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[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Re: Postel's law, exceptions
 Michael Champion wrote: > That makes XML 1.0 processing sortof a Big Bang that either produces a > fully formed Infoset, or an error message. As I've said, this seems to be now the usual understanding of 'infoset'. But Infoset is a concept unknown to XML 1.0, and parsing is the necessary first step of *every* instance of XML 1.0 processing. Many things might be built upon the output of a particular XML 1.0 parse, including a tree, or a graph, or the abstraction into Infoset form of the data items identified in that parse. Yet it seems to me that an 'infoset' would not usually be the desired final product of processing an XML instance, in large part because such an 'infoset' is a terminal output product: it cannot be passed or pipelined into any other context because it is utterly specific to the circumstances in which it is produced. What is required if the output of particular XML processing is to be passed to other XML processing is a document, which of course will first be parsed before any other processing is performed on it in that new environment. It is this conveyance of XML instances from context to context which is so well suited to the internetwork topology, and particularly to the Web-as-we-know-it. Instances are available as entity bodies which we may GET at a particular URL, process, and then republish as new instances at other URLs. It seems doubtful that the aggregation of RSS feed items should be considered a substantially different process. What is necessary for this process to work reliably is the draconian XML 1.0 parse as the first step in every new process. Whatever 'liberal' and 'conservative' might have meant in Postel's original usage, in the context of XML instances which we can GET at URLs 'liberal' in what we accept means that we acknowledge the instance is not likely to be in a form which our process can use directly. The only form which our process could use directly would be a very particular data structure. In their own terms, processes operate only upon specific data structures, and neither a concrete instance document nor an abstract infoset is what a process can use directly. The difference is that the process can be designed to be liberal in accepting numerous schematically differing concrete instances to parse and then to instantiate the output of the parse as the particular data structure which the process requires. That liberality cannot be extended to some sort of 'infoset' as input, because input which is not a parseable document must either correspond perfectly to a very specific and typed schematic or be useless to a particular process, and that is the most illiberal of demands. Respectfully, Walter Perry 
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