Meeting Pearls 1/Meeting Pearls Vol 1 (1994).iso/amok98-106/amok104/oberon-a/texts.lha/Oberon2.Report.Text

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The Programming Language Oberon-2

H. Mössenböck, N. Wirth
Institut für Computersysteme, ETH Zürich
October 1993



1. Introduction

Oberon-2 is a general-purpose language in the tradition of Oberon and
Modula-2. Its most important features are block structure, modularity,
separate compilation, static typing with strong type checking (also
across module boundaries), and type extension with type-bound
procedures.

Type extension makes Oberon-2 an object-oriented language. An object is
a variable of an abstract data type consisting of private data (its
state) and procedures that operate on this data. Abstract data types are
declared as extensible records. Oberon-2 covers most terms of
object-oriented languages by the established vocabulary of imperative
languages in order to minimize the number of notions for similar
concepts.

This report is not intended as a programmer's tutorial. It is
intentionally kept concise. Its function is to serve as a reference for
programmers, implementors, and manual writers. What remains unsaid is
mostly left so intentionally, either because it can be derived from
stated rules of the language, or because it would require to commit the
definition when a general commitment appears as unwise.

Appendix A defines some terms that are used to express the type checking
rules of Oberon-2. Where they appear in the text, they are written in
italics to indicate their special meaning (e.g. the same type).


2. Syntax

An extended Backus-Naur Formalism (EBNF) is used to describe the syntax
of Oberon-2: Alternatives are separated by |. Brackets [ and ] denote
optionality of the enclosed expression, and braces { and } denote its
repetition (possibly 0 times). Non-terminal symbols start with an
upper-case letter (e.g. Statement). Terminal symbols either start with a
lower-case letter (e.g. ident), or are written all in upper-case letters
(e.g. BEGIN), or are denoted by strings (e.g. ":=").


3. Vocabulary and Representation

The representation of (terminal) symbols in terms of characters is
defined using the ASCII set. Symbols are identifiers, numbers, strings,
operators, and delimiters. The following lexical rules must be observed:
Blanks and line breaks must not occur within symbols (except in
comments, and blanks in strings). They are ignored unless they are
essential to separate two consecutive symbols. Capital and lower-case
letters are considered as distinct.

1. Identifiers are sequences of letters and digits. The first character
must be a letter.

ident = letter {letter | digit}.

Examples:     x     Scan     Oberon2     GetSymbol     firstLetter

2. Numbers are (unsigned) integer or real constants. The type of an
integer constant is the minimal type to which the constant value belongs
(see 6.1). If the constant is specified with the suffix H, the
representation is hexadecimal otherwise the representation is decimal.

A real number always contains a decimal point. Optionally it may also
contain a decimal scale factor. The letter E (or D) means "times ten to
the power of". A real number is of type REAL, unless it has a scale
factor containing the letter D. In this case it is of type LONGREAL.

number   = integer | real.
integer   = digit {digit} | digit {hexDigit} "H".
real   = digit {digit} "." {digit} [ScaleFactor].
ScaleFactor   = ("E" | "D") ["+" | "-"] digit {digit}.
hexDigit   = digit | "A" | "B" | "C" | "D" | "E" | "F".
digit   = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9".

Examples:
1991  INTEGER  1991
0DH   SHORTINT  13
12.3  REAL  12.3
4.567E8   REAL  456700000
0.57712566D-6   LONGREAL  0.00000057712566

3. Character constants are denoted by the ordinal number of the
character in hexadecimal notation followed by the letter X.

character  = digit {hexDigit} "X".

4. Strings are sequences of characters enclosed in single (') or double
(") quote marks. The opening quote must be the same as the closing quote
and must not occur within the string. The number of characters in a
string is called its length. A string of length 1 can be used wherever a
character constant is allowed and vice versa.

string  = ' " ' {char} ' " ' | " ' " {char} " ' ".

Examples:     "Oberon-2"     "Don't worry!"     "x"

5. Operators and delimiters are the special characters, character pairs,
or reserved words listed below. The reserved words consist exclusively
of capital letters and cannot be used as identifiers.

+  :=  ARRAY  IMPORT  RETURN
-  ^  BEGIN  IN  THEN
*  =  BY  IS  TO
/  #  CASE  LOOP  TYPE
~  <  CONST  MOD  UNTIL
&  >  DIV  MODULE  VAR
.  <=  DO  NIL  WHILE
,  >=  ELSE  OF  WITH
;  ..  ELSIF  OR
|  :  END  POINTER
(  )  EXIT  PROCEDURE
[  ]  FOR  RECORD
{  }  IF  REPEAT

6. Comments may be inserted between any two symbols in a program. They
are arbitrary character sequences opened by the bracket (* and closed by
*). Comments may be nested. They do not affect the meaning of a program.


4. Declarations and scope rules

Every identifier occurring in a program must be introduced by a
declaration, unless it is a predeclared identifier. Declarations also
specify certain permanent properties of an object, such as whether it is
a constant, a type, a variable, or a procedure. The identifier is then
used to refer to the associated object.

  The scope of an object x extends textually from the point of its
declaration to the end of the block (module, procedure, or record) to
which the declaration belongs and hence to which the object is local. It
excludes the scopes of equally named objects which are declared in
nested blocks. The scope rules are:

  1.  No identifier may denote more than one object within a given scope
    (i.e. no identifier may be declared twice in a block);
  2.  An object may only be referenced within its scope;
  3.  A type T of the form POINTER TO T1 (see 6.4) can be declared at a
    point where T1 is still unknown. The declaration of T1 must follow
    in the same block to which T is local;
  4.  Identifiers denoting record fields (see 6.3) or type-bound
    procedures (see 10.2) are valid in record designators only.

An identifier declared in a module block may be followed by an export
mark (" * " or " - ") in its declaration to indicate that it is
exported. An identifier x exported by a module M may be used in other
modules, if they import M (see Ch.11). The identifier is then denoted as
M.x in these modules and is called a qualified identifier. Identifiers
marked with " - " in their declaration are read-only in importing
modules.

Qualident   = [ident "."] ident.
IdentDef   = ident [" * " | " - "].

