- Email: [email protected]
Advances in natural language generation—An interdisciplinary perspective
Computer Speech and Language ( 1990) 4,29 l-294
Book reviews Advances in Natural Language Generation-An interdisciplinary perspective edited by M. Zock and G. Sabah: Pinter, 1986. 2 ~01s. f59. Why is language generation such a “young field”, until recently taken seriously only by a few isolated researchers? The question, posed by David McDonald in his introduction to this collection of papers, seems a good place to begin. His answer is straightforwardthere has been little need for linguistic sophistication in computer messages; after all, computers are not currently capable of subtle thoughts. As many of us will have experienced, a lot may be achieved simply by the use of “canned texts”. But, McDonald argues, the need for programs which generate natural language utterances is now justifiable. He suggests that large application programs need to distance themselves from natural language, to increase the quality of their output, and to allow possibly independent influences to combine in the output message. The collection reviewed here, 21 papers in all, is the result of a workshop held at the Abbey of Royaumont, France, in 1986. Its interdisciplinary nature is partly evident in the organisation of the four sections, devoted to: (i) linguistic formalisms; (ii) implementational issues; (iii) psychological isues; and (iv) educational applications. The disciplines can all be broadly described as belonging to Cognitive Science, and in fact a majority of the papers present computational theories or implementations. It is not practical to review every contribution here: those selected reflect this reviewer’s interests. This being essentially then a collection of papers for computational linguists, the presence of a section in which grammatical formalisms are fought over is not a surprise. Two contributions, concerned with the application of Lexical Functional Grammar (LFG) to generation, may be of interest, especially in view of the tendency in much contemporary computational linguistics to make formalisms as declarative as possible, and thus indifferent to their direction of application, whether in parsing or generation. R. Block “Can a parsing grammar be used for natural language generation? The negative example of LFG” and H. Horacek “The application of unification for syntactic generation in German” take opposing views on this issue, Block comparing LFG disfavourably with Aspects-style syntax, while Horacek adapts the same formalism to handle problems specific to generation. H. Bunt “Discontinuous Phrase Structure grammar and its use in sentence generation” presents a grammatical formalism for dealing with discontinuous constituents, as in: “John talked, of course, about politics”. Such constructions present a difficulty for language understanding programs, especially those with compositional semantics. Bunt argues that describing syntax discontinuously (a discontinuous phrase-structure tree looks like any other, but with crossed branches) obviates the need for an intermediate continuous syntactic representation from which the surface is transformationally derived. Elsewhere in the collection, there is some attention, if less detailed, to the contemporary grammatical theories, including: GPSG and systemic grammar (Houghton and Pearson), Kay’s functional unification grammar (Mellish, Simonin). With the exception of systemic grammar, and possibly Dik’s functional grammar, these are formalisms which have evolved, like much in computational linguistics, in parallel with the
292
Book reviews
development of post-Chomskyan theories of syntax. I am left with the impression that for many contributors to this collection the choice of formalism is a pragmatic one, comparable to, say, choosing a programming language. It is possible though that some will find R. Fawcett’s contribution: “Language generation as choice in social interaction” a refreshing .attempt to counter what he sees as the orthodoxy imposed by generative grammarians. Whether this collection is an appropriate place for such a polemic is doubtful; in fact Fawcett’s treatment of issues concerning specifically generation is minimal. Several authors are concerned with generating from semantic representations. Case grammars, and frame-based representations, are popular (Simonin, Zock, Rosner). Other old favourites like Schank’s conceptual dependency notation have, predictably, found their way into the collection (Adorni). One writer (Bunt) goes some way towards providing Montague-style derivations, but elsewhere, with the dubious exception of S. Dik “Concerning the logical component of a natural language generator”, there is little attention to formal semantics. Of somewhat more interest to this reviewer, a few contributions are concerned with generation arising out of some computer-based model of the world. Noteworthy in this respect is H. Novak “Generating referring phrases in a dynamic environment”. The domain, computer descriptions of objects and events representing traffic situations, is, typically, a limited one. However, it provides a scope for treatment of events, and complex referring expressions, as in, for example: “the BMW which did not overtake the truck”. A related application, concerned with geometric diagrams and the process of drawing them, is described in D. RGsner “The generation system of the SEMSYN project”. The discussion there however is sketchy, possibly because Riisner is at pains to illustrate the SEMSYN generator working with a number of other applications. It is to be regretted that no space was found to provide English glosses for the examples (in German) of generated texts. Concerned also with referring