A Computational Lexeme-Based Treatment of Arabic Morphology

Arabic morphology represents a special type of morphological systems. It is generally considered to be of the nonconcatenative type which depends on manipulating root letters in a nonconcatenative manner. In addition to prefixation and suffixation, inflectional and derivational processes may cause stems to undergo infixational modification in the presence of different syntactic features as well as certain stem consonants. The basic problem, then, is the large number of variants that must be analyzed or generated. In this paper, we seek to reduce the complexity of Arabic morphology using the lexeme-based morphology theory to represent the linguistic resources and MORPHE as a computational tool to implement them. We show that the space of rules can be kept small if we consider the stem as the phonological domain of realisation rules. The reduction in the number of rules keeps the system small and also increases its understandability and maintainability. We primarily focus on generation of verbs and broken plurals. 1 Lexeme-based Morphology As Lexeme-Based Morphology (LBM) plays an important role in the background of our claim, a brief review is in order. In a LBM model as in (Aronoff, 1994), only lexemes and free morphemes are minimal grammatical elements. Inflectional or derivational morphemessuffixes, prefixes, infixes and reduplication are not themselves grammatical elements. Instead, these are merely the phonological expression of operations that apply to basic grammatical elements. This approach, thus, differs from the previous computational analyses of Arabic (Beesley, 1996; Kiraz, 1994) which have, essentially, granted equal status to all the constituents of a word (the root, the pattern and the vocalism) by placing them in separate lexicons. 1.1 The lexeme concept Lexemes are vocabulary items belonging to the major lexical categories of verb, noun, adjective and adverb. For example, given the forms cat and cats, we would say that there is a lexeme CAT which has two word forms cat and cats and that the description 'the singular/plural of CAT' is a grammatical word. A lexeme then is a complex representation linking a meaning with a set of word forms or grammatical words which are associated with corresponding word forms. From the point of view of the lexicon, the lexeme is a lexical entry. 1.2 The stem and the root While a lexeme is an assembly of an item's sound form, syntax and meaning, a stem and a root correspond only to the sound-form part of 1 We write lexemes in capital letters. this assembly. That is, both roots and stems are sound forms of a lexeme, the difference between them is that a root is defined with respect to a lexeme, while a stem is always defined with respect to realization rules. Only the stem is morphologically relevant in that realization rules act on it. A root is what is left when all morphologically added structure has been wrung out. 2 The Arabic Verbal System One of the most puzzling problems in the study of Arabic is its verbal system which is very rich in forms. An Arabic verb can be conjugated according to one of the traditionally recognized patterns (or conjugations). There are 15 triliteral forms, of which at least 9 are in common use. Within each conjugation/Form, an entire paradigm of word forms is found: two tenses (perfect and imperfect), two voices (active and passive) and five moods (indicative, subjunctive, jussive, imperative and energetic). The paradigms in each conjugation are highly regular. The attested irregularities are due to the phonological peculiarities of certain root consonants. The paradigms of weak roots, however, follow automatically if the nature and position of the weak consonants are known, as will be shown below. In this section, we argue for the derivational process of Arabic inflected verbs shown in Figure 1. We consider the Arabic binyanim to be simultaneously derivational and inflectional. The choice of a binyan for a particular root is part of lexeme-formation. The net effect of lexeme-formation, however, is to place the root into an inflectional class which determines its overall morphological realization (i.e., the stems a verb lexeme can take). Stem formation is morphomic, where a morphome is defined as a mapping function using purely morphological rules. Arabic verb stems can then be realized by a single morphological function F(fn), where fn ranges over the traditionally recognized patterns/Forms (form1, form2,.fn). The function F(f1), for example, will map the verb lexeme KATAB "write" onto form1 perfective stem katab and imperfective stem ktub. 2 The terms binyan, conjugation, pattern and Form are used interchangeably. The term "word form" or "grammatical word" refers to an inflectional form. Inflectional rules then act on the resulting stems to yield fully inflected forms. Figure 1. The Derivational Process of Arabic Inflected Verbs In order to develop the analysis further, it will be useful to make illustrative reference to an Arabic verb's inflectional paradigm. We will draw upon the indicative perfective/imperfective form I conjugation of a strong and a weak triliteral verb. Tables 1 and 2 set out the perfective and imperfective word forms for the lexeme KATAB, respectively. Given the paradigms in Tables 1 and 2 below, we note the following: (i) Form 1 perfective and imperfective stems have the templates CVCVC and CCVC (C=consonant, V=vowel), respectively, throughout the paradigm; (ii) There are cases of syncretism in both paradigms; that is certain combinations of features have the same realization as certain others: certain inflected verbs have the same word form as certain others. An examination of Arabic strong verbs reveals that Form 1 perfective and imperfective stem templates are valid for all verbs. Accordingly, we can associate form 1 conjugations with two stem templates: CVCVC for the perfective and CCVC for the imperfective. There is, however, a complication with the postulation of the two prosodic stem templates provided above. The complication relates to root lexeme-formation, or