[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Re[2]: Conditional actions in XSL?
My biggest concern with using *any* language with XSL would be the potential for odd things when language functions have side-effects. If a function or subroutine sets the state of an external variable, then it's possible that external operation might occur *every* time an element is redisplayed. I believe the spec (rightly) states that side-effects are not allowed and that XSL's behavior (and the state of the variables involved in the side-effecting function) will be undefined if they are used. [I suspect this is old info for those familiar with the history of DSSSL -- the choice of a language that supports functional (no side effect) programming language such as Scheme indicates that the spec developers were quite aware of the potential dangers of more conventional programming styles.] Reading external variables can be another problem since you have to hand-craft some sort of polling or event handling if you expect the external state might change during a document's use. It's too bad this is a tough problem with the current XSL (no value dependency information is maintained, I suspect, in any existing or planned implementation), since you can provide neat capabilities if you keep track of the information each layout variable depends upon. This "constraint-based" approach make it possible to support selective redisplays (depending on the changes that invalidate previous calculations) and dynamic displays that depend on information external to the formatter. For more on the difference between the usual approach (Random question: does DSSSL have an event model?) and the more powerful constraint-based approach, you might want to look at Prof. Ethan Munson's Proteus papers. (http://www.cs.uwm.edu/faculty/munson/) He describes an alternative to DSSSL that uses constraints to handle out-of-order layout, dynamic behavior, etc. The biggest drawback are the memory requirements for maintaining the dependency information in large documents. A less important concern would be the proliferation of scripting languages, which is a real annoyance from a general service developer's point of view. xml-dev: A list for W3C XML Developers. To post, mailto:xml-dev@i... Archived as: http://www.lists.ic.ac.uk/hypermail/xml-dev/ To (un)subscribe, mailto:majordomo@i... the following message; (un)subscribe xml-dev To subscribe to the digests, mailto:majordomo@i... the following message; subscribe xml-dev-digest List coordinator, Henry Rzepa (mailto:rzepa@i...)
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