Transfer and MT Modularity

The t cansfer components of t y p i c a l second generation (G2) MT systems do not fully conform to the principles o~ G2 modularity, incorporating extensive target language information while failing to separate translation facts from linguistic theory. The exclusion from transfer of all non-contrastive information [eads us to a system design in which the three major components operate in parallel, rather than in seqnence. We also propose that MT systems be designed to allow translators to express their knowledge in natural metalanguage statements. I. Modularity: a Basic Principle of G2 S~stems Modularity is a defining characteristic of second genera ti.on machine translation systems (hereafter C2 MT). G2 systems are claimed to be based on a mode] in which linguistic descriptions are clearly separated from the algorithms and programs that actually produce translations. Moreover, in this mode]., the linguistic facts that pertain solely to the source language (SL) are supposed to be clearly separated from the facts that pertain solely to the target language (TL), and from those facts that concern the lexlcai and structural contrasts between TL and S],. Such, at Least, are the principles of G2 design, as set fortb for example in Vauquois [i ] . This conception of MT gives rise to systems composed of three distinct and successive phases : a monolingual analysis component, which produces a SL-dependent structural description (SD) of the input text; a transfer component, which maps that SD onto a TL-dependent SD; and a monolingual synthesis component, which transforms that SD into a TL output text. As pointed out by Kay [2], this classical G2 design offers a number of advantages. Ideally, it should allow the formal[ description of a given language to serve the needs of analysis and synthesis indifferently. As well, it should allow a given analysis or synthesis component to be coupled onto other MT systems of simi.lar design to produce translations for other language pairs. And finally, because the algorithms are independent of the particular linguistic descriptions, they too should be reusable in other MT applications. il. Transfer in T_yj~ical G2 Systems Transl!er in G2 systems like TAUM-AVIATION [3], ARIANE-78 [4] and METAL [511 is essentially a treetransformation system, relating the SDs of two complete translation unlts (generally sentences). Lexieal units (LUs) are not translated in isolation; rather, transfer rules typically test the structural environment of each SL LU and, after inserting the appropriate TL equivalent, may rearrange that structure to accord with contextual[ constraints imposed by the TL LU. Details of formalization aside, transfer rules ia these systems will encode facts like the following :