Getting Idioms into a Lexicon Based Parser's Head

An account is given of flexible idiom processing within a lexicon based parser. The view is a compositional one. The parser's behaviour is basically the "literal" one, unless a certain threshold is crossed by the weight of a particular idiom. A new process will then be added. The parser, besides yielding all idiomatic and literal interpretations embodies some claims of human processing simulation. 1. M o t i v a t i o n a n d c o m p a r i s o n w i t h o t h e r a p p r o a c h e s Id ioms a re a p e r v a s i v e p h e n o m e n o n in n a t u r a l languages. For instance, the first page of this paper (even if writ ten by a non-native speaker) includes no less than halfdozen of them. Linguists have proposed different accounts for idioms, which are derived from two basic points of view: one point of view considers idioms as the basic units of language, wi th holistic characteristics, perhaps including wordsasa particular case; the other point of view emphasizes instead the fact that idioms are made up of normal parts of speech, that play a precise role in the complete idiom. An expl ici t s t a t e m e n t wi th in th is approach is the Principle of Decompositionality (Wasow, Sag and Nunberg 1982): "When an expression admits analysis as morphologically or syntactically complex, assume as an operating hypothesis that the sense of the expression arises from the composition of the senses of its constituent parts". The syntactic consequence is that idioms are not a different thing from "normal" forms. Our view is of the lat ter kind. We are aware of the fact tha t the f lexibi l i ty of an idiom, depends on how recognizable its metaphorical origin is. Within flexible word order languages the flexibility of idioms seems to be even more closely linked to the strengths of particular syntactic constructions. Let us now briefly discuss some computational approaches to idiom understanding. Applied computational systems must necessarily have a capacity for analyzing idioms. In some systems there is a preprocessor delegated to the recognition of idiomatic forms. This preprocessor replaces the group of words that make for one idiom with the word or words that convey the meaning involved. In ATN systems instead, specially if oriented towards a particular domain, sometimes there are sequences of particular arcs inserted in the network, which, if transited, lead to the recognition of a particular idiom (e.g. PLANES, Waltz 1978). LIFER (Hendrix 1977), one of the most successful applied systems, was based on a semantic grammar, and within this mechanism idiom recognition was easy to implement, without considering flexibility. Of course, in all these systems there is no intention to give an account of human processing. PHRAN (Wilensky and Arens 1980) is a system based entirely on pattern recognition. Idiom recognition, following Fillmore's view (Fillmore 1979) is considered the basic resource all the way down to replace the concept of grammar based parsing. PHRAN is based on a data base of patterns (including single words, at the same level), and proceeds deterministically, applying the two principles "when in doubt choose the more specific pattern'* and "choose the longest pattern'. The limits of this approach lie in the capacity of generating various alternative interpretations in case of ambiguity and in running the risk of having an eccessive spread of nonterminal symbols if the data base of idioms is large. A recent work on idioms with a similar perspective is Dyer and Zernik (1986). The approach we have followed is different. The goals we had with our work must be stated explicitly: I) to yield a cognitive model of idiom processing; 2) to integrate