Memoisation: Purely, Left-recursively, and with (Continuation Passing) Style

Memoisation, or tabling, is a well-known technique that yields large improvements in the performance of some recursive computations. Tabled resolution in Prologs such as XSB and B-Prolog can transform so called left-recursive predicates from non-terminating computations into finite and well-behaved ones. In the functional programming literature, memoisation has usually been implemented in a way that does not handle left-recursion, requiring supplementary mechanisms to prevent non-termination. A notable exception is Johnson's (1995) continuation passing approach in Scheme. This, however, relies on mutation of a memo table data structure and coding in explicit continuation passing style. We show how Johnson's approach can be implemented purely functionally in a modern, strongly typed functional language (OCaml), presented via a monadic interface that hides the implementation details, yet providing a way to return a compact represention of the memo tables at the end of the computation

Robust Probabilistic Predictive Syntactic Processing

This thesis presents a broad-coverage probabilistic top-down parser, and its application to the problem of language modeling for speech recognition. The parser builds fully connected derivations incrementally, in a single pass from left-to-right across the string. We argue that the parsing approach that we have adopted is well-motivated from a psycholinguistic perspective, as a model that captures probabilistic dependencies between lexical items, as part of the process of building connected syntactic structures. The basic parser and conditional probability models are presented, and empirical results are provided for its parsing accuracy on both newspaper text and spontaneous telephone conversations. Modifications to the probability model are presented that lead to improved performance. A new language model which uses the output of the parser is then defined. Perplexity and word error rate reduction are demonstrated over trigram models, even when the trigram is trained on significantly more data. Interpolation on a word-by-word basis with a trigram model yields additional improvements.Comment: Ph.D. Thesis, Brown University, Advisor: Mark Johnson. 140 pages, 40 figures, 27 table