Constituent-Based Morphological Parsing: A New Approach to the Problem of Word-Recognition.

Constituent-Based Morphological Parsing: A New Approach to the Problem of Word-Recognition. Richard Sproat Linguistics Department AT&T Bell Laboratories 600 Mountain Ave Murray Hill, NJ 07974. Barbara Brunson* AT&T Bell Laboratories and Department of Linguistics University of Toronto Toronto, Ontario, Canada M5S 1A1. Abstract We present a model of morphological processing which directly encodes prosodic constituency, a notion which is clearly crucial in many widespread morphological processes. The model has been implemented for the Australian language Warlpiri and has been successfully interfaced with a syntactic parser for that language (Brunson, 1986). We contrast our approach with approaches to morphological parsing in the KIMMO framework. 1. Introduction The "Two-Level" Model of morphological processing developed by Kimmo Koskenniemi (1983), henceforth KIMMO, has spawned much subsequent research in the same framework (Karttunen, 1983; inter alia). Important design features of this model include a set of morpheme lexicons and a set of parallel finite state transducers which implement phonological rules mapping surface strings to lexical representations. Not only are phonological rules finite state, but the control structure of the model is itself finite state. Two criticisms of this model can be put forth. First, KIMMO is not guaranteed to be computationally efficient (Barton, 1986). Second, there are many interesting morphological phenomena that KIMMO cannot cover without significantly redesigning the model. In this paper we will address the second point. We will present a model of word-structure recognition which, unlike the KIMMO model, makes heavy use of prosodic constituent structure. Not only is reference to prosodic constituency necessary to provide a principled way of dealing with certain morphological processes, but such an approach to phonological processing is crucial for any interface of current parsing systems with speech recognition systems (Church, 1983). The model has been implemented for the Australian language Warlpiri. We will describe how the parser works, and how it handles morphological phenomena that would, at best, require inelegant mechanisms within the KIMMO model. We will also show how we can handle morphological phenomena that are not exemplified in Warlpiri but which are of a similar ilk. 2. Two Facts about Morphology We will now consider two issues in morphology, namely prosody and the non- isomorphism of syntactic and phonological structure. We maintain that these are are central to the task of a morphological analyzer and, hence, have incorporated them into our model. 2.1 The Relevance of Prosody to Morphology It has become increasingly evident from research within Generative Linguistics that 65 morphology cannot be limited to the concatenation and subsequent modification of strings of segments, but must recognize prosodic constituents devoid of segmental content (McCarthy, 1979; Levin, 1985). Work on reduplication I by Marantz (1982) and by Levin (1985) has argued convincingly that reduplication involves the preftxation or suffixation of a prosodic constituent which is empty of segmental information but which receives segmental specification by copying the segmental melody from the base. Furthermore, it has been suggested that infLxation 2 must be viewed as prefixation or suffixation of an affix to a prescribed prosodic …

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