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
~ 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:
- No identifier may denote more than one object within a given scope (i.e. no
identifier may be declared twice in a block);
- An object may only be referenced within its scope;
- A typeT 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;
- 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 TOT, 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 adynamic 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
- v is a variable parameter of record type or v is a pointer, and
if
- 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
- v is a variable parameter of record type or v is a pointer, and
if
- 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:
- 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;
- if Tv and Te are pointer types, the dynamic type of v
becomes the dynamic type of e;
- 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 * 2^n)
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; LONGINT length of v in dimension n
n: integer const. (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; terminate program execution
n: integer constant if not x
COPY(x, v) x: character array, string; v := x
v: character array
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 allocate v^
fixed array
NEW(v,x0,...,xn) v: pointer to open array; allocate v^ with lengths
xi: integer type 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
- Ta and Tb are both denoted by the same type identifier, or
- Ta is declared to equal Tb in a type declaration of the form
Ta = Tb, or
- 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
- Ta and Tb are the same type, or
- Ta and Tb are open array types with equal element types,
or
- 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
- Ta and Tb are the same types, or
- 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:
- Te and Tv are the same type;
- Te and Tv are numeric types and Tv includes
Te;
- Te and Tv are record types and Te is an extension
of Tv and the dynamic type of v is Tv;
- Te and Tv are pointer types and Te is an
extension of Tv;
- Tv is a pointer or a procedure type and e is NIL;
- Tv is ARRAY n OF CHAR, e is a string constant with m
characters, and m <n;
- 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
- Tf and Ta are the same type, or
- Tf is an open array, Ta is any array, and their element types
are array compatible, or
- 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
type including
both operands
+ - * / SET SET SET
DIV MOD integer integer smallest integer type
type including
both operands
OR & ~ BOOLEAN BOOLEAN BOOLEAN
= # < numeric numeric BOOLEAN
<= > >= CHAR CHAR BOOLEAN
character array, character array, BOOLEAN
string string
= # BOOLEAN BOOLEAN BOOLEAN
SET SET BOOLEAN
NIL, pointer type NIL, pointer type BOOLEAN
T0 or T1 T0 or T1
procedure type T, procedure type T, BOOLEAN
NIL NIL
IN integer SET BOOLEAN
IS type T0 type T1 BOOLEAN
- Matching formal parameter lists
- Two formal parameter lists match if
- they have the same number of parameters, and
- they have either the same function result type or none, and
- parameters at corresponding positions have equal types, and
- 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 := Register n
v: any basic type, pointer,
procedure type
PUTREG(n, x) n: integer constant; Register n := x
x: any basic type, 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 loadingof 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.
- N.Wirth, J.Gutknecht: The Oberon System. Software Practice
and Experience 19, 9, Sept. 1989
- M.Reiser: The Oberon System. User Guide and Programming
Manual. Addison-Wesley, 1991
- C.Pfister, B.Heeb, J.Templ: Oberon Technical Notes. Report
156, ETH Zürich, March 1991
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