A Lexical Functional Grammar System in Prolog

A Lexical Functional Grammar System in Prolog Andreas Eisele and Jochen DOrre Department of Linguistics University of Stuttgart West Germany Abstract This paper describes a system in PROLOG for the automatic transforination of a grammar, written in LFG formalism, into a DCG-based parser. It demonstrates the main principles of the transformation, the representation of f-structures and constraints, the treatment of long-distance dependencies, and left rccursion. Finally some problem areas of the system and possibilities for overcoming them are discussed. Introduction In order to intprove our knowledge about natural language, it is desirable to have a high-level description language which can be used to test grammars on a computer system, but which is independent of the details of the implementation. For linguists without knowledge of programming languages, a system for writing and testing grammars on a computer should be offered. At the University of Stuttgart such a system has been imple- mented in PROLOG, which uses tile formalism of Lexical- Functional Grammar [Kaplan/Bresuan 82] as its description language. The system makes it possible for the user to enter grammar rules and lexical entries directly in the form described in [Kaplan/Bresnan 82]. The input is translated into PROLOG rules, which form a top down parser in defiuite clause grammar style. Equations and constraints associated with a grammar rule are evalnated as soon as the rule is used, thus allowing the rejection of incorrect parses as soon as constraints are violated. One of the main problems using DCG grammars - the prohibition of using left-recursive grammar rules is solved by a conversion to right-recursive rules that does not violate the semantics of the functional description. grammar implementors in two respects: i) Using DCGs for parsing (and overcoming the prohibition of left recursion) ii)Profiting from PROLOG's unification mechanism to implement LFG-Unification. Uuification LFG is a unification-based grammar formalism. To be more precise, any defining equation in LFG can be interpreted as the unification of certaiu f-structures. Unifying two f- structures is an operation very similar to set union. However, unification may fail, if the stuctures contain contradicting values for the same attribute. Otherwise the two structures become the same object, which contains the information of hoth structures. Consider for example the LFG rule S -.~ NP VP /t sueJ)=$ T==~ and take FS, FNP and FVP as the f-strnctures associated to tile S, NP and VP node, respectively. Then the two equations cau be interpreted as the unifications FS U - FVP and FS U [SUBJ = FNP]. The unification of f-structures is also closely related to the unification of PROLOG-Terms, yet there are two important differences: lu f-structures values are identified by labels (the attributes) and their number is potentially unlimited, whereas in PROLOG-terms the arguments are identified by their position and their number is fixed. In the following we show how we can model f-structure unification in PROLOG. We represent partial f-structures as an 'open ended' list of pairs: [ AI = VI , A2 = V2 ..... An = Vn I …

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