expressions is R. Dale “The generation of subsequent referring expressions in structured discourse”. Dale pays critical attention to the work of writers such as Grosz and Sidner, who relate observed referring expressions to underlying theories of discourse structure. Although concerned primarily with architectures and algorithms in general, rather than specific domains, Dale illustrates his approach to generation with the example of generating cookery recipes. Of particular value to readers interested in implementation, C. Mellish “Natural language generation from plans” demonstrates that the input to a generator is not always so congruent to the desired output as is often expected. Before linguistic generation, based on a grammar/lexicon is attempted, the input, which has been produced by a Tate-style non-linear planner, is subject to a series of rewrite rules, in a phase which Mellish dubs “message optimisation”. The resulting text demonstrates how the use of a discourse connective such as “meanwhile” can be motivated by an underlying representation incorporating parallel and sequential events. Mellish is modest about his output, calling it “disappointing”. This serves however to point to a significant problem, not I think very well addressed elsewhere: namely that current generation systems often lack the amount of world knowledge needed to produce felicitous text. Unique to this collection, and untypical of the generation literature is G. Houghton and M. Pearson “The production of spoken dialogue”. This paper addresses issues specifically of speech production; a timely reminder that one of the most promising applications of generation technology is that of voice output. Houghton and Pearson adapt Power’s computational theory of conversation, enabling them to simulate two
Book reviews
293
conversing robots, “Fred” and “Doris”, which are implemented as identical programs, but with differing initial beliefs and goals. On the language production side, the paper is extremely eclectic, in its mixture of formalisms: systemic networks and GPSG. The authors have a point however: GPSG, for all its declarativeness and economy of representation is a theory of well-formedness, and does not address issues of pragmatic choice. Insofar that the systemic choice component is concerned with what to put in a message and what to leave out, it may be compared with Mellish’s message optimisation component. The section on the generation of intonation contours begins well, with an account (somewhat brief) of how surface structure, which may be focussed, is realised at a phonological level; unfortunately the remainder tends to be over-general, and rather tutorial in nature. The account of melodic assignment, for example, has little to say about how it is actually achieved in Houghton and Pearson’s implementation. The section on psychological issues contains a number of interesting papers. T. Harley “Automatic and executive processing in semantic and syntactic planning” is concerned with building a cognitive model to account for evidence that human planning and execution of speech production may occur in variable order. He proposes a neural network model as underlying the automatic component, but suggests that different, “executive” mechanisms must be available to allow the speaker to replan or consciously interfere. H. Schriefers and T. Pechmann “Incremental production of referential nounphrases by human speakers”, is also concerned with non-linearity in language production; in particular, the interleaving of planning and linguistic ordering, which according to the naive view could be neatly described as two phases in a pipeline architecture. They describe an experiment in which subjects describe geometrical shapes, and which demonstrates that interaction takes place between conceptual planning and considerations of syntactic well-formedness. The collection contains several contributions of potential interest, mainly to linguists and computer scientists engaged in language generation research. It is somewhat uneven in content, and on the expensive side. The prospective buyer might want to consider acquiring just one of the volumes--I would hesitate to say which. Interestingly, the editors and publishers may have anticipated such a decision: both volumes contain roughly identical distributions of the various ‘discipline-based’ sections. Both contain identical forewords by McDonald, and fairly close permutations of the same introduction by the editors. The volumes are on the slim side; curiously, one contributor complains of space limitations. Some of the material is rather sketchy, not to say minimalist. An extreme example occurs in the paper by V. Zammuner, “Discourse planning and production”, where section 8.5.3 contains simply two words: “Lexical K”. K, if you hadn’t guessed, has been defined earlier to denote “knowledge”. This would be more appropriate in the context of an oral presentation with slides. Since this collection emanates from a workshop, it would have been pleasant to read about some of the interchange which took place. I can appreciate however that there are many constraints on the time of authors and editors. Somewhat less pardonable, though, is the number of contributors who indulge in literature reviews, many of these covering virtually identical ground. Especially since the tutorial by McKeown and Swartout which begins the collection is truly excellent in its coverage. In particular it highlights a major deficiency in this collection: the relative lack of contributions concerning the planning component of language generation. Any future achievements in that area, I suspect, as well as in the area of spoken language generation, will go a long way to satisfying those critics who are still wondering whether language generation has something to say. N. J. YOUD
