An important goal of computational linguistics has been to use linguistic theory to guide the construction of computationally efficient real-world natural language processing systems. At first glance, generalized phrase structure grammar (GPSG) appears to be a blessing on two counts. First, the precise formalisms of GPSG might be a direct and fransparent guide for parser design and implementation. Second, since GPSG has weak context-free generative power and context-free languages can be parsed in O(n ~) by a wide range of algorithms, GPSG parsers would appear to run in polynomial time. This widely-assumed GPSG "efficient parsability " result is misleading: here we prove that the universal recognition problem for current GPSG theory is exponential-polynomial time hard, and assuredly intractable. The paper pinpoints sources of complexity (e.g. metarules and the theory of syntactic features) in the current GPSG theory and concludes with some linguistically and computationally motivated restrictions on GPSG. 1

this paper. Because a GPSG is so closely related to a CFG, it was thought that the well-known efficient parsing techniques for CFGs could be applied, with modifications, to GPSGs, and that GPSGs would therefore be computationally tractable. Recently, however, Ristad (1985) has shown that this is not the case, and that the unrestricted GPSG parsing problem is NP-complete (on the total problem size, viz. grammar plus input sentence) . Even before Ristad's result was known, workers in this field had found the practical problems caused by the interaction of FCRs, FSDs, and the propagation conventions difficult to surmount. Briscoe's comments (1986) are typical: "Finally, the concept of privileged feature, its interaction with feature specification defaults and the bi-directionality of the head feature convention are all so complex that it is debatable how much use they would be in a practical system (even if we did manage to implement them)" (p. 1); "The interaction of feature co-occurrence restrictions, feature specification defaults and feature propagation proved very hard to implement/understand" (p. 2)

Because of the wide variety of contemporary practices used in the automatic syntactic parsing of natural languages, it has become necessary to analyze and evaluate the strengths and weaknesses of different approaches. This research is all the more necessary because there are currently no genre- and domain-independent parsers that are able to analyze unrestricted text with 100% preciseness (I use this term to refer to the correctness of analyses assigned by a parser). All these factors create a need for methods and resources that can be used to evaluate and compare parsing systems. This research describes: (1) A theoretical analysis of current achievements in parsing and parser evaluation. (2) A framework (called FEPa) that can be used to carry out practical parser evaluations and comparisons. (3) A set of new evaluation resources: FiEval is a Finnish treebank under construction, and MGTS and RobSet are parser evaluation resources in English. (4) The results of experiments in which the developed evaluation framework and the two resources for English were used for evaluating a set of selected parsers.

An efficient algorithm is described for parsing a dialect of generalised phrase structure grammar (GPSG). A practical parsing system, based on the algorithm, is presented. The dialect of GPSG which the parsing system accepts is smaller, but considerably "purer" (closer to the original definition of GPSG) and mathematically "cleaner" than that which is accepted by other practical parsing systems. In particular, the parsing system correctly implements feature co-occurrence restrictions, subject only to the restriction that the FCR set can be expressed in clausal form as a set of Horn clauses.

this paper have illustrated, context-free generarive power does not guarantee efficient parsability. Every ID/LP grammar technically generates a context-free language, but the potentially large size of the corresponding CFG means that we can't count on that fact to give us efficient parsing. Thus it is impossible to sustain this particular argument for the advantages of such formalisms as (early) GPSG over other linguistic theories; instead, GPSG and other modern theories seem to be (very roughly) in the same boat with respect to complexity. In such a situation, the linguistic merits of various theories are more important than complexity results. (See Berwick (1982), Berwick and Weinberg (1982, 1984), and Ristad (1985) for further discussion.) The reduction does not rule out the use of formalisms that decouple ID and LP constraints; note that Shieber's direct parsing algorithm wins out over the use of the object grammar. However, if we assume that natural languages are efficiently parsable (EP), then computational difficulties in parsing a formalism do indicate that the formalism itself does not tell the whole story. That is, they point out that the range of possible languages has been incorrectly characterized: the additional constraints that guarantee efficient parsability remain unstated. Since the general case of parsing ID/LP grammars is computationally difficult, if the linguistically relevant ID/LP grammars are to be efficiently parsable, there must be additional factors that guarantee a certain amount of constraint from some source. TM (Constraints beyond the bare ID/LP formalism are required on linguistic grounds as well.) Note that the subset principle of language acquisition (cf. Berwick and Weinberg 1984:233) would lead the language learner to initially hyp...