller standard library than Java.) As much of the effort of learning any language is learning the standard libraries, Ichbiah's objection above can be extended to a strategy of simplification by moving features from the core language into standard libraries. Wirth, and Oberon fans, argue that Oberon has essentially, and effectively, avoided this problem.

Example Oberon-2 code

The following Oberon-2 code would implement a simple list class: 

MODULE ListClass;

  TYPE List*    = POINTER TO ListNode;
       ListNode = RECORD
            value : INTEGER;
             next : List;
       END;

  PROCEDURE ( l : List ) Add*    ( v : INTEGER );
  BEGIN
       (* ... *)
  END Add;
  PROCEDURE ( l : List ) AddLast*( v : INTEGER );
  BEGIN
       (* ... *)
  END AddLast;
  PROCEDURE ( l : List ) AddAt*  ( i : INTEGER; v : INTEGER );
  BEGIN
       (* ... *)
  END AddAt;
  
  PROCEDURE ( l : List ) Remove*;
  BEGIN
       (* ... *)
  END Remove;
  PROCEDURE ( l : List ) RemoveLast*;
  BEGIN
       (* ... *)
  END RemoveLast;
  PROCEDURE ( l : List ) RemoveAt* ( i : INTEGER );
  BEGIN
       (* ... *)
  END RemoveAt;

END ListClass.

Features of the original Oberon

Key characteristics

The following features characterise the Oberon language :

  • Pascal-like, but more consistent syntax
  • Type-extension with strong type-checking
  • Modules with type-checked interfaces and separate compilation
  • Compatibility between all numeric types (mixed expressions)
  • Limited string operations
  • Support for system programming

Visibility flags

Global variables, types, constants, and procedures are by default only visible within the declaring module. They may be made public to other modules by suffixing them with a visibility flag, namely an asterisk (*) for read-write permission. The default was chosen to ensure safety in case a flag was inadvertently omitted.

Local variables, types, constants, and procedures are always visible only to the declaring procedure.

Call by reference or by value

Two possible modes are available for procedure parameters. Call-by-value (CBV) allows expressions to be used as parameters, so that the value of the expression is passed down to the procedure. Call-by-reference (CBR) allows variables to be used, so that the value of the variable may be changed by the procedure. A procedure may declare a CBR parameter by prefixing it with the VAR keyword.

Oberon-2 extensions to Oberon

Type-bound procedures

Procedures can be bound to a record (or pointer) type. They are equivalent to instance methods in object-oriented terminology.

Read-only export

The use of exported variables and record fields can be restricted to read-only access. This is shown with a "-" visibility flag.

Open arrays

Open arrays which previously could only be declared as formal parameter types may now be declared as pointer base types.

FOR statement

The FOR statement of Pascal and Modula-2 was not implemented in Oberon. It is reintroduced in Oberon-2.

Run-time type checking

Oberon-2 provides several mechanisms for checking the dynamic type of an object. For example, where a Bird object might be instantiated to either a Duck or a Cuckoo, Oberon-2 allows the programmer to respond to the actual type of the object at run-time.

The first, most conventional, approach is to rely on the type binding system. The second approach is to use the WITH statement, which allows the dynamic subtype of a variable to be checked directly. In both cases, once the subtype has been identified, the programmer can make use of any type-bound procedures or variables that are appropriate to the subtype. Examples of these approaches are shown below.

Note that the form of WITH statement used in Oberon-2 is unrelated to the Pascal and Modula-2 WITH statement. This method of abbreviating access to record fields is not implemented in Oberon or Oberon-2.

Type binding 

MODULE Birds;

    TYPE Bird* = RECORD
                     sound* : ARRAY 10 OF CHAR;
                 END;
END Birds.
(*-------------------------------------*)
MODULE Ducks;
    IMPORT Birds;
    
    TYPE Duck* = RECORD(Birds.Bird) END;
            
    PROCEDURE makeSound*( VAR bird: Duck );
    BEGIN COPY("Quack!", bird.sound); END makeSound;

END Ducks.
(*-------------------------------------*)
MODULE Cuckoos;
    IMPORT Birds;

    TYPE Cuckoo* = RECORD(Birds.Bird) END;
            
    PROCEDURE makeSound*( VAR bird: Cuckoo );
    BEGIN COPY("Cuckoo!", bird.sound); END makeSound;

END Cuckoos.

