/ CASE STUDY
/ The Source Language
Summary of the Language
A computer program consists of two essential parts, a description of actions which are to be performed, and a description of the data, which are manipulated by these actions. Actions are described by statements, and data are described by declarations.
The data are represented by constants and values of variables. Every variable occurring in a statement must be introduced by a variable declaration which associates an identifier and a data type with that variable. The data type essentially defines the set of values which may be assumed by that variable. The data type is directly described in the variable declaration.
There exist three basic types: Boolean, integer, and real. The values of the type Boolean are denoted by reserved identifiers, the numeric values are denoted by numbers.
Array types are defined by describing the types of their components and an integer range. A component of an array value is selected by an integer index. The type of the component is the component type of the corresponding array type.
The most fundamental statement is the assignment statement. It specifies that a newly computed value be assigned to a variable (or a component of a variable). The value is obtained by evaluating an expression. Expressions consist of variables, constants and operators operating on the denoted quantities and producing new values. MiniLAX defines a fixed set of operators, each of which can be regarded as describing a mapping from the operand types onto the result type. The set of operators is subdivided into
The result of a comparison is of type Boolean. The procedure statement causes the execution of the designated procedure (see below). Assignment and procedure statements are the components or building blocks of structured statements, which specify sequential, selective, or repeated execution of their components. Sequential execution of statements is specified by statement sequences, selective execution by the if statement, and repeated execution by the while statement. The if statement serves to make the execution of two alternative statements dependent on the value of a Boolean expression. The while statement serves to execute a statement while a Boolean expression is true.
A statement sequence can be given a name (identifier), and be referenced through that identifier. The statement sequence is then called a procedure, and its declaration a procedure declaration. Such a declaration may additionally contain a set of variable declarations and further procedure declarations. The variables and procedures thus declared can be referenced only within the procedure itself, and are therefore called local to the procedure. Their identifiers have significance only within the program text which constitutes the procedure declaration and which is called the scope of these identifiers. Since procedures may be declared local to other procedures, scopes may be nested. Entities which are declared in the main program, i.e. not local to some procedure, are called global. A procedure has a fixed number of parameters, each of which is denoted within the procedure by an identifier called the formal parameter. Upon an activation of the procedure statement, an actual quantity has to be indicated for each parameter, which can be referenced from within the procedure through the formal parameter. This quantity is called the actual parameter. There are two kinds of parameters: value parameters and variable parameters. In the first case, the actual parameter is an expression which is evaluated once. The formal parameter represents a local variable to which the result of this evaluation is assigned before the execution of the procedure. In the case of a variable parameter, the actual parameter is a variable, and the formal parameter stands for this variable. Possible indices are evaluated before execution of the procedure.