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CHAPTER 27 A COMPARISON BETWEEN ODMG AND THE THIRD MANIFESTO
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Next, structured literals. As noted earlier, ODMG supports a STRUCT type generator, analogous (somewhat) to the TUPLE type generator of The Third Manifesto; hence, struct or tuple types can be generated, and instances of such types then seem to be called structured literals, regardless of whether (in Manifesto terms) they are values or variables. A structured literal ... has a fixed number of elements, each of which has a ... name and can contain either a literal value [sic!] or an object [i.e., an object ID]. The idea of a literal including a component that is an object (i.e., variable) seems to muddle some very basic notions that would surely be better kept separate. Note: ODMG requires support for certain built-in structured literal types, namely DATE, TIME, TIMESTAMP, and INTERVAL (not to be confused with the structured object types of the same names). Finally, collection literals. Here again the situation seems unnecessarily confusing. Collection literals are really collection type generators. The ODMG Object Model supports collection literals of the following types [sic!]: SET, BAG, ... (etc.). These type generators [sic!] are analogous to those of collection objects ... Their elements ... can be of literal types or object types. Apparently, therefore, we can have, e.g., a list object whose elements are literals, and a list literal whose elements are objects ( ). Note: The ODMG book also mentions a user-defined UNION literal type, the semantics of which it does not explain (we suspect it refers to heterogeneous collections, although collections are elsewhere stated to be homogeneous). It does not say whether there is an analogous UNION object type. In sum, we agree wholeheartedly that there is a vast and important logical difference between values and variables, but that particular difference does not seem to be the one that ODMG draws between literals and objects nor does the precise nature of the difference that ODMG does draw in this connection seem very clear, at least to us. At the same time, ODMG also makes use of the value vs. variable terminology, but it does not give definitions of these latter concepts. We conclude this subsection by noting that: An ODMG literal (of any kind) is not an object and does not have an object ID. Although as noted earlier the Object Model prescribes certain operators for certain objects (especially collection objects), it does not appear to prescribe any such operators for literals. Indeed, if it is really true that object types and literal types are different kinds of things, it would seem that (e.g.) if operator Op is defined to work on, say, integer objects, it cannot be invoked on integer literals ( ). ODMG also supports a special null literal. For every literal type ... there exists another literal type supporting a null value [sic] ... This nullable type is the same as the literal type augmented by the ... value nil. The semantics of null are the same as those defined by [the SQL standard]. (This last claim is contradicted later, in the chapter on OQL.)
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Types, Classes, and Interfaces
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On the face of it, it might have seemed more logical to explain these concepts prior to this point. We deliberately did not do so, however, because (a) the concepts are not as clear as they might be, and (b) it therefore seems necessary to have some appreciation of the material discussed in the foregoing subsections in order to understand them. Consider the following series of quotes:
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CHAPTER 27 A COMPARISON BETWEEN ODMG AND THE THIRD MANIFESTO
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A type has an external specification and one or more implementations. The specification defines the external characteristics of the type. These are the aspects that are visible to users of the type: the operations that can be invoked on its instances [and] the properties ... whose values can be accessed ... By contrast, a type s implementation defines the internal aspects of the objects of the type [or of literals of the type, presumably, if the type in question is a literal type instead of an object type]: the implementation of the type s operations and other internal details. An external specification of a type consists of an abstract, implementation-independent description of the operations ... and properties that are visible to users of the type. An interface definition is a specification that defines only the abstract behavior of an object type. A class definition is a specification that defines the abstract behavior and abstract state of an object type. A literal definition defines only the abstract state of a literal type. Observe, therefore, that the terms type and class are being used here in a somewhat unusual way: Class is being used to refer to what in the Manifesto book, following more orthodox usage, we call a type, and type is being used to refer to the combination of class in this unorthodox sense together with the implementation of the class in question.8 Note: We will come back to literal definitions in a few moments, and to interface definitions in the next subsection. But then what are we to make of the following, which appears a couple of pages later Classes are types ... Interfaces are types ... Most of the rest of the ODMG book either ignores the distinction between types and classes or observes it in inconsistent ways. In what follows, we will stay with the term type (mostly). As for the assertion that a literal definition defines only the abstract state of a literal type : This statement seems to suggest that no operations can be performed on literals, which cannot be correct. We suspect that what the ODMG book really means here is that a literal definition that is actually an application of some type generator (see the previous subsection) defines the unencapsulated logical structure of some generated type but does not define any operators that apply to instances of that generated type (the only operators that do apply being inherited built-in ones).
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