WITH statement 

MODULE Test;
  IMPORT Out, Birds, Cuckooo, Ducks;

    TYPE SomeBird* = RECORD ( Birds.Bird ) END;

    VAR sb : SomeBird;
    VAR c  : Cuckoos.Cuckoo;
    VAR d  : Ducks.Duck;
    
    PROCEDURE setSound*( VAR bird : Birds.Bird );
    BEGIN
        WITH bird : Cuckoos.Cuckoo DO
                    bird.sound := "Cuckoo!";
           | bird : Ducks.Duck DO
                    bird.sound := "Quack!";
        ELSE
                    bird.sound := "Tweet!";
        END;
    END setSound;

    PROCEDURE MakeSound* ( VAR b : Birds.Bird );
    BEGIN
    Out.Ln;
    Out.String( b.sound );
    Out.Ln;
    END MakeSound;

BEGIN

    setSound(c);
    setSound(d);
    setSound(sb);
    
    MakeSound(c);
    MakeSound(d);
    MakeSound(sb);

END Test.

POINTER 

MODULE PointerBirds;
   IMPORT Out;

   TYPE BirdRec*   = RECORD sound* : ARRAY 10 OF CHAR; END;
        DuckRec*   = RECORD(BirdRec) END;
        CuckooRec* = RECORD(BirdRec) END;

   TYPE Bird   = POINTER TO BirdRec;
        Cuckoo = POINTER TO CuckooRec;
        Duck   = POINTER TO DuckRec;

    VAR pb : Bird;
        pc : Cuckoo;
        pd : Duck;

    PROCEDURE makeDuckSound*( bird : Duck );
    BEGIN COPY("Quack!", bird.sound) END makeDuckSound;
    
    PROCEDURE makeCuckooSound*( bird : Cuckoo );
    BEGIN COPY("Cuckoo!", bird.sound) END makeCuckooSound;

    PROCEDURE makeSound*( bird : Bird );
    BEGIN
        WITH bird : Cuckoo DO makeCuckooSound(bird);
           | bird : Duck   DO makeDuckSound(bird)
          ELSE
            COPY("Tweet!", bird.sound);
        END;
    END makeSound;

BEGIN
   NEW(pc);
   NEW(pd);
   
   makeCuckooSound( pc );
   makeDuckSound( pd );
   
   Out.Ln;Out.String( pc^.sound );Out.Ln;
   Out.Ln;Out.String( pd^.sound );Out.Ln;

   makeSound( pc );
   makeSound( pd );

   Out.Ln;Out.String( pc^.sound );Out.Ln;
   Out.Ln;Out.String( pd^.sound );Out.Ln;
(* -------------------------------------- *)
(* Pass dynamic type to procedure         *)

   pb := pd;

   makeDuckSound( pb(Duck) );
   Out.Ln;Out.String( pb^.sound );Out.Ln;

   pb := pc;

   makeCuckooSound( pb(Cuckoo) );
   Out.Ln;Out.String( pb^.sound );Out.Ln;
(* -------------------------------------- *)

   makeSound(pb);
   Out.Ln;Out.String( pb^.sound );Out.Ln;

   pb := pd;
   
   makeSound(pb);
   Out.Ln;Out.String( pb^.sound );Out.Ln;
(* -------------------------------------- *)
   NEW(pb);
   
   makeSound(pb);
   Out.Ln;Out.String( pb^.sound );Out.Ln;
   
END PointerBirds.

A third approach is possible using the IS operator. This is a relation operator with the same precedence as equals (=), greater(>), etc. but which tests dynamic type. Unlike the two other approaches, however, it does not allow the programmer access to the subtype that has been detected.

Syntax

The development of the ALGOL - Pascal - Modula-2 - Oberon - Component Pascal language family is marked by a reduction in the complexity of the language syntax. The entire Oberon-2 language is described (Mössenböck & Wirth, 1993) using only 33 grammatical productions in the extended Backus–Naur form, as shown below. 

