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CHAPTER 26 BASIC CONCEPTS IN UML: A REQUEST FOR CLARIFICATION
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I began my detailed investigations into OCL by studying a book titled The Object Constraint Language: Precise Modeling with UML (Addison-Wesley, 1999), by Jos Warmer and Anneke Kleppe (referred to throughout this chapter as the OCL book ). Here s another quote from the introduction to that book (page xix): [This] book is meant to be a textbook and reference manual for practitioners of object technology who find a need for more precise modeling. More precise than what, I m tempted to ask. More precise than UML without OCL, I suppose which doesn t say much for UML without OCL, I fear. Anyway, since it s meant, at least in part, as a reference manual, the book is presumably also meant to be both accurate and complete and since one of the authors, Warmer, is described on the back cover as the primary author of the OCL standard [sic], it s presumably meant to be authoritative as well. Unfortunately, however, I very quickly ran into difficulties in trying to understand it; I found myself going round in circles, stumbling up against what seemed to be contradictions, and generally finding a lack of clarity and precision in what is, after all, a field in which such qualities should surely be prized above all others. While I had numerous problems at the detail level with the OCL book, my biggest difficulties all had to do with the question of types, which are, of course, fundamental. I mean, it seemed to me that I didn t have a hope of understanding OCL constraints as such until I understood the types of things that could thereby be constrained ... Here are a few quotes, with commentary, that illustrate some of the difficulties I encountered:3 Page 4: Invariants are always coupled to classes, types, or interfaces ... An invariant is a constraint that states a condition that must always be met by all instances of the class, type, or interface [with which it is coupled]. This first quote clearly suggests that class, type, and interface are three different kinds of things (though I should say immediately that I m not sure that the idea of interfaces in particular having instances makes much sense, but let that pass for the moment). Page 23: The context of an invariant is always a class, an interface, or a type. In this book, the convention is to show the type of the model element underlined on the line above the [invariant]. The first sentence here lends further support to the idea that class, interface, and type are different kinds of things. But then shouldn t type in the second sentence really be class, interface, or type After considerable thought, however, I realized that the phrase the type of the model element in that sentence should be abbreviated to just the model element ; the term type is being used formally in the first sentence but only very informally in the vague sense of kind in the second.
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3. I apologize ahead of time if some of the criticisms that follow seem excessively convoluted or are otherwise hard to understand. In my defense, I appeal to Chris Date s Principle of Incoherence (sometimes referred to, unkindly, as The Incoherent Principle): It s hard to criticize something coherently if what you re trying to criticize is itself not very coherent in the first place.
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CHAPTER 26 BASIC CONCEPTS IN UML: A REQUEST FOR CLARIFICATION
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Note: A model element is just a component of the applicable UML diagram (e.g., Customer ). Accordingly, I think the foregoing interpretation of the second sentence is charitable at best! In fact, I see no good reason to include the phrase the type of in that sentence at all. Anyway, the book goes on to give the following example of an invariant, together with its context : Customer -------Name = 'Edward' Meaning: All customers must be named Edward hardly a very realistic example, I would have thought, but never mind that. The context here is the model element (not the type of the model element!) called Customer. So far, so good ( ). But then I encountered the following: Page 29: The classes, types, interfaces, and datatypes defined in the UML model are, within OCL, considered to be classes of which instances can be made. In other words, all these UML model elements define types within OCL. First of all, we now apparently have a fourth category, datatypes. Second, classes, types, and interfaces (and datatypes ) are now apparently all just classes anyway! so why all the different terms (Note in particular that types are classes, a point I want to come back to in a few moments.) Third, classes are types! (The first sentence says types ... are ... classes. The second sentence says all these UML model elements [classes in particular] define types. ) So all types are classes and all classes are types. Hence, type and class mean exactly the same thing, and the terms are interchangeable ( ). In addition to the foregoing, I have to say that I have a severe problem with the idea that value types in particular see below can be regarded as classes of which instances can be made. I ll come back to this point in my final comment on the quote immediately following. Now I need to back up a little way ... Elsewhere in the OCL book, we find the following: Page 22: OCL has value types and object types. Both are types (that is, both specify instances), but there is an important difference. Value types define instances that never change their value ... Object types or classes represent types that define instances that can change their value(s) ... Martin Fowler ... calls object types reference objects and value types value objects. It seems to me that here the authors of the OCL book are trying to get at the fundamental distinction between values and variables, but, frankly, they aren t making a very good job of it. Note: That distinction which truly is fundamental is discussed at length in a book by Hugh Darwen and myself titled Databases, Types, and the Relational Model: The Third Manifesto, 3rd edition, Addison-Wesley, 2006. As I say, that distinction truly is fundamental; indeed, it underpins all of our thinking in this field, both in The Third Manifesto itself and elsewhere. Let me elaborate. It seems to me that it would be possible to interpret the passage just quoted from page 22, somewhat charitably, as meaning that (a) any given value is an instance of some given value type, and (b) any given variable is an instance of some given object type. But then, if Edward is a character string value and Name is a character string variable, they re apparently of different types! and so we need some rules to say whether they can be
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