The following identifiers are predeclared; their meaning is defined in
the indicated sections:

ABS  (10.3)  LEN  (10.3)
ASH  (10.3)  LONG  (10.3)
BOOLEAN  (6.1)  LONGINT  (6.1)
CAP  (10.3)  LONGREAL  (6.1)
CHAR  (6.1)  MAX  (10.3)
CHR  (10.3)  MIN  (10.3)
COPY  (10.3)  NEW  (10.3)
DEC  (10.3)  ODD  (10.3)
ENTIER  (10.3)  ORD  (10.3)
EXCL  (10.3)  REAL  (6.1)
FALSE  (6.1)  SET  (6.1)
HALT  (10.3)  SHORT  (10.3)
INC  (10.3)  SHORTINT  (6.1)
INCL  (10.3)  SIZE  (10.3)
INTEGER  (6.1)  TRUE  (6.1)


5. Constant declarations

A constant declaration associates an identifier with a constant value.

ConstantDeclaration   = IdentDef "=" ConstExpression.
ConstExpression   = Expression.

A constant expression is an expression that can be evaluated by a mere
textual scan without actually executing the program. Its operands are
constants (Ch.8) or predeclared functions (Ch.10.3) that can be
evaluated at compile time. Examples of constant declarations are:

N = 100
limit = 2*N - 1
fullSet = {MIN(SET) .. MAX(SET)}


6. Type declarations

A data type determines the set of values which variables of that type
may assume, and the operators that are applicable. A type declaration
associates an identifier with a type. In the case of structured types
(arrays and records) it also defines the structure of variables of this
type. A structured type cannot contain itself.

TypeDeclaration   = IdentDef "=" Type.
Type   = Qualident | ArrayType | RecordType | PointerType | ProcedureType.

Examples:

Table = ARRAY N OF REAL
Tree = POINTER TO Node
Node =  RECORD
  key : INTEGER;
  left, right: Tree
END
CenterTree = POINTER TO CenterNode
CenterNode = RECORD (Node)
  width: INTEGER;
  subnode: Tree
END
Function = PROCEDURE(x: INTEGER): INTEGER


6.1 Basic types

The basic types are denoted by predeclared identifiers. The associated
operators are defined in 8.2 and the predeclared function procedures in
10.3. The values of the given basic types are the following:

1.  BOOLEAN  the truth values TRUE and FALSE
2.  CHAR  the characters of the extended ASCII set (0X .. 0FFX)
3.  SHORTINT  the integers between MIN(SHORTINT) and MAX(SHORTINT)
4.  INTEGER  the integers between MIN(INTEGER) and MAX(INTEGER)
5.  LONGINT  the integers between MIN(LONGINT) and MAX(LONGINT)
6.  REAL  the real numbers between MIN(REAL) and MAX(REAL)
7.  LONGREAL  the real numbers between MIN(LONGREAL) and MAX(LONGREAL)
8.  SET  the sets of integers between 0 and MAX(SET)

Types 3 to 5 are integer types, types 6 and 7 are real types, and
together they are called numeric types. They form a hierarchy; the
larger type includes (the values of) the smaller type:

  LONGREAL  >=  REAL  >=  LONGINT  >=  INTEGER  >=  SHORTINT


6.2 Array types

An array is a structure consisting of a number of elements which are all
of the same type, called the element type. The number of elements of an
array is called its length. The elements of the array are designated by
indices, which are integers between 0 and the length minus 1.

ArrayType   = ARRAY [Length {"," Length}] OF Type.
Length   = ConstExpression.

A type of the form

  ARRAY L0, L1, ..., Ln OF T

is understood as an abbreviation of

  ARRAY L0 OF
    ARRAY L1 OF
    ...
      ARRAY Ln OF T

Arrays declared without length are called open arrays. They are
restricted to pointer base types (see 6.4), element types of open array
types, and formal parameter types (see 10.1). Examples:

  ARRAY 10, N OF INTEGER
  ARRAY OF CHAR


6.3 Record types

A record type is a structure consisting of a fixed number of elements,
called fields, with possibly different types. The record type
declaration specifies the name and type of each field. The scope of the
field identifiers extends from the point of their declaration to the end
of the record type, but they are also visible within designators
referring to elements of record variables (see 8.1). If a record type is
exported, field identifiers that are to be visible outside the declaring
module must be marked. They are called public fields; unmarked elements
are called private fields.

RecordType   = RECORD ["("BaseType")"] FieldList {";" FieldList} END.
BaseType   = Qualident.
FieldList   = [IdentList ":" Type ].

Record types are extensible, i.e. a record type can be declared as an
extension of another record type. In the example

  T0 = RECORD x: INTEGER END
  T1 = RECORD (T0) y: REAL END

T1 is a (direct) extension of T0 and T0 is the (direct) base type of T1
(see App. A). An extended type T1 consists of the fields of its base
type and of the fields which are declared in T1. All identifiers
declared in the extended record must be different from the identifiers
declared in its base type record(s).

Examples of record type declarations:

RECORD
  day, month, year: INTEGER
END

RECORD
  name, firstname: ARRAY 32 OF CHAR;
  age: INTEGER;
  salary: REAL
END


6.4 Pointer types

Variables of a pointer type P assume as values pointers to variables of
some type T. T is called the pointer base type of P and must be a record
or array type. Pointer types adopt the extension relation of their
pointer base types: if a type T1 is an extension of T, and P1 is of type
POINTER TO T1, then P1 is also an extension of P.

  PointerType = POINTER TO Type.

If p is a variable of type P = POINTER TO T, a call of the predeclared
procedure NEW(p) (see 10.3) allocates a variable of type T in free
storage. If T is a record type or an array type with fixed length, the
allocation has to be done with NEW(p); if T is an n-dimensional open
array type the allocation has to be done with NEW(p, e0, ..., en-1)
where T is allocated with lengths given by the expressions e0, ...,
en-1. In either case a pointer to the allocated variable is assigned to
p. p is of type P. The referenced  variable p^ (pronounced as
p-referenced) is of type T. Any pointer variable may assume the value
NIL, which points to no variable at all.


6.5 Procedure types

Variables of a procedure type T have a procedure (or NIL) as value. If a
procedure P is assigned to a variable of type T, the formal parameter
lists (see Ch. 10.1) of P and T must match (see App. A). P must not be a
predeclared or type-bound procedure nor may it be local to another
procedure.

  ProcedureType = PROCEDURE [FormalParameters].


7. Variable declarations

Variable declarations introduce variables by defining an identifier and
a data type for them.

  VariableDeclaration = IdentList ":" Type.

Record and pointer variables have both a static type (the type with
which they are declared - simply called their type) and a dynamic type
(the type of their value at run time). For pointers and variable
parameters of record type the dynamic type may be an extension of their
static type. The static type determines which fields of a record are
accessible. The dynamic type is used to call type-bound procedures (see
10.2).

