On Parsing Preferences

It is argued that syntactic preference principles such as Right Association and Minimal Attachment are unsatisfactory as usually formulated. Among the difficulties are: (I) dependence on ill-specified or implausible principles of parser operation; (2) dependence on questionable assumptions about syntax; (3) lack Of provision, even in principle, for integration with semantic and pragmatic preference principles; and (4) apparent counterexamples, even when discounting (I)-(3). A possible approach to a solution is sketched. I. Some preference principles The following are some standard kinds of sentences illustrating the role of syntactic preferences. (I) John bought the book which I had selected for Mary (2) John promised to visit frequently (3) The girl in the chair with the spindly legs looks bored (4) John carried the groceries for Mary (5) She wanted the dress on that rack (6) The horse raced past the Darn fell (7) The boy got fat melted (I) (3) illustrate Right Association of PP's and adverbs, i.e., the preferred association of these modifiers with the rightmost verb (phrase) or noun (phrase) they can modify (Kimball 1973). Some variants of Right Association (also characterized as Late Closure or Low Attachment) which have Dean proposed are Final Arguments (Ford et al. 1982) and Shifting Preference (Shieber 1983); the former is roughly Late Closure restricted to the last obligatory constituent and any following optional constituents of verb phrases, while the latter is Late Closure within the context of an LR(1) shiftreduce parser. Regarding (4), it would seem that according to Right Association the PP for Mar~ should be preferred as postmodifier of groceries rather than carried; yet the opposite is the case. Frazier & Fodor's (1979) explanation is based on the assumed phrase structure rules VP -> V NP PP, and NP -> NP PP: attachment of the PP into the VP minimizes the resultant number of nodes. This principle of Minimal Attachment is assumed to take precedence over Right Association. Ford et al's (1982) variant is Invoked Attachment, and Shieber's (1983) variant is Maximal Reduction; roughly speaking, the former amounts to early closure of no___nn-final constituents, while the latter chooses the longest reduction among those possible reductions whose initial constituent is "strongest" (e.g., reducing V NP PP to VP is preferred to reducing NP PP to PP). In (5), Minimal Attachment would predict association of the PP on that rack with wanted, while the actual preference is for association with dress. Both Ford et al. and Shieber account for this fact by appeal to lexical preferences: for Ford et al., the strongest form of want takes an NP complement only, so that Final Arguments prevails; for Shieber, the NP the dress is stronger than wanted, viewed as a V requiring NP and PP complements, so that the shorter reduction prevails. sentence (6) leads most people "down the garden path", a fact explainable in terms of Minimal Attachment or its variants. The explanation also works for (7) (in the case of Ford et al. with appeal to the additional principle that re-analysis of complete phrases requiring re-categorization of lexical constituents is not possible). Purportedly, this is an advantage over Marcus' (1980) parsing model, whose three-phrase buffer should allow trouble-free parsing of (7). 2. Problems with the preference principles 2.1 Dependence on ill-specified or implausible principles of parser operation. Frazier & Fodor's (1979) model does not completely specify what structures are built as each new word is accommodated. Consequently it is hard to tell exactly what the effects Of their preference principles are. Shieber's (1983) shift-reduce parser is welldefined. However, it postulates complete phrases only, whereas human parsing appears to involve integration of completely analyzed phrases into larger, incomplete phrases. Consider for example the following sentence Deginnings: (8) So I says to the ... (9) The man reconciled herself to the ... (10) The news announced on the ... (11) The reporter announced on the ... (12) John beat a rather hasty and undignified ... People presented with complete, spoken sentences beginning like (8) and (9) are able to signal detection of the errors about two or three syllables after their occurrence. Thus agreement