"Open the pod bay doors, please, HAL." (2001: A Space Odyssey).
Programming languages are a useful way of communicating information to a computer, but there are a variety of situations such as software specification testing and machine learning where a programming language can be too cumbersome for the intended audience. Natural languages involve the interaction of too many complex systems to process, but it is possible to come up with a "controlled language" subset that can be disambiguated systematically. Xcel is a modular system designed to make translating between controlled English and prolog flexible and easily extensible. This full length student paper deals with the characteristics of the xcel system.
The system is most easily conceptualized as a pipeline that takes a document written in Attempto Controlled English (ACE) at one end and produces prolog from the other. Each section of pipe has a meaningful input and output that is coupled in a generic way. These couplings not only make it possible to debug the program incrementally, but also make extensions simpler by allowing simple splitting points.
The source of the process is a lexical analysis performed by a tokenizer chain loaded dynamically from a tokenizer chain markup language (TCML) specification using java reflection. The base is a a tokenizer which breaks apart the source based on regular expressions into symbolic tokens that have both a type and value. This stream of tokens then passes through a set of filters that perform secondary operations (discarding whitespace, identifying parts of speech, etc.)
The token stream is fed into a LR(k) parser generated from a symbolic Backus-Naur form (SBNF) grammar. SBNF is syntactically equivalent to ISO EBNF except that its productions are the token types from the token stream rather than characters. As output, the parser generates a events for the start and finish of rules and the passage of terminals in the parse.
Cocoon is a framework from the Apache XML project used to generate content for the web by passing simple api for XML parsing (SAX) events through a set of transformations. The next part of the system translates the events from the SBNF parser into SAX events and sets them up as a Cocoon "generator".
Cocoon allows the application of extensible stylesheet transformations (XSLT) to the SAX events from the generator. The next three parts of the system are XSLT's. The first disambiguates the parse tree by performing operations like establishing the referents of pronouns and anaphors in a regular way. The unambiguous tree is then converted into a XML representation of first-order predicate calculus (FOPC) by the second XSLT. The third XSLT then converts the FOPC markup into ISO prolog.
The separation of concerns in xcel works well. Lexical information is captured in the tokenization, then semantic information in the parsing. The use of XML in the subsequent transformations allows for semantic information to be maintained in an easily accessible form. Having only to deal with the parse trees makes alterations to the grammar much easier to propagate through the system.
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