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In functional and logic programming, parsers can be built as modular executable specifications of grammars, using parser combinators and definite clause grammars respectively. These techniques are based on top-down backtracking search. Commonly used implementations are inefficient for ambiguous languages, cannot accommodate left-recursive grammars, and require exponential space to represent parse trees for highly ambiguous input. Memoization is known to improve efficiency, and work by other researchers has had some success in accommodating left recursion. This paper combines aspects of previous approaches and presents a method by which parsers can be built as modular and efficient executable specifications of ambiguous grammars containing unconstrained left recursion.

Top-down backtracking language processors are highly modular, can handle ambiguity, and are easy to implement with clear and maintainable code. However, a widely-held, and incorrect, view is that top-down processors are inherently exponential for ambiguous grammars and cannot accommodate left-recursive productions. It has been known for many years that exponential complexity can be avoided by memoization, and that left-recursive productions can be accommodated through a variety of techniques. However, until now, memoization and techniques for handling left recursion have either been presented independently, or else attempts at their integration have compromised modularity and clarity of the code.

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Few voice-in/voice-out applications are available on the web. One problem appears to be the lack of appropriate open-source tools. More speech applications would increase the functionality of the web for people with visual, cognitive, and motor disabilities. Our research group has developed open-source tools for the creation and deployment of speech applications by nonexpert as well as expert users, and an open-source software platform to deploy those applications on the web. In addition, a suite of exemplar speech applications, together with documentation, has been built to facilitate the creation and deployment of similar applications by others.

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Abstract.A system based on a general top-down parsing algorithm has been developed which allows language processors to be created as executable specifications of arbitrary attribute grammars. Declarative notation of attribute grammars allows modular construction of executable language definitions. Syntax is defined through general context-free grammar rules, and meaning is defined by associated semantic rules with arbitrary dependencies. An innovative technique allows parses to be pruned by arbitrary semantic constraints. This new technique is useful in modelling natural-language phenomena by imposing unification-like restrictions, and accommodating long-distance and cross-serial dependencies, which cannot be handled by context-free rules alone.