The Programming Language Oberon

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Declarations and scope rules

Every identifier occurring in a program must be introduced by a declaration, unless it is a predefined identifier. Declarations also serve to 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. This is possible in those parts of a program only which are within the scope of the declaration. No identifier may denote more than one object within a given scope. The scope extends textually from the point of the declaration to the end of the block (procedure or module) to which the declaration belongs and hence to which the object is local. The scope rule has the following amendments:
1. If a type T is defined as POINTER TO T1 (see 6.4), the identifier T1 can be declared textually following the declaration of T, but it must lie within the same scope.
2. Field identifiers of a record declaration (see 6.3) are valid in field designators only.
In its declaration, an identifier in the global scope may be followed by an export mark (*) to indicate that it be exported from its declaring module. In this case, the identifier may be used in other modules, if they import the declaring module. The identifier is then prefixed by the identifier designating its module (see Ch. 11). The prefix and the identifier are separated by a period and together are called a qualified identifier.
qualident = [ident « . »] ident.
identdef = ident [« * »].

Constant declarations

A constant declaration associates an identifier with a constant value.
ConstantDeclaration = identdef « = » ConstExpression.
ConstExpression = expression.
A constant expression can be evaluated by a mere textual scan without actually executing the program. Its operands are constants (see Ch. 8).

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 is used to associate an identifier with the type. Such association may be with unstructured (basic) types, or it may be with structured types, in which case it defines the structure of variables of this type and, by implication, the operators that are applicable to the components. There are two different structures, namely arrays and records, with different component selectors.

Array types

An array is a structure consisting of a fixed 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 declaration of the form
ARRAY N0, N1, … , Nk OF T
is understood as an abbreviation of the declaration
ARRAY N0 OF
ARRAY N1 OF
…
ARRAY Nk OF T
Examples of array types:
ARRAY N OF INTEGER
ARRAY 10, 20 OF REAL

Record types

A record type is a structure consisting of a fixed number of elements of possibly different types. The record type declaration specifies for each element, called field, its type and an identifier which denotes the field. The scope of these field identifiers is the record definition itself, but they are also visible within field designators (see 8.1) referring to elements of record variables.