Morphonology in the Lexicon

In this paper we present a means of defining morphonological phenomena in an inheritance based lexicon. We make use of the theory behind the formal language MOLUSC, in which morphological alternations were defined as mappings between sequences of tree-structured syllables. We discuss how the alternations can be defined in the inheritance-based lexical representation language DATR, and how the phonological aspects can be built upon to bring it closer to an integrated lexicon with representations which can be used by both the morphology and phonology of a language. 1 I n t r o d u c t i o n The use of inheritance mechanisms in computational linguistics has become wide-ranging, with applications in semantics, syntax, morphology and phonology. In this paper, we shall examine the applicability of such mechanisms to phonological aspects of morphology. The inheritance-based lexical representation language, DATR, has become widely used for various aspects of linguistic description, and previous treatments of both morphological and phonological phenomena in DATR have shown its applicability to this area, both for its handling of inheritance by default, and for its ability to define hierarchical structures. For example, [Gibbon, 1990] describes how Kikuyu tone displacement and Arabic non-concatenative morphology can be defined in *Thanks are d u e to Roger Evans and Dafydd Gibbon for c o m m e n t s on previous drafts of this paper. DATR and [Reinhard, 1990] describes a hierarchical approach to German umlaut. In this paper we assume a knowledge of DATR and refer the reader to the introductions in [Cahill and Evans, 1990] and [Evans and Gazdar, 1990]. MOI_USC ([Cahill, 1990a],[Cahill, 1990b],[Cahill and Gazdar, 1990]), is a formal language for defining morphological alternations as mappings between sequences of tree-structured syllables. It is based on the theory that (many) morphological alternations are phonologically based, and can best be described as operating on hierarchical structures, such as the syllable. However, there are fundamentally linear aspects of morphological alternations, which require reference to concepts such as "initial", "final" and "penultimate". An account of English verbal morphology was discussed in [Cahill, 1990b] which was expressed in a combined DATR/MOI_USC lexicon fragment. The morphological distribution was defined by the DATR while the morphological realisation was defined by a set of MOIUSC functions. In this paper, we discuss an account derived from this (see appendix), which expresses the distribution of alternations involved in the same underlying way, but which does not require a separate language to define them. In doing this, we can reduce the two-tiered DATR/MOLUSC approach originally used, to a single-tiered account. This has the obvious advantage of reducing the "mechanisms" needed. More importantly, however, we shall demonstrate, with discussion of how the morphonological information may be generalised to be more useful to the phonology proper, it also has the advantage of moving the account towards a fully-integrated lexicon, in which ultimately all levels of description morphology, phonology, orthography, syntax, semantics are combined.