Examples of variable declarations (refer to examples in Ch. 6):

i, j, k: INTEGER
x, y: REAL
p, q: BOOLEAN
s: SET
F: Function
a: ARRAY 100 OF REAL
w: ARRAY 16 OF RECORD
    name: ARRAY 32 OF CHAR;
    count: INTEGER
  END
t, c: Tree


8. Expressions

Expressions are constructs denoting rules of computation whereby
constants and current values of variables are combined to compute other
values by the application of operators and function procedures.
Expressions consist of operands and operators. Parentheses may be used
to express specific associations of operators and operands.


8.1 Operands

With the exception of set constructors and literal constants (numbers,
character constants, or strings), operands are denoted by designators. A
designator consists of an identifier referring to a constant, variable,
or procedure. This identifier may possibly be qualified by a module
identifier (see Ch. 4 and 11) and may be followed by selectors if the
designated object is an element of a structure.

Designator   = Qualident {"." ident | "[" ExpressionList "]" | "^" |
    "(" Qualident ")"}.
ExpressionList   = Expression {"," Expression}.

If a designates an array, then a[e] denotes that element of a whose
index is the current value of the expression e. The type of e must be an
integer type. A designator of the form a[e0, e1, ..., en] stands for
a[e0][e1]...[en]. If r designates a record, then r.f denotes the field f
of r or the procedure f bound to the dynamic type of r (Ch. 10.2). If p
designates a pointer, p^ denotes the variable which is referenced by p.
The designators p^.f and p^[e] may be abbreviated as p.f and p[e], i.e.
record and array selectors imply dereferencing. If a or r are read-only,
then also a[e] and r.f are read-only.

  A type guard v(T) asserts that the dynamic type of v is T (or an
extension of T), i.e. program execution is aborted, if the dynamic type
of v is not T (or an extension of T). Within the designator, v is then
regarded as having the static type T. The guard is applicable, if

  1.  v is a variable parameter of record type or v is a pointer, and if
  2.  T is an extension of the static type of v

If the designated object is a constant or a variable, then the
designator refers to its current value. If it is a procedure, the
designator refers to that procedure unless it is followed by a (possibly
empty) parameter list in which case it implies an activation of that
procedure and stands for the value resulting from its execution. The
actual parameters must correspond to the formal parameters as in proper
procedure calls (see 10.1).

Examples of designators (refer to examples in Ch.7):

i  (INTEGER)
a[i]  (REAL)
w[3].name[i]  (CHAR)
t.left.right  (Tree)
t(CenterTree).subnode  (Tree)


8.2 Operators

Four classes of operators with different precedences (binding strengths)
are syntactically distinguished in expressions. The operator ~ has the
highest precedence, followed by multiplication operators, addition
operators, and relations. Operators of the same precedence associate
from left to right. For example, x-y-z stands for (x-y)-z.

Expression   = SimpleExpression [Relation SimpleExpression].
SimpleExpression  = ["+" | "-"] Term {AddOperator Term}.
Term   = Factor {MulOperator Factor}.
Factor   = Designator [ActualParameters] |
      number | character | string | NIL | Set | "(" Expression ")" |
      "~" Factor.
Set   = "{" [Element {"," Element}] "}".
Element   = Expression [".." Expression].
ActualParameters   = "(" [ExpressionList] ")".
Relation   = "=" | "#" | "<" | "<=" | ">" | ">=" | IN | IS.
AddOperator   = "+" | "-" | OR.
MulOperator   = "*" | "/" | DIV | MOD | "&".

The available operators are listed in the following tables. Some
operators are applicable to operands of various types, denoting
different operations. In these cases, the actual operation is identified
by the type of the operands. The operands must be expression compatible
with respect to the operator (see App. A).

8.2.1 Logical operators

OR  logical disjunction   p OR q    "if p then TRUE, else q"
&  logical conjunction   p & q    "if p then q, else FALSE"
~  negation   ~ p    "not p"

These operators apply to BOOLEAN operands and yield a BOOLEAN result.

8.2.2 Arithmetic operators

+  sum
-  difference
*  product
/  real quotient
DIV  integer quotient
MOD  modulus

The operators +, -, *, and / apply to operands of numeric types. The
type of the result is the type of that operand which includes the type
of the other operand, except for division (/), where the result is the
smallest real type which includes both operand types. When used as
monadic operators, - denotes sign inversion and + denotes the identity
operation. The operators DIV and MOD apply to integer operands only.
They are related by the following formulas defined for any x and
positive divisors y:

x = (x DIV y) * y + (x MOD y)
0 <= (x MOD y) < y

Examples:
x  y  x DIV y  x MOD y
5  3  1  2
-5  3  -2  1

8.2.3 Set Operators

+  union
-  difference (x - y = x * (-y))
*  intersection
/  symmetric set difference (x / y = (x-y) + (y-x))

Set operators apply to operands of type SET and yield a result of type
SET. The monadic minus sign denotes the complement of x, i.e. -x denotes
the set of integers between 0 and MAX(SET) which are not elements of x.
Set operators are not associative ((a+b)-c # a+(b-c)).

A set constructor defines the value of a set by listing its elements
between curly brackets. The elements must be integers in the range
0..MAX(SET). A range a..b denotes all integers in the interval [a, b].

8.2.4 Relations

=  equal
#  unequal
<  less
<=  less or equal
>  greater
>=  greater or equal
IN  set membership
IS  type test

Relations yield a BOOLEAN result. The relations =, #, <, <=, >, and >=
apply to the numeric types, CHAR, strings, and character arrays
containing 0X as a terminator. The relations = and # also apply to
BOOLEAN and SET, as well as to pointer and procedure types (including
the value NIL). x IN s stands for "x is an element of s". x must be of
an integer type, and s of type SET. v IS T stands for "the dynamic type
of v is T (or an extension of T)" and is called a type test. It is
applicable if

1.  v is a variable parameter of record type or v is a pointer, and if
2.  T is an extension of the static type of v

Examples of expressions (refer to examples in Ch.7):

1991  INTEGER
i DIV 3  INTEGER
~p OR q  BOOLEAN
(i+j) * (i-j)  INTEGER
s - {8, 9, 13}  SET
i + x  REAL
a[i+j] * a[i-j]  REAL
(0<=i) & (i<100)  BOOLEAN
t.key = 0  BOOLEAN
k IN {i..j-1}  BOOLEAN
w[i].name <= "John"  BOOLEAN
t IS CenterTree  BOOLEAN


9. Statements

Statements denote actions. There are elementary and structured
statements. Elementary statements are not composed of any parts that are
themselves statements. They are the assignment, the procedure call, the
return, and the exit statement. Structured statements are composed of
parts that are themselves statements. They are used to express
sequencing and conditional, selective, and repetitive execution. A
statement may also be empty, in which case it denotes no action. The
empty statement is included in order to relax punctuation rules in
statement sequences.

