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CHAPTER 6 ON THE LOGICAL DIFFERENCES BETWEEN TYPES, VALUES, AND VARIABLES
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1. First of all, integer values (or just integers for short) are self-explanatory. Note carefully, however, that if some given value is of type INTEGER, then it isn t of any other type. For example, the value denoted by the numeral 3 is of type INTEGER, and type INTEGER only. 2. Here s a possible definition for an integer variable called N: VAR N INTEGER ; N here has explicitly been declared to be of type INTEGER; at any given time, the current value of N is some integer value. Note: Barring explicit statements to the contrary, all coding examples in this chapter are based on a language called Tutorial D, which is the language we use for examples in the Manifesto book. 3. Here s a possible definition for an operator called DOUBLER that takes an integer and doubles it: OPERATOR DOUBLER ( I INTEGER ) RETURNS INTEGER ; RETURN ( I + I ) ; END OPERATOR ; DOUBLER is a read-only operator (it doesn t update anything); it s explicitly declared to be of type INTEGER (see the RETURNS clause), meaning the value it returns when it s invoked is an integer. The sole parameter I is also explicitly declared to be of type INTEGER, meaning the argument that corresponds to that parameter when the operator is invoked must be an integer value. 4. Alternatively, we could make the doubling operator an update operator instead: OPERATOR DOUBLEU ( I INTEGER ) UPDATES { I } ; I := ( I + I ) ; END OPERATOR ; Operator DOUBLEU has no declared type and returns no value when it s invoked (there s no RETURN statement, and the RETURNS clause has been replaced by an UPDATES clause, showing that arguments corresponding to the parameter I are subject to update). The argument that corresponds to the sole parameter I when the operator is invoked must be an integer variable specifically. The assignment operation causes that variable to be updated appropriately. Note: It follows that DOUBLEU(V), where V is an integer variable, doesn t have a type, doesn t have a value, and more generally isn t regarded as an expression. Hence the operator must be invoked by means of an explicit CALL statement e.g., CALL DOUBLEU(V); or something equivalent. 5. In contrast to the foregoing, DOUBLER(v), where v is an integer value, does have a type (INTEGER), does have a value (an integer), and is regarded as an expression in fact, it can appear wherever an integer literal is allowed. Here s an example: ( 5 * DOUBLER ( J ) ) + DOUBLER ( K - 4 ) This expression has a type, too: namely, the type of the outermost operator involved, which happens to be PLUS ( + ) in this example.
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CHAPTER 6 ON THE LOGICAL DIFFERENCES BETWEEN TYPES, VALUES, AND VARIABLES
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Note: To avoid some possible confusion, I should explain that simple literals and simple variable names are both legal expressions. The outermost operator involved in, e.g., the expression 3 is a literal reference, and it s of the obvious type. Likewise, the outermost operator involved in, e.g., the expression N is a variable reference, and it too is of the obvious type. 6. Finally, consider the following relation type: RELATION { P# P#, PNAME NAME, COLOR COLOR, WEIGHT WEIGHT, CITY CHAR } This relation type involves five attributes P#, PNAME, COLOR, WEIGHT, and CITY, of types P#, NAME, COLOR, WEIGHT, and CHAR, respectively. (Don t worry if this example doesn t make too much sense to you yet. I ll have more to say about relation types later in this chapter, in the section Relation Values and Variables. )
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