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CHAPTER 11 MULTIPLE ASSIGNMENT
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<assign> ::= <scalar assign> | <nonscalar assign> <scalar assign> ::= <scalar target> := <scalar exp> <scalar target> ::= <scalar var ref> | <scalar pseudovar ref> Comment: A <scalar var ref> is just a scalar variable name. We omit further details of <scalar pseudovar ref>. <nonscalar assign> ::= <relation assign> Comment: The only kind of nonscalar assignment we discuss here is relational assignment, but we could clearly include other kinds if we wanted to (e.g., <tuple assign>, <array assign>, and so on). <relation ::= | | | assign> <relation <relation <relation <relation target> := <relation exp> insert> update> delete>
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<relation target> ::= <relation var ref> | <relation pseudovar ref> Comment: It is not our aim in this chapter to get into details of what <relation pseudovar ref>s might look like. <relation insert> ::= INSERT <relation target> <relation exp> <relation delete> ::= DELETE <relation target> [ WHERE <bool exp> ] Comment: The <bool exp> is allowed to include an <attribute ref> wherever a literal would be allowed. An analogous remark applies to the production rule immediately following as well. <relation update> ::= UPDATE <relation target> [ WHERE <bool exp> ] ( <attribute update> [ , <attribute update commalist> ] ) <attribute update> ::= <attribute ref> := <exp> Comment: The <exp> is allowed to include an <attribute ref> wherever a literal would be allowed.
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CHAPTER 11 MULTIPLE ASSIGNMENT
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Assigning to Several Components at Once
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We have a significant piece of unfinished business to attend to. Suppose we re given a type ELLIPSE, defined as follows: TYPE ELLIPSE POSSREP { A INTEGER, B INTEGER, CTR POINT CONSTRAINT A B } ; Explanation: Ellipses can possibly be represented by their major semiaxis a, their minor semiaxis b, and their center (where we assume for simplicity that semiaxes are just integers and centers are points in two-dimensional space). For brevity, we will refer to the semiaxes and center of a given ellipse as its components. Please note immediately, however, that this usage is more than a little sloppy; the components in question aren t really components of the ellipse as such, they re components of the specified possible representation of that ellipse. But the usage is convenient, and we ll stay with it in this section. Observe now that ellipses are subject to a type constraint: namely, the constraint that a must not be less than b. (It s the presence of that type constraint that makes this example different in kind from the POINT examples we ve been using in earlier sections of the chapter.) Now let E be a variable of declared type ELLIPSE VAR E ELLIPSE ; and consider the following multiple assignment: THE_A ( E ) := 7 , THE_B ( E ) := 3 ; The intended effect, presumably, is to make the current value of E an ellipse with a = 7, b = 3, and center unchanged (i.e., the center should be the same as it was before the assignment). In other words, we want to zap the a and b components of E while leaving the center component alone. But what actually happens Well, let s expand the two constituent assignments: E := ELLIPSE ( 7, THE_B ( E ), THE_CTR ( E ) ) , E := ELLIPSE ( THE_B ( E ), 3, THE_CTR ( E ) ) ; Now we remove the first assignment and replace the second by: E := WITH ELLIPSE ( 7, THE_B ( E ), THE_CTR ( E ) ) AS E : ELLIPSE ( THE_B ( E ), 3, THE_CTR ( E ) ) ; Suppose the previous value of E had a = 10 and b = 4; then this assignment will work perfectly. But suppose the previous value of E had a = 10 and b = 8. Then the expression in the WITH clause ELLIPSE ( 7, THE_B ( E ), THE_CTR ( E ) ) will raise a run-time error! To be specific, it will attempt to produce an ellipse with a = 7 and b = 8, thereby violating the type constraint on ellipses to the effect that a must not be less than b. To elaborate: The expression in question is a selector invocation once again. As you ll recall, selectors are operators that are used to select, or specify, values of some specified type. By definition, such operators cannot yield a value that violates any type constraint that applies to the type in question. Type constraints are thus an exception to the general rule that constraint
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