  Statement =
    [ Assignment | ProcedureCall | IfStatement | CaseStatement |
      WhileStatement | RepeatStatement | ForStatement | LoopStatement |
      WithStatement | EXIT | RETURN [Expression] ].


9.1 Assignments

Assignments replace the current value of a variable by a new value
specified by an expression. The expression must be assignment compatible
with the variable (see App. A). The assignment operator is written as
":=" and pronounced as becomes.

  Assignment = Designator ":=" Expression.

If an expression e of type Te is assigned to a variable v of type Tv,
the following happens:

  1.  if Tv and Te are record types, only those fields of Te are
    assigned which also belong to Tv (projection); the dynamic type of v
    must be the same as the static type of v and  is not changed by the
    assignment;
  2.  if Tv and Te are pointer types, the dynamic type of v becomes the
    dynamic type of e;
  3.  if Tv is ARRAY n OF CHAR and e is a string of length m<n, v[i]
    becomes ei for i = 0..m-1 and v[m] becomes 0X.

Examples of assignments (refer to examples in Ch.7):

i := 0
p := i = j
x := i + 1
k := log2(i+j)
F := log2    (* see 10.1 *)
s := {2, 3, 5, 7, 11, 13}
a[i] := (x+y) * (x-y)
t.key := i
w[i+1].name := "John"
t := c

9.2 Procedure calls

A procedure call activates a procedure. It may contain a list of actual
parameters which replace the corresponding formal parameters defined in
the procedure declaration (see Ch. 10). The correspondence is
established by the positions of the parameters in the actual and formal
parameter lists. There are two kinds of parameters: variable and value
parameters.

If a formal parameter is a variable parameter, the corresponding actual
parameter must be a designator denoting a variable. If it denotes an
element of a structured variable, the component selectors are evaluated
when the formal/actual parameter substitution takes place, i.e. before
the execution of the procedure. If a formal parameter is a value
parameter, the corresponding actual parameter must be an expression.
This expression is evaluated before the procedure activation, and the
resulting value is assigned to the formal parameter (see also 10.1).

  ProcedureCall = Designator [ActualParameters].

Examples:

WriteInt(i*2+1)  (* see 10.1 *)
INC(w[k].count)
t.Insert("John")  (* see 11 *)


9.3 Statement sequences

Statement sequences denote the sequence of actions specified by the
component statements which are separated by semicolons.

  StatementSequence = Statement {";" Statement}.


9.4 If statements

  IfStatement =
    IF Expression THEN StatementSequence
    {ELSIF Expression THEN StatementSequence}
    [ELSE StatementSequence]
    END.

If statements specify the conditional execution of guarded statement
sequences. The Boolean expression preceding a statement sequence is
called its guard. The guards are evaluated in sequence of occurrence,
until one evaluates to TRUE, whereafter its associated statement
sequence is executed. If no guard is satisfied, the statement sequence
following the symbol ELSE is executed, if there is one.

Example:

IF (ch >= "A") & (ch <= "Z") THEN ReadIdentifier
ELSIF (ch >= "0") & (ch <= "9") THEN ReadNumber
ELSIF (ch = " ' ") OR (ch = ' " ') THEN ReadString
ELSE SpecialCharacter
END


9.5 Case statements

Case statements specify the selection and execution of a statement
sequence according to the value of an expression. First the case
expression is evaluated, then that statement sequence is executed whose
case label list contains the obtained value. The case expression must
either be of an integer type that includes the types of all case labels,
or both the case expression and the case labels must be of type CHAR.
Case labels are constants, and no value must occur more than once. If
the value of the expression does not occur as a label of any case, the
statement sequence following the symbol ELSE is selected, if there is
one, otherwise the program is aborted.

CaseStatement   = CASE Expression OF Case {"|" Case} [ELSE
  StatementSequence] END.
Case   = [CaseLabelList ":" StatementSequence].
CaseLabelList   = CaseLabels {"," CaseLabels}.
CaseLabels   = ConstExpression [".." ConstExpression].

Example:

CASE ch OF
  "A" .. "Z": ReadIdentifier
|  "0" .. "9": ReadNumber
|  " ' ", ' " ': ReadString
ELSE SpecialCharacter
END


9.6 While statements

While statements specify the repeated execution of a statement sequence
while the Boolean expression (its guard) yields TRUE. The guard is
checked before every execution of the statement sequence.

  WhileStatement = WHILE Expression DO StatementSequence END.

Examples:
WHILE i > 0 DO i := i DIV 2; k := k + 1 END
WHILE (t # NIL) & (t.key # i) DO t := t.left END


9.7 Repeat statements

A repeat statement specifies the repeated execution of a statement
sequence until a condition specified by a Boolean expression is
satisfied. The statement sequence is executed at least once.

  RepeatStatement = REPEAT StatementSequence UNTIL Expression.


9.8 For statements

A for statement specifies the repeated execution of a statement sequence
while a progression of values is assigned to an integer variable called
the control variable of the for statement.

  ForStatement = FOR ident ":=" Expression TO Expression [BY
    ConstExpression] DO StatementSequence END.

The statement

  FOR v := beg TO end BY step DO statements END

is equivalent to

temp := end; v := beg;
IF step > 0 THEN
  WHILE v <= temp DO statements; v := v + step END
ELSE
  WHILE v >= temp DO statements; v := v + step END
END

temp has the same type as v. step must be a nonzero constant expression.
If step is not specified, it is assumed to be 1.

Examples:
FOR i := 0 TO 79 DO k := k + a[i] END
FOR i := 79 TO 1 BY -1 DO a[i] := a[i-1] END


9.9 Loop statements

A loop statement specifies the repeated execution of a statement
sequence. It is terminated upon execution of an exit statement within
that sequence (see 9.10).

  LoopStatement = LOOP StatementSequence END.

Example:
LOOP
  ReadInt(i);
  IF i < 0 THEN EXIT END;
  WriteInt(i)
END

Loop statements are useful to express repetitions with several exit
points or cases where the exit condition is in the middle of the
repeated statement sequence.