Book reviews Advances in Natural Language Generation-An interdisciplinary perspective edited by M. Zock and G. Sabah: Pinter, 1986. 2 ~01s. f59. Why is language generation such a “young field”, until recently taken seriously only by a few isolated researchers? The question, posed by David McDonald in his introduction to this collection of papers, seems a good place to begin. His answer is straightforwardthere has been little need for linguistic sophistication in computer messages; after all, computers are not currently capable of subtle thoughts. As many of us will have experienced, a lot may be achieved simply by the use of “canned texts”. But, McDonald argues, the need for programs which generate natural language utterances is now justifiable. He suggests that large application programs need to distance themselves from natural language, to increase the quality of their output, and to allow possibly independent influences to combine in the output message. The collection reviewed here, 21 papers in all, is the result of a workshop held at the Abbey of Royaumont, France, in 1986. Its interdisciplinary nature is partly evident in the organisation of the four sections, devoted to: (i) linguistic formalisms; (ii) implementational issues; (iii) psychological isues; and (iv) educational applications. The disciplines can all be broadly described as belonging to Cognitive Science, and in fact a majority of the papers present computational theories or implementations. It is not practical to review every contribution here: those selected reflect this reviewer’s interests. This being essentially then a collection of papers for computational linguists, the presence of a section in which grammatical formalisms are fought over is not a surprise. Two contributions, concerned with the application of Lexical Functional Grammar (LFG) to generation, may be of interest, especially in view of the tendency in much contemporary computational linguistics to make formalisms as declarative as possible, and thus indifferent to their direction of application, whether in parsing or generation. R. Block “Can a parsing grammar be used for natural language generation? The negative example of LFG” and H. Horacek “The application of unification for syntactic generation in German” take opposing views on this issue, Block comparing LFG disfavourably with Aspects-style syntax, while Horacek adapts the same formalism to handle problems specific to generation. H. Bunt “Discontinuous Phrase Structure grammar and its use in sentence generation” presents a grammatical formalism for dealing with discontinuous constituents, as in: “John talked, of course, about politics”. Such constructions present a difficulty for language understanding programs, especially those with compositional semantics. Bunt argues that describing syntax discontinuously (a discontinuous phrase-structure tree looks like any other, but with crossed branches) obviates the need for an intermediate continuous syntactic representation from which the surface is transformationally derived. Elsewhere in the collection, there is some attention, if less detailed, to the contemporary grammatical theories, including: GPSG and systemic grammar (Houghton and Pearson), Kay’s functional unification grammar (Mellish, Simonin). With the exception of systemic grammar, and possibly Dik’s functional grammar, these are formalisms which have evolved, like much in computational linguistics, in parallel with the
292
Book reviews
development of post-Chomskyan theories of syntax. I am left with the impression that for many contributors to this collection the choice of formalism is a pragmatic one, comparable to, say, choosing a programming language. It is possible though that some will find R. Fawcett’s contribution: “Language generation as choice in social interaction” a refreshing .attempt to counter what he sees as the orthodoxy imposed by generative grammarians. Whether this collection is an appropriate place for such a polemic is doubtful; in fact Fawcett’s treatment of issues concerning specifically generation is minimal. Several authors are concerned with generating from semantic representations. Case grammars, and frame-based representations, are popular (Simonin, Zock, Rosner). Other old favourites like Schank’s conceptual dependency notation have, predictably, found their way into the collection (Adorni). One writer (Bunt) goes some way towards providing Montague-style derivations, but elsewhere, with the dubious exception of S. Dik “Concerning the logical component of a natural language generator”, there is little attention to formal semantics. Of somewhat more interest to this reviewer, a few contributions are concerned with generation arising out of some computer-based model of the world. Noteworthy in this respect is H. Novak “Generating referring phrases in a dynamic environment”. The domain, computer descriptions of objects and events representing traffic situations, is, typically, a limited one. However, it provides a scope for treatment of events, and complex referring expressions, as in, for example: “the BMW which did not overtake the truck”. A related application, concerned with geometric diagrams and the process of drawing them, is described in D. RGsner “The generation system of the SEMSYN project”. The discussion there however is sketchy, possibly because Riisner is at pains to illustrate the SEMSYN generator working with a number of other applications. It is to be regretted that no space was found to provide English glosses for the examples (in German) of generated texts. Concerned also with referring expressions is R. Dale “The generation of subsequent referring expressions in