XML as a scripting language

John R. Nestor

Xml For All, Inc.

Pittsburgh

USA

nestor@xmlforall.com

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XML as a scripting language

XML as a scripting language

Most XML applications have been developed using one of the widely-used web development languages such as Java, Perl, or Visual Basic. Since these languages all predate XML, they had to be augmented with extensions for processing XML. The most common extensions, XML parsers, are widely available.

An alternative is to use XML itself as the scripting language. I focus here on XFA Script (http://www.xmlforall.com), a language I developed, but most of my remarks apply equally well to other XML-based scripting systems such as XML Script (http://www.xmlscript.org).

Unlike languages where XML support was an afterthought, XFA Script was designed from the start around XML, making it ideally suited for processing XML data. There are however even more important advantages of XML-based scripting.

1. Most XML processing languages are double-level where a program written in a language such as Java produces output in a second language, XML. This is accomplished with formatted I/O or string processing that obscures the structure of the output. In XFA Script output is directly interleaved in the processing program making it easier to write, check, and understand.
2. XFA scripts are themselves just XML. This means that XFA code can be treated as data, making it easy to write scripts that manipulate other scripts. XFA Script also can make any XML data into a program by specifying the active behavior associated with each data element type.
3. The XML community continues to produce ever more specialized XML processing standards. With DTD's and schemas for validating data, XSL and XSLT for rendering and transforming data, and with pointers, links, paths and RDF to connect data, the overall complexity continues to soar. XFA Script is a single simple general-purpose language that can be used for all these purposes.
4. Although DTD's and schemas allow XML syntax to be formally described, very little has been done on the more important problem of formal specification of XML semantics. Since XFA Script is itself XML and has a clean clear semantics, it is an ideal notation for defining the semantics of XML data. An added bonus is that XFA semantic specifications can easily become executable semantic checkers.

XML scripting is not without some problems. The XML syntax does not lend itself well to scripting. XFA Script handles this with extended XML syntax that adds expression forms to XML attribute values. A one-to-one mapping is defined between this form and a less readable strict XML form. Another problem is what should be the exact set of tags used for XML scripting? Although I believe the XFA Script tags are well chosen, I make no claims to their unique utility. To accommodate diversity, XFA Script makes it easy to define interpretation rules for other tag sets.

XFA Script has been used for over three years to produce a wide range of XML applications. Experience has strongly confirmed all the advantages of XML-based scripting discussed here.

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