XML in a Nutshell, 3rd Edition

What XML Is Not

XML is a markup language, and it is only a markup language. It’s important to remember that. The XML hype has gotten so extreme that some people expect XML to do everything up to and including washing the family dog.

First of all, XML is not a programming language . There’s no such thing as an XML compiler that reads XML files and produces executable code. You might perhaps define a scripting language that used a native XML format and was interpreted by a binary program, but even this application would be unusual. XML can be used as a format for instructions to programs that do make things happen, just like a traditional program may read a text config file and take different actions depending on what it sees there. Indeed, there’s no reason a config file can’t be XML instead of unstructured text. Some more recent programs use XML config files; but in all cases, it’s the program taking action, not the XML document itself. An XML document by itself simply is. It does not do anything.

Tip

At least one XML application, XSL Transformations (XSLT), has been proven to be Turing complete by construction. See http://www.unidex.com/turing/utm.htm for one universal Turing machine written in XSLT.

Second, XML is not a network transport protocol . XML won’t send data across the network, any more than HTML will. Data sent across the network using HTTP, FTP, NFS, or some other protocol might be encoded in XML; but again there has to be some software outside the XML document that actually ...