Compiling HPSG type constraints into definite clause programs

Compiling HPSG type constraints into definite clause programs Thilo G~tz and Walt Detmar Meurers* SFB 340, Universit£t Tfibingcn Kleine Wilhelmstrat~e 113 72074 Tfibingen Germany ~tg, dm}©sf s. nphil, uni-tuebingen, de Abstract We present a new approach to HPSG pro- cessing: compiling HPSG grammars ex- pressed as type constraints into definite clause programs. This provides a clear and computationally useful correspondence between linguistic theories and their im- plementation. The compiler performs off- line constraint inheritance and code opti- mization. As a result, we are able to effi- ciently process with HPSG grammars with- out haviog to hand-translate them into def- inite clause or phrase structure based sys- tems. 1 Introduction The HPSG architecture as defined in (Pollard and Sag, 1994) (henceforth HPSGII) is being used by an increasing number of linguists, since the formally well-defined framework allows for a rigid and ex- plicit formalization of a linguistic theory. At the same time, the feature logics which provide the for- mal foundation of HPSGII have been used as basis for several NLP systems, such as ALE (Carpenter, 1993), CUF (DSrre and Dorna, 1993), Troll (Gerde- mann and King, 1993) or TFS (Emele and Zajac, 1990). These systems are - at least partly - intended as computational environments for the implementa- tion of HPSG grammars. HPSG linguists use the description language of the logic to express their theories in the form of im- plicative constraints. On the other hand, most of the computational setups only allow feature descriptions as extra constraints with a phrase structure or defi- nite clause based language. 1 From a computational point of view the latter setup has several advantages. It provides access to the pool of work done in the *The authors are listed alphabetically. 1One exception is the TFS system. However, the pos- sibility to express recursive relations on the level of the description language leads to serious control problems in that system. area of natural language processing, e.g., to efficient control strategies for the definite clause level based on tabelling methods like Earley deduction, or differ- ent parsing strategies in the phrase structure setup. The result is a gap between the description lan- guage theories of HPSG linguists and the definite clause or phrase structure based NLP systems pro- vided to implement these theories. Most grammars currently implemented therefore have no clear corre- spondence to the linguistic theories they originated from. To be able to use implemented grammars to provide feedback for a rigid and complete formal- ization of linguistic theories, a clear and computa- tionMly useful correspondence has to be established. This link is also needed to stimulate further devel- opment of the computational systems. Finally, an HPSGII style setup is also interesting to model from a software engineering point of view, since it permits a modular development and testing of the grammar. The purpose of this paper is to provide the de- sired link, i.e., to show how a HPSG theory formu- lated as implicative constraints can be modelled on the level of the …

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