| The difference between relations and relvars is actually a special case of the difference between values and variables in general, and I'd like to take a few moments to look at the more general case. (It's a bit of a digression, but I think it's worth taking the time because clear thinking here can be a tremendous help in so many areas.) Here then are some definitions: Definition: A value is an "individual constant": for example, the individual integer 3. A value has no location in time or space. However, values can be represented in memory by means of some encoding, and those representations do have locations in time and space. Indeed, distinct representations of the same value can appear at any number of distinct locations in time and space, meaning, loosely, that any number of different variables see the definition below can have the same value, at the same time or different times. Observe in particular that, by definition, a value can't be updated; for if it could, then after such an update it would no longer be that value.
Definition: A variable is a holder for a representation of a value. A variable does have location in time and space. Also, of course, variables, unlike values, can be updated; that is, the current value of the variable can be replaced by another value.
Please note very carefully that it isn't just simple things like the integer 3 that are legitimate values. On the contrary, values can be arbitrarily complex; for example, a value might be a geometric point, or a polygon, or an X ray, or an XML document, or a fingerprint, or an array, or a stack, or a list, or a relation (and on and on). Analogous remarks apply to variables too, of course. I'll have more to say on these matters in the next two chapters. Now, it might be hard to imagine people getting confused over a distinction as obvious as that between values and variables, but in fact it's all too easy to fall into traps in this area. By way of illustration, consider the following extract from a tutorial on object-oriented databases (the italicized portions in brackets are comments by myself): We distinguish the declared type of a variable from . . . the type of the object that is the current value of the variable [so an object is a value] . . . We distinguish objects from values [so an object isn't a value after all] . . . A mutator [is an operator such that it's] possible to observe its effect on some object [so in fact an object is a variable].
|