structured discourse”. Dale pays critical attention to the work of writers such as Grosz and Sidner, who relate observed referring expressions to underlying theories of discourse structure. Although concerned primarily with architectures and algorithms in general, rather than specific domains, Dale illustrates his approach to generation with the example of generating cookery recipes. Of particular value to readers interested in implementation, C. Mellish “Natural language generation from plans” demonstrates that the input to a generator is not always so congruent to the desired output as is often expected. Before linguistic generation, based on a grammar/lexicon is attempted, the input, which has been produced by a Tate-style non-linear planner, is subject to a series of rewrite rules, in a phase which Mellish dubs “message optimisation”. The resulting text demonstrates how the use of a discourse connective such as “meanwhile” can be motivated by an underlying representation incorporating parallel and sequential events. Mellish is modest about his output, calling it “disappointing”. This serves however to point to a significant problem, not I think very well addressed elsewhere: namely that current generation systems often lack the amount of world knowledge needed to produce felicitous text. Unique to this collection, and untypical of the generation literature is G. Houghton and M. Pearson “The production of spoken dialogue”. This paper addresses issues specifically of speech production; a timely reminder that one of the most promising applications of generation technology is that of voice output. Houghton and Pearson adapt Power’s computational theory of conversation, enabling them to simulate two
Book reviews
293
conversing robots, “Fred” and “Doris”, which are implemented as identical programs, but with differing initial beliefs and goals. On the language production side, the paper is extremely eclectic, in its mixture of formalisms: systemic networks and GPSG. The authors have a point however: GPSG, for all its declarativeness and economy of representation is a theory of well-formedness, and does not address issues of pragmatic choice. Insofar that the systemic choice component is concerned with what to put in a message and what to leave out, it may be compared with Mellish’s message optimisation component. The section on the generation of intonation contours begins well, with an account (somewhat brief) of how surface structure, which may be focussed, is realised at a phonological level; unfortunately the remainder tends to be over-general, and rather tutorial in nature. The account of melodic assignment, for example, has little to say about how it is actually achieved in Houghton and Pearson’s implementation. The section on psychological issues contains a number of interesting papers. T. Harley “Automatic and executive processing in semantic and syntactic planning” is concerned with building a cognitive model to account for evidence that human planning and execution of speech production may occur in variable order. He proposes a neural network model as underlying the automatic component, but suggests that different, “executive” mechanisms must be available to allow the speaker to replan or consciously interfere. H. Schriefers and T. Pechmann “Incremental production of referential nounphrases by human speakers”, is also concerned with non-linearity in language production; in particular, the interleaving of planning and linguistic ordering, which according to the naive view could be neatly described as two phases in a pipeline architecture. They describe an experiment in which subjects describe geometrical shapes, and which demonstrates that interaction takes place between conceptual planning and considerations of syntactic well-formedness. The collection contains several contributions of potential interest, mainly to linguists and computer scientists engaged in language generation research. It is somewhat uneven in content, and on the expensive side. The prospective buyer might want to consider acquiring just one of the volumes--I would hesitate to say which. Interestingly, the editors and publishers may have anticipated such a decision: both volumes contain roughly identical distributions of the various ‘discipline-based’ sections. Both contain identical forewords by McDonald, and fairly close permutations of the same introduction by the editors. The volumes are on the slim side; curiously, one contributor complains of space limitations. Some of the material is rather sketchy, not to say minimalist. An extreme example occurs in the paper by V. Zammuner, “Discourse planning and production”, where section 8.5.3 contains simply two words: “Lexical K”. K, if you hadn’t guessed, has been defined earlier to denote “knowledge”. This would be more appropriate in the context of an oral presentation with slides. Since this collection emanates from a workshop, it would have been pleasant to read about some of the interchange which took place. I can appreciate however that there are many constraints on the time of authors and editors. Somewhat less pardonable, though, is the number of contributors who indulge in literature reviews, many of these covering virtually identical ground. Especially since the tutorial by McKeown and Swartout which begins the collection is truly excellent in its coverage. In particular it highlights a major deficiency in this collection: the relative lack of contributions concerning the planning component of language generation. Any future achievements in that area, I suspect, as well as in the area of spoken language generation, will go a long way to satisfying those critics who are still wondering whether language generation has something to say. N. J. YOUD