9.10 Return and exit statements

A return statement indicates the termination of a procedure. It is
denoted by the symbol RETURN, followed by an expression if the procedure
is a function procedure. The type of the expression must be assignment
compatible (see App. A) with the result type specified in the procedure
heading (see Ch.10).

  Function procedures require the presence of a return statement
indicating the result value. In proper procedures, a return statement is
implied by the end of the procedure body. Any explicit return statement
therefore appears as an additional (probably exceptional) termination
point.

  An exit statement is denoted by the symbol EXIT. It specifies
termination of the enclosing loop statement and continuation with the
statement following that loop statement. Exit statements are
contextually, although not syntactically associated with the loop
statement which contains them.


9.11 With statements

With statements execute a statement sequence depending on the result of
a type test and apply a type guard to every occurrence of the tested
variable within this statement sequence.

WithStatement   = WITH Guard DO StatementSequence {"|" Guard DO StatementSequence}
    [ELSE StatementSequence] END.
Guard  = Qualident ":" Qualident.

If v is a variable parameter of record type or a pointer variable, and
if it is of a static type T0, the statement

  WITH v: T1 DO S1 | v: T2 DO S2 ELSE S3 END

has the following meaning: if the dynamic type of v is T1, then the
statement sequence S1 is executed where v is regarded as if it had the
static type T1; else if the dynamic type of v is T2, then S2 is executed
where v is regarded as if it had the static type T2; else S3 is
executed. T1 and T2 must be extensions of T0. If no type test is
satisfied and if an else clause is missing the program is aborted.

Example:
  WITH t: CenterTree DO i := t.width; c := t.subnode END


10. Procedure declarations

A procedure declaration consists of a procedure heading and a procedure
body. The heading specifies the procedure identifier and the formal
parameters. For type-bound procedures it also specifies the receiver
parameter. The body contains declarations and statements. The procedure
identifier is repeated at the end of the procedure declaration.

  There are two kinds of procedures: proper procedures and function
procedures. The latter are activated by a function designator as a
constituent of an expression and yield a result that is an operand of
the expression. Proper procedures are activated by a procedure call. A
procedure is a function procedure if its formal parameters specify a
result type. The body of a function procedure must contain a return
statement which defines its result.

  All constants, variables, types, and procedures declared within a
procedure body are local to the procedure. Since procedures may be
declared as local objects too, procedure declarations may be nested. The
call of a procedure within its declaration implies recursive activation.

  Objects declared in the environment of the procedure are also visible
in those parts of the procedure in which they are not concealed by a
locally declared object with the same name.

ProcedureDeclaration  = ProcedureHeading ";" ProcedureBody ident.
ProcedureHeading   = PROCEDURE [Receiver] IdentDef [FormalParameters].
ProcedureBody   = DeclarationSequence [BEGIN StatementSequence] END.
DeclarationSequence   =
  {CONST {ConstantDeclaration ";"} | TYPE {TypeDeclaration ";"} | VAR {VariableDeclaration
";"} }
  {ProcedureDeclaration ";" | ForwardDeclaration ";"}.
ForwardDeclaration   = PROCEDURE " ^ " [Receiver] IdentDef [FormalParameters].

If a procedure declaration specifies a receiver parameter, the procedure
is considered to be bound to a type (see 10.2). A forward declaration
serves to allow forward references to a procedure whose actual
declaration appears later in the text. The formal parameter lists of the
forward declaration and the actual declaration must match (see App. A).


10.1 Formal parameters

Formal parameters are identifiers declared in the formal parameter list
of a procedure. They correspond to actual parameters specified in the
procedure call. The correspondence between formal and actual parameters
is established when the procedure is called. There are two kinds of
parameters, value and variable parameters, indicated in the formal
parameter list by the absence or presence of the keyword VAR. Value
parameters are local variables to which the value of the corresponding
actual parameter is assigned as an initial value. Variable parameters
correspond to actual parameters that are variables, and they stand for
these variables. The scope of a formal parameter extends from its
declaration to the end of the procedure block in which it is declared. A
function procedure without parameters must have an empty parameter list.
It must be called by a function designator whose actual parameter list
is empty too. The result type of a procedure can be neither a record nor
an array.

FormalParameters  = "(" [FPSection {";" FPSection}] ")" [":" Qualident].
FPSection   = [VAR] ident {"," ident} ":" Type.

Let Tf be the type of a formal parameter f (not an open array) and Ta
the type of the corresponding actual parameter a. For variable
parameters, Ta must be the same as Tf, or Tf must be a record type and
Ta an extension of Tf. For value parameters, a must be assignment
compatible with f (see App. A).

  If Tf is an open array , then a must be array compatible with f (see
App. A). The lengths of f are taken from a.

Examples of procedure declarations:

PROCEDURE ReadInt(VAR x: INTEGER);
  VAR i: INTEGER; ch: CHAR;
BEGIN i := 0; Read(ch);
  WHILE ("0" <= ch) & (ch <= "9") DO
    i := 10*i + (ORD(ch)-ORD("0")); Read(ch)
  END;
  x := i
END ReadInt

PROCEDURE WriteInt(x: INTEGER); (*0 <= x <100000*)
  VAR i: INTEGER; buf: ARRAY 5 OF INTEGER;
BEGIN i := 0;
  REPEAT buf[i] := x MOD 10; x := x DIV 10; INC(i) UNTIL x = 0;
  REPEAT DEC(i); Write(CHR(buf[i] + ORD("0"))) UNTIL i = 0
END WriteInt

PROCEDURE WriteString(s: ARRAY OF CHAR);
  VAR i: INTEGER;
BEGIN i := 0;
  WHILE (i < LEN(s)) & (s[i] # 0X) DO Write(s[i]); INC(i) END
END WriteString;


PROCEDURE log2(x: INTEGER): INTEGER;
  VAR y: INTEGER; (*assume x>0*)
BEGIN
  y := 0; WHILE x > 1 DO x := x DIV 2; INC(y) END;
  RETURN y
END log2


10.2 Type-bound procedures

Globally declared procedures may be associated with a record type
declared in the same module. The procedures are said to be bound to the
record type. The binding is expressed by the type of the receiver in the
heading of a procedure declaration.  The receiver may be either a
variable parameter of record type T or a value parameter of type POINTER
TO T (where T is a record type). The procedure is bound to the type T
and is considered local to it.

ProcedureHeading  = PROCEDURE [Receiver] IdentDef [FormalParameters].
Receiver   = "(" [VAR] ident ":" ident ")".

