Simplifying the Complexity of Machine Translation

A description of the CAT2 machine translation system is presented as an example of a model which stresses simplicity over complexity. The design of the formalism encompasses a minimum of formal constructs, yet is powerful enough to describe complex linguistic and translational phenomena. The paper presents an overview of the formalism, followed by examples of its usage in linguistic applications. A critical evaluation is presented, in which the authors discuss the shortcomings of the system and present the directions that are being taken to achieve a more realistic, and more simpli ed, model of machine translation. Le prototype d'un syst eme de traduction automatique CAT2 est present e comme exemple d'un syst eme qui met l'accent sur la notion de simplicit e. Les quelques bases formelles du formalisme sont expliqu ees et illustr ees par des applications linguistiques. Au moyen de quelques exemples, il est montr e comment un tel formalisme permet avec des moyens tr es simples de r ealiser des grammaires aptes a traiter des ph enom enes complexes et linguistiquement int eressants. Sont egalement esquiss es et motiv es les futurs d eveloppements du formalisme actuel. Simplifying the Complexity of Machine Translation Randall Sharp and Oliver Streiter IAI Martin-Luther-Stra e 14 D-6600 Saarbr ucken 3 Germany In developing any natural language processing (NLP) system, particularly for machine translation (MT) applications, it is crucial that the basic constructs of the system become, and remain, simple. Otherwise the complexity of the system becomes overwhelming (1) for the system developers, who have to maintain the system over its lifetime, (2) for the linguists and translators who maintain the grammars, lexicons, and translation dictionaries, and (3) for the end-users who actually make use of the system in some application. The notion of simplicity is often at odds with notions of power, expressiveness, e ciency, which one also obviously needs but which seem to promote complexity, and often, always unwittingly, metamorphoses into complicatedness, where the internal and external workings of the system are no longer well understood. This tension between simplicity and appropriate complexity underscores the design and development of an experimental MT system known as CAT2. CAT2, a transfer-based MT system, was rst developed in 1987 as a sideline to Eurotra, the massive MT project sponsored by the CEC in Luxembourg encompassing nine languages in twelve countries. In 1985 the CEC sponsored a rst design based on the ,T framework (Arnold et al. 1985,1986), in which a \rapid prototyping" approach was advocated. However, given the scope of the undertaking and the intense and varied use to which the prototype was subjected, its inadequacy as a working system soon became apparent. In 1987, the CEC cancelled this attempt and developed the Engineering Framework (Bech & Nygaard 1988) which has been in development up to the present time. Although some of the shortcomings of the original prototype were overcome, the \E-Framework" also introduced a number of new complexities into the framework (see Sharp (1991) for a review), to the point where the CEC has once again undertaken a new study to develop a third, even more powerful, prototype (Alshawi et al. 1991), scheduled for completion in 1994. In the meantime, a number of sideline alternatives have sprung up in order to continue the research into machine translation, among them the MiMo system (Arnold & Sadler 1990), CLG (Balari et al. 1990), and the CAT2 system (Sharp 1988). The CAT2 system, adhering to the original ,T methodology and abiding by the maxim of controlled simplicity, has continued to develop into a viable MT prototype in which current theories of computation, linguistics, computational linguistics, and translation are being tested, re ned and applied. To date, experimental translation systems have been developed for English, French, German, Spanish, Greek, Portuguese, Russian, Czech, and Japanese. In this article we will look at the nature of the CAT2 formalism, and try to re ect the notion of simplicity behind its design. The rst section describes the formalism, the second illustrates its application, and the third covers some of the intended extensions to the formalism which will further enhance and underline its inherent simplicity.