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[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Re: attribute order (RE: Syntax Sugar and XML information models)
 From: Simon St.Laurent <simonstl@s...> >Remember that we don't all have access to the archives, and I can imagine a >number of reasons for arguing either way. (It's pretty easy to argue about >how useful the various distinctions between elements and attributes really >are in general - this is just one case.) I think to have this discussion would require access to ISO minutes from the mid 1980s. When ADA was being developed they decided that the feature of command-line interfaces (e.g. UNIX commands) where you provided named arguments rather than relying on positions was pretty useful: not only did it make the program more self documenting, it provided a clean alternative to varargs and reduced error rates. (There is an interesting article at Rational's site, based on a large SW development on which there had been lots of measurements taken, that ADA was about twice as easy to maintain as C. ADA probably gets a too hard time, on those facts at least. There are a lot of assertions, and programmers have voted with their feet (or with their preference for UNIX and curly braces), but one shouldn't dismiss measured results out of hand.) So SGML seems to have adopted named arguments, like command-line interfaces and ADA and Common LISP, as part of best-practise sofware engineering as seen in the early 1980s. SGML also allowed anonymous attributes ( e.g. <size large>) but did that by enforcing a unique token rule on attribute values rather than positional dependence. DSSSL reintroduced keyword arguments into Scheme, but the mix of some named, some positional means that "positional" becomes "relative order of anonymous arguments" rather than "absolute order" which has always seemed a little messy to me (I think many people would agree that command line interfaces which use relative order for more than two meanings are rather difficult) So those are some design axes: named/anonymous, fixed/free position, where anonymous+free implies that the value tells you the effective name. Of course, the advent of IDEs and strongly typed languages means that the IDE performs implicit labelling and has reduced the need for explicit labels, but this is only reliable when the arguments have different types discernable from the values. I wonder if the use of pre-conditions (asserts) as espoused in Meyer's "programming by contract" is too some extent prudence in the face of anonymous function arguments: if you have named arguments you can use a built-in type-declaration system, but if you have anonymous arguments whose type differences cannot be resolved lexically then you need an ad-hoc type like precondition-checks. (Of course, Meyers has other reasons for programming by contract, and certainly not all "argument-in-the-wrong-position" can be detected either by lexical space or value space checks...only the ones that are square pegs in round holes...which in turn tends to support SGML's unique enumeration value rule for attributes, however inconvenient it is.) Cheers Rick Jelliffe 
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