If a procedure P is bound to a type T0, it is implicitly also bound to
any type T1 which is an extension of T0. However, a procedure P' (with
the same name as P) may be explicitly bound to T1 in which case it
overrides the binding of P. P' is considered a redefinition of P for T1.
The formal parameters of P and P' must match (see App. A). If P and T1
are exported (see Chapter 4) P' must be exported too.

  If v is a designator and P is a type-bound procedure, then v.P denotes
that procedure P which is bound to the dynamic type of v. Note, that
this may be a different procedure than the one bound to the static type
of v. v is passed to P's receiver according to the parameter passing
rules specified in Chapter 10.1.

  If r is a receiver parameter declared with type T, r.P^ denotes the
(redefined) procedure P bound to the base type of T. In a forward
declaration of a type-bound procedure the receiver parameter must be of
the same type as in the actual procedure declaration. The formal
parameter lists of both declarations must match (App. A).

Examples:

PROCEDURE (t: Tree) Insert (node: Tree);
  VAR p, father: Tree;
BEGIN p := t;
  REPEAT father := p;
    IF node.key = p.key THEN RETURN END;
    IF node.key < p.key THEN p := p.left ELSE p := p.right END
  UNTIL p = NIL;
  IF node.key < father.key THEN father.left := node ELSE father.right := node
END;
  node.left := NIL; node.right := NIL
END Insert;

PROCEDURE (t: CenterTree) Insert (node: Tree);  (*redefinition*)
BEGIN
  WriteInt(node(CenterTree).width);
  t.Insert^ (node)  (* calls the Insert procedure bound to Tree *)
END Insert;


10.3 Predeclared procedures

The following table lists the predeclared procedures. Some are generic
procedures, i.e. they apply to several types of operands. v stands for a
variable, x and n for expressions, and T for a type.

Function procedures

Name  Argument type  Result type  Function

ABS(x)  numeric type  type of x  absolute value
ASH(x, n)  x, n: integer type  LONGINT  arithmetic shift (x * 2n)
CAP(x)  CHAR  CHAR  x is letter: corresponding capital letter
CHR(x)  integer type  CHAR  character with ordinal number x
ENTIER(x)  real type  LONGINT  largest integer not greater than x
LEN(v, n)  v: array; n: integer const.  LONGINT  length of v in dimension n
(first dimension = 0)
LEN(v)  v: array  LONGINT  equivalent to LEN(v, 0)
LONG(x)  SHORTINT  INTEGER  identity
  INTEGER  LONGINT
  REAL  LONGREAL
MAX(T)  T = basic type  T  maximum value of type T
  T = SET  INTEGER  maximum element of a set
MIN(T)  T = basic type  T  minimum value of type T
  T = SET  INTEGER  0
ODD(x)  integer type  BOOLEAN  x MOD 2 = 1
ORD(x)  CHAR  INTEGER  ordinal number of x
SHORT(x)  LONGINT  INTEGER  identity
  INTEGER  SHORTINT  identity
  LONGREAL  REAL  identity (truncation possible)
SIZE(T)  any type  integer type  number of bytes required by T

Proper procedures

Name  Argument types  Function

ASSERT(x)  x: Boolean expression  terminate program execution if not x
ASSERT(x, n)   x: Boolean expression; n: integer constant  terminate program
execution if not x
COPY(x, v)  x: character array, string; v: character array  v := x
DEC(v)  integer type  v := v - 1
DEC(v, n)  v, n: integer type  v := v - n
EXCL(v, x)  v: SET; x: integer type  v := v - {x}
HALT(n)  integer constant  terminate program execution
INC(v)  integer type  v := v + 1
INC(v, n)  v, n: integer type  v := v + n
INCL(v, x)  v: SET; x: integer type  v := v + {x}
NEW(v)  pointer to record or fixed array  allocate v ^
NEW(v, x0, ..., xn) v: pointer to open array; xi: integer type  allocate v ^
with lengths x0.. xn

COPY allows the assignment of a string or a character array containing a
terminating 0X to another character array. If necessary, the assigned
value is truncated to the target length minus one. The target will
always contain 0X as a terminator. In ASSERT(x, n) and HALT(n), the
interpretation of n is left to the underlying system implementation.


11. Modules

A module is a collection of declarations of constants, types, variables,
and procedures, together with a sequence of statements for the purpose
of assigning initial values to the variables. A module constitutes a
text that is compilable as a unit.

Module   = MODULE ident ";" [ImportList] DeclarationSequence
      [BEGIN StatementSequence] END ident ".".
ImportList   = IMPORT Import {"," Import} ";".
Import   = [ident ":="] ident.

The import list specifies the names of the imported modules. If a module
A is imported by a module M and A exports an identifier x, then x is
referred to as A.x within M. If A is imported as B := A, the object x
must be referenced as B.x. This allows short alias names in qualified
identifiers. A module must not import itself. Identifiers that are to be
exported (i.e. that are to be visible in client modules) must be marked
by an export mark in their declaration (see Chapter 4).

  The statement sequence following the symbol BEGIN is executed when the
module is added to a system (loaded), which is done after the imported
modules have been loaded. It follows that cyclic import of modules is
illegal. Individual (parameterless and exported) procedures can be
activated from the system, and these procedures serve as commands (see
Appendix D1).

MODULE Trees;   (* exports: Tree, Node, Insert, Search, Write, Init *)
  IMPORT Texts, Oberon;  (* exports read-only: Node.name *)

  TYPE
    Tree* = POINTER TO Node;
    Node* = RECORD
      name-: POINTER TO ARRAY OF CHAR;
      left, right: Tree
    END;

  VAR w: Texts.Writer;

  PROCEDURE (t: Tree) Insert* (name: ARRAY OF CHAR);
    VAR p, father: Tree;
  BEGIN p := t;
    REPEAT father := p;
      IF name = p.name^ THEN RETURN END;
      IF name < p.name^ THEN p := p.left ELSE p := p.right END
    UNTIL p = NIL;
    NEW(p); p.left := NIL; p.right := NIL; NEW(p.name, LEN(name)+1); COPY(name,
p.name^);
    IF name < father.name^ THEN father.left := p ELSE father.right := p END
  END Insert;

  PROCEDURE (t: Tree) Search* (name: ARRAY OF CHAR): Tree;
    VAR p: Tree;
  BEGIN p := t;
    WHILE (p # NIL) & (name # p.name^) DO
      IF name < p.name^ THEN p := p.left ELSE p := p.right END
    END;
    RETURN p
  END Search;

  PROCEDURE (t: Tree) Write*;
  BEGIN
    IF t.left # NIL THEN t.left.Write END;
    Texts.WriteString(w, t.name^); Texts.WriteLn(w); Texts.Append(Oberon.Log,
w.buf);
    IF t.right # NIL THEN t.right.Write END
  END Write;

  PROCEDURE Init* (t: Tree);
  BEGIN NEW(t.name, 1); t.name[0] := 0X; t.left := NIL; t.right := NIL
  END Init;

BEGIN Texts.OpenWriter(w)
END Trees.

