A Description of the Oberon-2 Language

Written by Paul Floyd

Introduction

Usually, Oberon grammar is presented in EBNF (the Extended Backus-Naur Formalism).

The main building block of Oberon applications is the module. Each module has a name. You can import other modules into a module, and can export components of the module so that other modules may import them (i.e., make them public). Variables and functions are tagged as exported by appending a "*" at the end of the name. Read-only export is also possible, this time by appending a "-" at the end of the name. The module can contain the other building blocks described later - type definitions, constants, variables and functions.

Comments are opened by "(*" and closed by "*)".

Here is an example of the module syntax:

MODULE MyModule; (* MyModule is the name of this module *)
   IMPORT S := SYSTEM, OS2;
   (* Here we import 2 modules, SYSTEM for low-level Oberon functions *)
   (* and OS2, an interface to the OS/2 API *)
   (* Note "S := SYSTEM" allows us to use S as an alias of SYSTEM, so *)
   (* we can call functions like S.CC() *)

(* type definitions, constants, variables, procedures *)
BEGIN
   (* body of the module goes here *)
END MyModule.

Constants

A section of declared constants starts with the keyword CONST. Constants may be built up from other constants. The assignment of a constant uses "=", unlike that of a variable which uses ":=".

Example:

CONST
   a = 31;
   m = 12*a; (* will be 372 *)
   s- = "string"; (* read-only exported, as an ARRAY 6 of CHAR *)

Types

You can define your own types from the base types defined in Oberon, and types you have defined. Type declarations begin with the keyword TYPE.

Oberon has single inheritance. A RECORD may have a base type by putting the name of the base type in parentheses after the RECORD declaration of the derived type.

The built in types are: BOOLEAN, CHAR, SHORTINT, INTEGER, LONGINT, REAL, LONGREAL and SET. Most of these are implementation-specific. A SET is a SET of integers.

Examples of type declarations:

TYPE
   Flag = BOOLEAN;
   String = ARRAY 32 of CHAR;
   Date = RECORD
             Day, Month, Year: INTEGER
          END;
   DatePtr = POINTER TO Date;
   SecondOfYear = RECORD (Date) (* this shows inheritance *)
                     Hour, Minute, Second: INTEGER
                  END;
   SecondPtr = POINTER TO SecondOfYear;

You can use ARRAYs and RECORDs (structures) for compound types. You can declare POINTERs, but they must point to structured types only. ARRAYs need not have their dimension specified, but in this case they must be ARRAYS of POINTERs.

Example:

TYPE
   Coordinate = RECORD
                END;

   CoordinateCartesian = RECORD(Coordinate)
      x, y, z : REAL
                         END;

   CoordinateCylindrical = RECORD(Coordinate)
      r, h, theta : REAL;
                           END;

   CoordinateSpherical = RECORD(Coordinate)
      r, phi, theta : REAL;
                         END;

   ThreeDim = POINTER TO ARRAY OF ARRAY OF Coordinate;

ThreeDim is now a three dimensional ARRAY of Coordinate values, where the representation might be Cartesian, Spherical or Cylindrical. This is only useful for parameters/methods.

Which representation you have might be checked by a type guard, which is something completely different than a type cast. It checkes which extension of a base type is the current runtime type.

Variables

Declarations of variables start with the VAR keyword, and use either built in types of types created in the TYPE declarations.

Examples:

VAR
   a, b, c : INTEGER;
   Matrix : ARRAY 10, 10 OF REAL; (* 2 dimensional array *)
   SP : SecondPtr; (* still need to NEW this before using it *)

Procedures

Procedures basically come in two types, function procedures and proper procedures. Function procedures return a value when called.

Examples:

PROCEDURE CopyString(instr: ARRAY OF CHAR; VAR outstr ARRAY OF CHAR) ;
(* proper procedure *)
BEGIN
   (* body of procedure *)
   RETURN
END CopyString

PROCEDURE Atan2(x, y: REAL): REAL
(* function procedure *)
BEGIN
   (* body of procedure *)
   RETURN answer
END Atan2

The parameters that the procedures take are either passed by value (default) or passed by reference (indicated by the VAR keyword). Whether or not a procedure is a function procedure depends on whether the PROCEDURE line is followed by the return type. Note however that the return type cannot be compound. You must either return a pointer or a simple type.

Procedure forward declaration

Oberon compilers are generally one-pass (thus blindingly fast), so if a procedure is used before it is defined, it must be declared first. The caret symbol is used to indicate definition of a procedure. The forward declaration and actual declaration must have the same name, type binding and parameters. Example:

PROCEDURE ^Atan2(x, y: REAL): REAL

PROCEDURE MathFunction (* a procedure that calls Atan2 *)

(* actual declaration of Atan2, as above *)

Local procedures

Unlike C/C++, Oberon can have procedures within procedures. In the case of local and greater scope procedures having the same name, the local one will take precedence. The local and greater scope procedures with the same name can't be differentiated by parameter lists. If two such functions exist, and you try to call the outer procedure, then either the compiler will perform type conversion and call the local procedure, or it will say that the parameter lists do not match.

Type bound procedures

The concept of type bound procedures is very important in Oberon-2. They are the equivalent of class methods in C++.

Firstly, the syntax. Type bound procedures are indicated by declaring the record to which the procedure in parentheses before the procedure name. A name for the module type is also given, which is used for dereferencing components of the module (as compared to the anonymous this in C++). Type bound procedures are not declared with the record itself, but can be declared at any point.