Module        = MODULE ident ";" [ImportList] DeclSeq [BEGIN StatementSeq] END ident ".".
ImportList    = IMPORT [ident ":="] ident {"," [ident ":="] ident} ";".
DeclSeq       = { CONST {ConstDecl ";" } | TYPE {TypeDecl ";"} | VAR {VarDecl ";"}} {ProcDecl ";" | ForwardDecl ";"}.
ConstDecl     = IdentDef "=" ConstExpr.
TypeDecl      = IdentDef "=" Type.
VarDecl       = IdentList ":" Type.
ProcDecl      = PROCEDURE [Receiver] IdentDef [FormalPars] ";" DeclSeq [BEGIN StatementSeq] END ident.
ForwardDecl   = PROCEDURE "^" [Receiver] IdentDef [FormalPars].
FormalPars    = "(" [FPSection {";" FPSection}] ")" [":" Qualident].
FPSection     = [VAR] ident {"," ident} ":" Type.
Receiver      = "(" [VAR] ident ":" ident ")".
Type          = Qualident
              | ARRAY [ConstExpr {"," ConstExpr}] OF Type
              | RECORD ["("Qualident")"] FieldList {";" FieldList} END
              | POINTER TO Type
              | PROCEDURE [FormalPars].
FieldList     = [IdentList ":" Type].
StatementSeq  = Statement {";" Statement}.
Statement     = [ Designator ":=" Expr
              | Designator ["(" [ExprList] ")"]
              | IF Expr THEN StatementSeq {ELSIF Expr THEN StatementSeq} [ELSE StatementSeq] END
              | CASE Expr OF Case {"|" Case} [ELSE StatementSeq] END
              | WHILE Expr DO StatementSeq END
              | REPEAT StatementSeq UNTIL Expr
              | FOR ident ":=" Expr TO Expr [BY ConstExpr] DO StatementSeq END
              | LOOP StatementSeq END
              | WITH Guard DO StatementSeq {"|" Guard DO StatementSeq} [ELSE StatementSeq] END
              | EXIT
              | RETURN [Expr]
      ].	
Case          = [CaseLabels {"," CaseLabels} ":" StatementSeq].
CaseLabels    = ConstExpr [".." ConstExpr].
Guard         = Qualident ":" Qualident.
ConstExpr     = Expr.
Expr          = SimpleExpr [Relation SimpleExpr].
SimpleExpr    = ["+" | "-"] Term {AddOp Term}.
Term          = Factor {MulOp Factor}.
Factor        = Designator ["(" [ExprList] ")"] | number | character | string | NIL | Set | "(" Expr ")" | " ~ " Factor.
Set           = "{" [Element {"," Element}] "}".
Element       = Expr [".." Expr].
Relation      = "=" | "#" | "<" | "<=" | ">" | ">=" | IN | IS.
AddOp         = "+" | "-" | OR.
MulOp         = " * " | "/" | DIV | MOD | "&".
Designator    = Qualident {"." ident | "[" ExprList "]" | " ^ " | "(" Qualident ")"}.
ExprList      = Expr {"," Expr}.
IdentList     = IdentDef {"," IdentDef}.
Qualident     = [ident "."] ident.
IdentDef      = ident [" * " | "-"].

Implementations

Oberon-2 compilers maintained by ETH include versions for Windows, Linux, Solaris, Mac OS X.

There is an Oberon-2 Lex scanner and Yacc parser by Stephen J Bevan of Manchester University, UK, based on the one in the Mössenböck and Wirth reference. It is at version 1.4.

There is a release called Native Oberon which includes an operating system, and can directly boot on PC class hardware.

A .NET implementation of Oberon with the addition of some minor .NET-related extensions has also been developed at ETHZ.

Programmer's Open Workbench (POW!) is a very simple integrated development environment, which is provided with editor, linker and Oberon-2 compiler. This compiles to Windows executables. Full source code is provided - the compiler is written in Oberon-2.

The Java to Oberon Compiler (JOB) was written at the University of Vologda in Russia. It produces object code in the form of Java class files (bytecode). Some JOB-specific classes are provided which are Java compatible, but which use a more Oberon-like component hierarchy.

The Optimizing Oberon-2 Compiler compiles to C, using the gcc toolchain for program generation.

Oberon Script is a compiler that translates the full Oberon language into JavaScript. The compiler proper is written in JavaScript and can therefore be called from Web pages in order to process HTML script sections written in Oberon.

References

  • "Second International Modula-2 Conference", September 1991.

Evolution of Oberon and Oberon-2

Detailed papers

Books

can be found here (including electronic online versions).

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