Appendix A: Definition of terms


Integer types  SHORTINT, INTEGER, LONGINT
Real types  REAL, LONGREAL
Numeric types  integer types, real types


Same types

Two variables a and b with types Ta and Tb are of the same type if

1.  Ta and Tb are both denoted by the same type identifier, or
2.  Ta is declared to equal Tb in a type declaration of the form Ta =
Tb, or
3.  a and b appear in the same identifier list in a variable, record
field, or formal parameter declaration
  and are not open arrays.


Equal types

Two types Ta and Tb are equal if

1.  Ta and Tb are the same type,  or
2.  Ta and Tb are open array types with equal element types, or
3.  Ta and Tb are procedure types whose formal parameter lists match.


Type inclusion

Numeric types include (the values of) smaller numeric types according to
the following hierarchy:

  LONGREAL >= REAL >= LONGINT >= INTEGER >= SHORTINT


Type extension (base type)

Given a type declaration Tb = RECORD (Ta) ... END, Tb is a direct
extension of Ta, and Ta is a direct base type of Tb. A type Tb is an
extension of a type Ta (Ta is a base type of Tb) if

1.  Ta and Tb are the same types, or
2.  Tb is a direct extension of an extension of Ta

If Pa = POINTER TO Ta and Pb = POINTER TO Tb, Pb is an extension of Pa
(Pa is a base type of Pb) if Tb is an extension of Ta.


Assignment compatible

An expression e of type Te is assignment compatible with a variable v of
type Tv if one of the following conditions hold:

1.  Te and Tv are the same type;
2.  Te and Tv are numeric types and Tv includes Te;
3.  Te and Tv are record types and Te is an extension of Tv and the
dynamic type of v is Tv ;
4.  Te and Tv are pointer types and Te is an extension of Tv;
5.  Tv is a pointer or a procedure type and e is NIL;
6.  Tv is ARRAY n OF CHAR, e is a string constant with m characters, and
m < n;
7.  Tv is a procedure type and e is the name of a procedure whose formal
parameters match those of Tv.


Array compatible

An actual parameter a of type Ta is array compatible with a formal
parameter f of type Tf if

1.  Tf and Ta are the same type, or
2.  Tf is an open array, Ta is any array, and their element types are
array compatible, or
3.  Tf is ARRAY OF CHAR and a is a string.


Expression compatible

For a given operator, the types of its operands are expression
compatible if they conform to the following table (which shows also the
result type of the expression). Character arrays that are to be compared
must contain 0X as a terminator. Type T1 must be an extension of type
T0:

operator   first operand  second operand   result type
+ - *  numeric  numeric  smallest numeric type including both operands
/  numeric  numeric  smallest real type including both operands
+ - * /  SET  SET  SET
DIV MOD  integer  integer  smallest integer type including both operands
OR & ~  BOOLEAN  BOOLEAN  BOOLEAN
= # < <= > >=  numeric  numeric  BOOLEAN
  CHAR  CHAR  BOOLEAN
  character array, string  character array, string  BOOLEAN
= #  BOOLEAN  BOOLEAN  BOOLEAN
  SET  SET  BOOLEAN
  NIL, pointer type T0 or T1  NIL, pointer type T0 or T1  BOOLEAN
  procedure type T, NIL  procedure type T, NIL  BOOLEAN
IN  integer  SET  BOOLEAN
IS  type T0  type T1  BOOLEAN


Matching formal parameter lists
Two formal parameter lists match if
1.  they have the same number of parameters, and
2.  they have either the same function result type or none, and
3.  parameters at corresponding positions have equal types, and
4.  parameters at corresponding positions are both either value or variable
parameters.


Appendix B: Syntax of Oberon-2


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 [" * " | "-"].


Appendix C: The module SYSTEM

The module SYSTEM contains certain types and procedures that are
necessary to implement low-level operations particular to a given
computer and/or implementation. These include for example facilities for
accessing devices that are controlled by the computer, and facilities to
break the type compatibility rules otherwise imposed by the language
definition. It is strongly recommended to restrict their use to specific
modules (called low-level modules). Such modules are inherently
non-portable, but easily recognized due to the identifier SYSTEM
appearing in their import list. The following specifications hold for
the implementation of Oberon-2 on the Ceres computer.

  Module SYSTEM exports a type BYTE with the following characteristics:
Variables of type CHAR or SHORTINT can be assigned to variables of type
BYTE. If a formal variable parameter is of type ARRAY OF BYTE then the
corresponding actual parameter may be of any type.

  Another type exported by module SYSTEM is the type PTR. Variables of
any pointer type may be assigned to variables of type PTR. If a formal
variable parameter is of type PTR, the actual parameter may be of any
pointer type.

  The procedures contained in module SYSTEM are listed in the following
tables. Most of them correspond to single instructions compiled as
in-line code. For details, the reader is referred to the processor
manual. v stands for a variable, x, y, a, and n for expressions, and T
for a type.

Function procedures

Name  Argument types  Result type  Function

ADR(v)  any  LONGINT  address of variable v
BIT(a, n)  a: LONGINT  BOOLEAN  bit n of Mem[a]
  n: integer
CC(n)  integer constant  BOOLEAN  condition n (0 <= n <= 15)
LSH(x, n)  x: integer, CHAR, BYTE  type of x  logical shift
  n: integer
ROT(x, n)  x: integer, CHAR, BYTE  type of x  rotation
  n: integer
VAL(T, x)  T, x: any type  T  x interpreted as of type T

Proper procedures

Name  Argument types  Function

GET(a, v)  a: LONGINT; v: any basic type,  v := Mem[a]
  pointer, procedure type
PUT(a, x)  a: LONGINT; x: any basic type,  Mem[a] := x
  pointer, procedure type
GETREG(n, v)  n: integer constant; v: any basic type,  v := Register n
  pointer, procedure type
PUTREG(n, x)  n: integer constant; x: any basic type,  Register n := x
  pointer, procedure type
MOVE(a0, a1, n)  a0, a1: LONGINT; n: integer  Mem[a1.. a1+n-1] := Mem[a0.. a0+n-1]
NEW(v, n)  v: any pointer; n: integer  allocate storage block of n bytes
    assign its address to v

Appendix D: The Oberon Environment

Oberon-2 programs usually run in an environment that provides command
activation, garbage collection, dynamic loading of modules, and certain
run time data structures. Although not part of the language, this
environment contributes to the power of Oberon-2 and is to some degree
implied by the language definition. Appendix D describes the essential
features of a typical Oberon environment and provides implementation
hints. More details can be found in [1], [2], and [3].


