The Proper Treatment Of Word Order In HPSG

This paper describes a possibility of expressing ordering constraints among non-sister constituents in binary branching syntactic structures on a local basis, supported by viewing the binary branching structure as a list (rather than a tree) of constituents within HPSGstyle grammars. The core idea of such a description of ordering is constituted by creating a type lattice for lists. The possibilities of expressing different approaches to word order in the framework are briefly discussed, exemplified and compared to other methods. In the standard immediate-constituent based approaches, the "free" word order I is described either directly in the phrase-structure (PS) rules, which thus express simultaneously both dominance (mother/daughter) relations and precedence (ordering) relations between syntactic categories, or, in more recent formalisms such as GPSG or HPSG, by the linear precedence (LP) rules creating a separate component of the grammar, whose other component is the set of immediate-dominance (ID) rules. In both these cases, the ordering constraints are limited to sister constituents, i.e. they are strictly local. One of the problems of both these variations of the standard PSapproach is the description of adjuncts (free modifiers). On the usual assumption that their number per clause is in principle not limited (though finite for a particular clause), an approach to their ordering presupposing them to be all sister constituents laust necessarily presuppose also an (at least potentially) infinite set of generative rules (e.g., a set induced by a Kleene star used in a "basic" variant of one of the rules). In languages whose grammar allows for more word order freedom than English 2, it is often the case that adjuncts and complements of a head (typically, but not solely, of a finite verb) can be freely intermixed, which makes the approach where the locality of LP constraints forces the head as well as its modifiers (both complements and adjuncts) be expanded as sisters still less attractive. Another possibility of description of word order is the "topological" approach used predominantly in more traditionally oriented German linguistics. Applied to German, this approach divides a clause into several word order "fields" ("Vorfeld", "Mittelfeld", "Nachfeld", "linke/rechte Satzklammer") whose mutual position is fixed, and studies mainly the word order regularities within these "fields". Though a lot of work has been done and many valuable insights presented within this paradigm, seen from the viewpoint of computational linguistics this approach has the fatal disadvantage that it is extremely difficult to formalize within the standard frameworks (e.g., none of the "fields" with the possible exception of "Vorfeld" creates a constituent in any usual sense etc.). AS an alternative to the two basic approaches mentioned above (as a modification of the first one, in fact), the description based on binary right-branching structures has been proposed independently in several works concerned with languages exAca~s DE COL1NG-92, NANTES, 23-28 AO~'T 1992 1 8 4 PROC. OF COLING-92, NnbrrES, AUG. 23-28, 1992 hibiting a considerable share of free word order: in (Uszkoreit,1986) for the description of verb-final clauses Jn German, in (Gunji, 1987) for Japanese and in (Avgustinova and Ol/va,1990) for (mainly) Slavic languages. However, the price paid for the removal of some problems, mainly, the free intermixing of heads, complements and adjuncts, of the abovementioned more standard descriptions is rather high at least two problematic points arise due to the strict binarity of the structure. The first of them is the fact that in binary structures no LP-rules relying on the relation "being sister constituent" can be used for ordering heads, complements and adjuncts in cases this is required, since these are not sisters any more. The second problematic point can be seen at best at the variant of the formalism given in (Avgustinova and Oliva, 1990) the occurrence of the phonologically empty rightmost element of the branching 3 (cf. the structure (1) for the string "John kissed Mary yesterday"). 6 The former problem concerning word order is in the majority of the binarybranching approaches (as far as they are at all concerned with it) solved by introducing word order mechanisms which are either of non-local nature or which burden the syntactic categories (understood as feature bundles) with otherwise unmotivated features used solely for the purpose of imposing ordering constraints (and most often, with a combination of the two). Neither approach is more fortunate than the other nonulocality is surely an unwished phenomenon in the description, and the presence of special ordering features in the categories is hardly better, i.a. also because order is a property of the syntactic structure (made of categories) rather than of the categories themselves 4 . This paper will try to show that in spite of the abovementioned reservations the "binary branching" can be a correct and fruitful approach to syntactic description if seen from a slightly different viewpoint. In order to get the proper perspective, let us observe the "binary branching" structure for the example sentence "The small boy ate an apple" shown in (2) . There are several things to be taken into consideration here. The most obvious among them is the division of the structure into "levels" contiguous sequences.of nodes with identical marking. Thus one ,,dj( .... 1,1/ \ ~f ~o NP ~ x ~ P