Simple example:

TYPE
   Point = RECORD
      x, y: INTEGER
   END;

PROCEDURE (pt: Point)Add(DeltaX, DeltaY: INTEGER)
(* procedure bound to record type Point *)
BEGIN
   (* pt is the instance of the record itself *)
   pt.x := pt.x + DeltaX;
   pt.y := pt.y + DeltaY;
   RETURN
END Add;

Type bound procedures may be redefined for records that are extensions. For instance, if we define a three dimensional co-ordinate:

TYPE
   Point3D = RECORD(Point)
              z: INTEGER
             END;

It is then possible to re-define Add:

PROCEDURE (pt: Point3D)Add(DeltaX, DeltaY: INTEGER)
BEGIN
   (* caret indicates that we are calling the base class type bound
      procedure *)
   pt.Add^(DeltaX, DeltaY);
   RETURN
END Add;

It is important to note that the redefined procedure must have the same parameters as the base class one.

Built-in function procedures

ABS(x)

Absolute value (accepts and returns any numeric type)

ASH(x, n)

Arithmetic shift left (LONGINT)

CAP(x)

Capitalize (CHAR)

CHR(x)

Letter from ASCII value (CHAR)

ENTIER(x)

Integral part of a real number (LONGINT)

LEN(v, n)

Length of vector n of array v (LONGINT)

LEN(v)

= LEN(v, 0)

LONG(x)

Promotes to next greatest numeric type (INTEGER, LOGINT or LONGREAL)

MAX(T)

If T is a type, returns the maximum value for that type

If T is a SET, the maximum value in that set (INTEGER)

MIN(T)

As MAX but minimum

ODD(x)

Detects odd integers (BOOLEAN)

ORD(x)

ASCII value from character (INTEGER)

SHORT(x)

Demotes to next lowest numeric type (SHORTINT, INTEGER or REAL)

SIZE(T)

Size of type in bytes (integral type)

Built in proper procedures

COPY(source, dest)      Copy string
DEC(i)                  Decrement integer
DEC(i, n)               Subtract n from i
EXCL(v, x)              Remove x from SET v
HALT(x)                 Terminate application
INC(i)                  Increment
INC(i, n)               Add n to i
NEW(p)                  Allocate a fixed-size object to pointer p
NEW(p, i0, i1, ... in)  Allocate n+1 objects of size ix to pointer p

System functions

Most Oberon compilers have a module SYSTEM which contains low-level functions.

ADR(v)      Address of variable (LONGINT)
BIT(a, n)   Test bit n of a (BOOLEAN)
CC(n)       Condition n (16 bit)
LSH(x, n)   Logical shift n (type of x)
ROT(x, n)   Rotate n (type of x)
VAL(T, x)   Force x to be expressed as type T (type of T)

Expressions

Expressions consist of operands and operators. Operands consist of procedure calls, string/numeric values, constants or variables. Variables may be simple types, arrays or pointers. A type guard (like a C/C++ cast) may be applied to a variable. Unlike C, where you can do 'useful' things like cast a double to a function call, Oberon type guards only apply to extended records. Using the example from the section on RECORDs above:

  VAR
     Calendar: DatePtr;
     ExactCalendar: SecondPtr;
  ...
     NEW(ExctCalendar); (* points to SecondOfYear record *)
     Calendar := ExactCalendar; (* Calendar is a pointer to Date, the base
                                class of SecondOfYear *)
     Calendar(ExactCalendar).Hour := 11; (* need the typeguard else the
                                         compiler will assume the static
                                         type Date which does not include
                                         Hour *)

The operators may be logical (OR & ~) (Why it isn't AND/OR or &/| I'll never know!), arithmetic (+ - * / DIV MOD), set (+ - * /) or relational (= # [not equal] < <= > >= IN [test for set membership] IS [test for type]).

Statements

Statements may be assignment (x := 1), procedure calls or a compound statement. Compound statements are either the contents of BEGIN and END or one of the built in language statements. Statements are always separated by semicolons.

The built in statements are as follows. Items in square brackets are optional.

If

IF expression THEN
   statements
[ELSIF expression THEN
   statements]
[ELSE
   statements]
END

Case

Here 'label' may be an integral type (or character), a range, indication by two dots (e.g., 0..59), or a CONST. The ELSE clause acts like the default: for a C/C++ switch.

CASE expression OF
   label:
      statements
[|  label:
      statements]
[ELSE
      statements]
END

While

WHILE expression DO
   statements
END

Repeat

REPEAT
   statements
UNTIL expression

Loop

LOOP
   statements (* must include an EXIT statement *)
END

For

The FOR statement does not exist at all in Oberon-1. WHILE, REPEAT or LOOP statements must be used instead.

FOR variable := expression TO expression [BY expression] DO
   statements
END

With

This performs a test on the type of a variable, and executes statements similarly to the CASE statement. If no ELSE clause is supplied and none of the supplied label types match the variable, the program will end.

WITH guard DO
   statements
[| guard DO
   statements]
[ELSE
   statements]
END

Example:

WITH Calendar: DatePtr DO
   (* statements appropriate to an object of type Date *)
| Calendar: SecondOfYear DO
   (* statements appropriate to an object of type SecondOfYear *)
ELSE
   (* default statements *)
END

Note that Oberon-1 does not have the option of having more than one guard test in the WITH statement.