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CHAPTER 6 ON THE LOGICAL DIFFERENCES BETWEEN TYPES, VALUES, AND VARIABLES
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A class consists of a type and possibly one or more functions or procedures ... that can be executed on objects of that class. The objects of a class are either values of that type (called immutable objects) or variables whose value is of that type (called mutable objects). But if they re truly just variables and values as suggested, then why drag in the new terminology What does it buy us, other than additional complexity and confusion (In any case, writers on object matters often talk as if objects are always mutable, and forget about the immutable ones.) Note: I said earlier in this chapter that to say that V is a variable is to say that V is assignable to, no more and no less. It might therefore be claimed that a mutable object isn t really a variable because the available methods typically don t include one for assigning a value (sorry, an immutable object) to it.12 As we saw in the section Pseudovariables, however, any method that allows us to zap any component of an object is really shorthand for one that allows us to assign to the entire object (and if there aren t any such methods, then the object isn t mutable in the first place!). So I stand by my position that mutable and immutable objects are really just variables and values, respectively. Other object systems use different terminology in an attempt to get at the same value vs. variable distinction. ODMG, for example, regards all objects as mutable and uses the term literal for an immutable object.13 But this latter term illustrates a confusion of a different kind; as we saw earlier in this chapter, a literal (at least as that term is conventionally understood) just isn t a value; rather, it s a symbol that denotes some value. While I m on the topic of ODMG, incidentally, I note that ODMG regards objects and literals as being fundamentally different things, in the sense that no object is of the same type as any literal! The (weird) implications of this state of affairs are well beyond the scope of the present discussion, however. Back to objects per se. Here s another extract from that same object database tutorial by Zdonik and Maier that manages to confuse values and variables and several other things besides: A class (sometimes called a type) is a template for its instances. Often the terms type and class are used interchangeably, but when the two terms are used in the same system, type usually refers to specifications, whereas class refers to the extension (i.e., all current instances) of the corresponding type. Every object is an instance of some class ... [Later:] Some types might also support operations that will alter the state of [their] instances.
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12. And why that term method, anyway Considered purely as a regular English word, it doesn t mean what the object advocates seem to want it to mean not to mention the fact that there are already several perfectly good terms for the concept, including the term operator in particular. 13. In this connection, you might care to meditate on the following quotes from the first edition of the ODMG book (R. G. G. Cattell, ed., The Object Database Standard: ODMG-93, Morgan Kaufmann, 1994). Page 16: [The] type Denotable_Object [has disjoint subtypes] Object and Literal [so a denotable object isn t necessarily an object, and no object is a literal, and no literal is an object] ... Objects are mutable; literals are immutable. Page 20: Literals are objects ... [ ].
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CHAPTER 6 ON THE LOGICAL DIFFERENCES BETWEEN TYPES, VALUES, AND VARIABLES
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Points arising: First, notice that we now have another term for object ( instance ),14 and another term for value ( state ), and a certain amount of confusion over the terms type and class. I see no need for either instance or state at all. As for type vs. class, let me remind you of the brief discussion of that topic in 4. Let me also draw your attention to the slightly different ( ) meanings those terms seem to have in the extract from Garcia-Molina, Ullman, and Widom quoted earlier. Second, the term extension is usually understood to refer to a set of values: to be specific, the set of values that satisfy some given predicate, often called a membership predicate. For example, the extension of the predicate i is an integer such that 0 < i < 6 is precisely the set of values 1, 2, 3, 4, 5. Thus, the phrase extension (i.e., all current instances) strongly suggests that an instance is a value. But the final sentence in the extract clearly implies that an instance is a variable. By the way, a type that didn t support operations that will alter the state of [its] instances would seem not to be very useful, since apparently we wouldn t be able to define variables of that type. (Well, perhaps we could define them, but we could never assign anything to them!) This next quote is from the same source once again: An intensional specification is a template specifying all possible objects with a given structure ... [Objects have] an object identity ... that remains invariant across all possible modifications of the object s value. The phrase all possible objects with a given structure means an object is a value (for example, think of all possible integers or all possible relations that can be assigned to the relvar P ).15 The phrase all possible modifications of the object s value means an object is a variable. Note: That same phrase also confuses values and variables (you can t modify values), but perhaps this is nitpicking; perhaps we re to understand the phrase to mean simply all possible modifications of the object. The next two quotes are taken from an interview with Mary Loomis in Data Base Newsletter 22, No. 6 (November/December 1994): One of the simplest ways to think about state is as the current values of the properties of an object much like the values of a record s variables at a particular point in time. Comment: A record must be either a record value or a record variable. If it s the former, it obviously can t contain any variables. If it s the latter it can t either! a record variable, like an array variable or a tuple variable or a relvar, is a single variable. So what does the phrase a record s variables mean Note: I think I know what s going on here, though. Objects in the object world are supposed to include a set of components, variously known as properties or attributes or members or instance variables, and those components are indeed thought of as variables in their own right. (At least, they are if the objects in question are mutable. I don t know what the instance variables
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14. The same tutorial elsewhere uses yet a third term: object instance (!). 15. The same phrase also confuses type and representation! still another logical difference that I plan to discuss in a future paper.
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