Interleaving Universal Principles and Relational Constraints over Typed Feature Logic

Interleaving Universal Principles And Relational Constraints Over Typed Feature Logic Thilo Giitz and Detmar Meurers SFB 340, Universit~t Tiibingen, Kleine Wilhelmstrafle 113, 72074 Tiibingen, Germany. {tg, dm}~sf s. nphil, uni-tuebingen, de Abstract We introduce a typed feature logic system providing both universal implicational principles as well as defi- nite clauses over feature terms. We show that such an architecture supports a modular encoding of linguistic theories and allows for a compact representation using underspecification. The system is fully implemented and has been used as a workbench to develop and test large HPSG grammars. The techniques described in this paper are not restricted to a specific implementa- tion, but could be added to many current feature-based grammar development systems. Introduction A significant part of the development of formalisms for computational linguistics has been concerned with find- ing the appropriate data structures to model the lin- guistic entities. The first order terms of Prolog and DCGs were replaced by feature structures in PATR style systems, 1 and in recent years systems using typed fea- ture structures have been developed. Following the tradition of DCG and PATR, these typed feature systems are generally definite clause based, e.g., CUF (D5rre and Dorna 1993), or phrase structure based, e.g., ALE (Carpenter and Penn 1994). Instead of per- mitting the grammar writer to express universal well- formedness constraints directly, the systems require the grammar writer to express relational constraints and attach them locally at the appropriate places in the grammar. 2 We believe there are several reasons why the advances in the linguistic data structures should entail the de- velopment of systems offering more expressive means for designing grammars. Using universal implicative constraints, or universal principles as they are usually called in the linguistic literature, grammatical generali- 1Cf. Shieber (1986) for a discussion of these formalisms. 2ALE has a restricted form of universal constraints, see the comparison section. sations can be expressed in a more compact and modu- lar way. Another advantage of an architecture includ- ing principles is that it computationally realizes the ar- chitecture assumed in Pollard and Sag (1994) for HPSG. It thus becomes possible to develop and test HPSG gram- mars in a computational system without having to re- code them as phrase structure or definite clause gram- mars. The architecture can also serve as extended ar- chitecture for principle based parsing (e.g., Stabler and Johnson 1993) since it facilitates the implementation of GB-style universal principles. Offering both more per- spicuous grammar code and closeness to linguistic the- ory, it seems well motivated to explore an architecture which allows both relational constraints and universal restrictions to be expressed. Our implementation is based on the idea to compile implicational constraints into a relational representa- tion (GStz and Meurers 1995) where calls to the con- straint solver are made explicit. This allows for an integration of implicational and relational constraints and a uniform evaluation strategy. Efficient processing is achieved through user-supplied delay patterns that work on both relations and implicational constraints, as well as …

Content truncated to 500 words