Active Schemata And Their Role In Semantic Parsing

In the past years we have been applying semantic ATN-grammars as introduced by Brown & Burton (1974) to natural language question-answering tasks (e.g. a LISP-Tutor [Barth, 1977], s question-answering system about the micro-world of soccer [Rathke & Sonntag, 1979]). We found that semantic grammars execute efficiently, but become large very quickly even with moderate domains of discourse. We therefore looked for ways to support parsing by domain-dependent knowledge represented in an inheritance network [Lauhsch,IQTQT. In this paper we first briefly describe our representation language Ohjtalk, and then illustrate how it is used for building an understanding system for processing German newspaper texts about the jobmarket situation. keywords: newspaper processing, ATN, frames semantic grammar, object-oriented programming, 0hj~alk Concepts as active schemata We have developped an object-oriented representation language called 0bjTalk in which objects are frame-like data structures which have behavioral traits and communicate through message passing. The objects (classes and instances) are organized into a (multiple) inheritance hierarchy. 0bjTalk is an extension to Lisp and was inspired by SMALLTAL~ [Goldberg&~ay, 1976], VRL [~oberts&Coldstein;, 1977], and KLONE [Braehman, 1978]. An inheritance net of objects (with the root class Object) is constructed by sending the message NEW: to the class CLASS, i.e. a concept definition in ObjTalk has the form: (ASK CLASS NEW: SUPERC (...) GENERIC-PROPERTIES : ME~HODS : [ => ] ~RIGCER-ATN ; attached ATN [ ] ...) The effect of sending NEW: to CLASS is to define a class with the given as a subclass of the named superelass(es). Instances are made by sending the concept a message that causes to fill its slots with fillers which satisfy the filler description. If an object receives a message, this will be matched against the method-filters. In case of success the corresponding method's body will be evaluated. Concepts inherit methods and generic properties fro~ their superclass(es) recursively upto the root node Object (which implements the system-defined messages). Objects may be further specified dynamically by adding slots or methods, or filling slots. For schema-driven parsing, a semantic suhATN should be activated whenever a particular word is found, or concept is expected. The basic mechanism is to attach trigger-key8 to a schema that state which words