Monday Jul 02, 2007

refactoring xml

Refactoring is defined as "Improving a computer program by reorganising its internal structure without altering its external behaviour". This is incredibly useful in OO programming, and is what has led to the growth of IDEs such as Netbeans, IntelliJ and Eclipse, and is behind very powerful software development movements such as Agile and Xtreeme programming. It is what helps every OO programmer get over the insidious writers block. Don't worry too much about the model or field names now, it will be easy to refactor those later!

If maintaining behavior is what defines refactoring of OO programs - change the code, but maintain the behavior - what would the equivalent be for XML? If XML is considered a syntax for declarative languages, then refactoring XML would be changing the XML whilst maintaining its meaning. So this brings us right to the question of meaning. Meaning in a procedural language is easy to define. It is closely related to behavior, and behavior is what programming languages do their best to specify very precisely. Java pushes that very far, creating very complex and detailed tests for every aspect of the language. Nothing can be called Java if it does not pass the JCP, if it does not act the way specified.
So again what is meaning of an XML document? XML does not define behavior. It does not even define an abstract semantics, how the symbols refer to the world. XML is purely specified at the syntactic level: how can one combine strings to form valid XML documents, or valid subsets of XML documents. If there is no general mapping of XML to one thing, then there is nothing that can be maintained to retain its meaning. There is nothing in general that can be said to be preserved by transformation one XML document into another.
So it is not really possible to define the meaning of an XML document in the abstract. One has to look at subsets of it, such at the Atom syndication format. These subset are given more or less formal semantics. The atom syndication format is given an english readable one for example. Other XML formats in the wild may have none at all, other than what an english reader will be able to deduce by looking at it. Now it is not always necessary to formally describe the semantics of a language for it to gain one. Natural languages for example do not have formal semantics, they evolved one. The problem with artificial languages that don't have a formal semantics is that in order to reconstruct it one has to look at how they are used, and so one has to make very subtle distinction between appropriate and inappropriate uses. This inevitably ends up being time consuming and controversial. Nothing that is going to make it easy to build automatic refactoring tools.

This is where Frameworks such as RDF come in very handy. The semantics of RDF, are very well defined using model theory. This defines clearly what every element of an RDF document means, what it refers to. To refactor RDF is then simply any change that preserves the meaning of the document. If two RDF names refer to the same resource, then one can replace one name with the other, the meaning will remain the same, or at least the facts described by the one will be the same as the one described by the other, which may be exactly what the person doing the refactoring wishes to preserve.

In conclusion: to refactor a document is to change it at the syntactic level whilst preserving its meaning. One cannot refactor XML in general, and in particular instances it will be much easier to build refactoring tools for documents with clear semantics. XML documents that have clear RDF interpretations will be very very easy to refactor mechanically. So if you are ever asking yourself what XML format you want to use: think how useful it is to be able to refactor your Java programs. And consider that by using a format with clear semantics you will be able to make use of similar tools for your data.

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