D1. Commands

A command is any parameterless procedure P that is exported from a
module M. It is denoted by M.P and can be activated under this name from
the shell of the operating system. In Oberon, a user invokes commands
instead of programs or modules. This gives him a finer grain of control
and allows modules with multiple entry points. When a command M.P is
invoked, the module M is dynamically loaded unless it is already in
memory (see D2) and the procedure P is executed. When P terminates, M
remains loaded. All global variables and data structures that can be
reached from global pointer variables in M retain their values. When P
(or another command of M) is invoked again, it may continue to use these
values.

  The following module demonstrates the use of commands. It implements
an abstract data structure Counter that encapsulates a counter variable
and provides commands to increment and print its value.

MODULE Counter;
  IMPORT Texts, Oberon;

  VAR
    counter: LONGINT;
    w: Texts.Writer;

  PROCEDURE Add*;   (* takes a numeric argument from the command line *)
    VAR s: Texts.Scanner;
  BEGIN
    Texts.OpenScanner(s, Oberon.Par.text, Oberon.Par.pos);
    Texts.Scan(s);
    IF s.class = Texts.Int THEN INC(counter, s.i) END
  END Add;

  PROCEDURE Write*;
  BEGIN
    Texts.WriteInt(w, counter, 5); Texts.WriteLn(w);
    Texts.Append(Oberon.Log, w.buf)
  END Write;

BEGIN counter := 0; Texts.OpenWriter(w)
END Counter.

The user may execute the following two commands:

Counter.Add   n   adds the value n to the variable counter
Counter.Write   writes the current value of counter to the screen

Since commands are parameterless they have to get their arguments from
the operating system. In general, commands are free to take arguments
from everywhere (e.g. from the text following the command, from the most
recent selection, or from a marked viewer). The command Add uses a
scanner (a data type provided by the Oberon system) to read the value
that follows it on the command line.

  When Counter.Add is invoked for the first time, the module Counter is
loaded and its body is executed. Every call of Counter.Add n increments
the variable counter by n. Every call of Counter.Write writes the
current value of counter to the screen.

  Since a module remains loaded after the execution of its commands,
there must be an explicit way to unload it (e.g. when the user wants to
substitute the loaded version by a recompiled version.) The Oberon
system provides a command to do that.


D2. Dynamic Loading of Modules

A loaded module may invoke a command of a still unloaded module by
specifying its name as a string. The specified module is then
dynamically loaded and the designated command is executed. Dynamic
loading allows the user to start a program as a small set of basic
modules and to extend it by adding further modules at run time as the
need becomes evident.

  A module M0 may cause the dynamic loading of a module M1 without
importing it. M1 may of course import and use M0, but M0 need not know
about the existence of M1. M1 can be a module that is designed and
implemented long after M0.


D3. Garbage Collection

In Oberon-2, the predeclared procedure NEW is used to allocate data
blocks in free memory. There is, however, no way to explicitly dispose
an allocated block. Rather, the Oberon environment uses a garbage
collector to find the blocks that are not used any more and to make them
available for allocation again. A block is in use as long as it can be
reached from a global pointer variable via a pointer chain. Cutting this
chain (e.g., setting a pointer to NIL) makes the block collectable.

  A garbage collector frees a programmer from the non-trivial task of
deallocating data structures correctly and thus helps to avoid errors.
However, it requires information about dynamic data at run time (see
D5).


D4. Browser

The interface of a module (the declaration of the exported objects) is
extracted from the module by a so-called browser which is a separate
tool of the Oberon environment. For example, the browser produces the
following interface of the module Trees from Ch. 11.

DEFINITION Trees;
  TYPE
    Tree = POINTER TO Node;
    Node = RECORD
      name: POINTER TO ARRAY OF CHAR;
      PROCEDURE (t: Tree) Insert (name: ARRAY OF CHAR);
      PROCEDURE (t: Tree) Search (name: ARRAY OF CHAR): Tree;
      PROCEDURE (t: Tree) Write;
    END;
  PROCEDURE Init (VAR t: Tree);
END Trees.

For a record type, the browser also collects all procedures bound to
this type and shows their declaration in the record type declaration.


D5. Run Time Data Structures

Certain information about records has to be available at run time: The
dynamic type of records is needed for type tests and type guards. A
table with the addresses of the procedures bound to a record is needed
for calling them. Finally, the garbage collector needs information about
the location of pointers in dynamically allocated records. All that
information is stored in so-called type descriptors of which there is
one for every record type at run time. The following paragraphs show a
possible implementation of type descriptors.

  The dynamic type of a record corresponds to the address of its type
descriptor. For dynamically allocated records this address is stored in
a so-called type tag which precedes the actual record data and which is
invisible for the programmer. If t is a variable of type CenterTree (see
example in Ch. 6) Figure D5.1 shows one possible implementation of the
run time data structures.


Fig. D5.1  A variable t of type CenterTree, the record t^ it points to,
and its type descriptor

Since both the table of procedure addresses and the table of pointer
offsets must have a fixed offset from the type descriptor address, and
since both may grow when the type is extended and further procedures and
pointers are added, the tables are located at the opposite ends of the
type descriptor and grow in different directions.

  A type-bound procedure t.P is called as t^.tag^.ProcTab[IndexP]. The
procedure table index of every type-bound procedure is known at compile
time. A type test v IS T is translated into
v^.tag^.BaseTypes[ExtensionLevelT] = TypeDescrAdrT. Both the extension
level of a record type and the address of its type descriptor are known
at compile time. For example, the extension level of Node is 0 (it has
no base type), and the extension level of CenterNode is 1.

[1]  N.Wirth, J.Gutknecht: The Oberon System. Software Practice and
Experience 19, 9, Sept. 1989

[2]  M.Reiser: The Oberon System. User Guide and Programming Manual.
Addison-Wesley, 1991

[3]  C.Pfister, B.Heeb, J.Templ: Oberon Technical Notes. Report 156, ETH
Zürich, March 1991