--- title: "Bidirectional Grammars and the Design of — Appelt 1987" description: "This is a position paper on the architecture of systems that both understand and generate natural language." authors: ["Douglas E. Appelt"] year: 1987 venue: "Theoretical Issues in Natural Language Processing 3" source: "https://aclanthology.org/T87-1042/" licence: "CC BY-NC-SA 3.0 (ACL Anthology, before 2016)" licence_url: "https://creativecommons.org/licenses/by-nc-sa/3.0/" summary_licence: "Elamur Cosmic License" canonical: "https://research.elamur.ai/free/en/papers/appelt-1987-bidirectional-grammars-design-natural-language/" --- [Elamur Research](https://md.research.elamur.ai/free/en/) · [this page in HTML](https://research.elamur.ai/free/en/papers/appelt-1987-bidirectional-grammars-design-natural-language/) [home](https://md.research.elamur.ai/free/en/) / [papers](https://md.research.elamur.ai/free/en/papers/) / [1987](https://md.research.elamur.ai/free/en/papers/1987/) / Bidirectional Grammars and the Design of Natural Language Generation Systems # Bidirectional Grammars and the Design of Natural Language Generation Systems **Authors:** Douglas E. Appelt · **Year:** 1987 · **Venue:** Theoretical Issues in Natural Language Processing 3 · **Licence:** [CC BY-NC-SA 3.0 (ACL Anthology, before 2016)](https://creativecommons.org/licenses/by-nc-sa/3.0/) **Links:** [Publisher's page](https://aclanthology.org/T87-1042/) · [PDF](https://aclanthology.org/T87-1042.pdf) · [OpenAlex](https://openalex.org/W1970027833) **Citations:** 46 citations (openalex-snapshot · 2026-10-14) · 13 in our library, of the 140673 papers whose bibliographies we read ## Summary This is a position paper on the architecture of systems that both understand and generate natural language. The author examines what changes in such a system when it relies on a bidirectional grammar: a single declarative specification of the correspondence between sentences and their meanings that both the parser and the generator use. This approach has several advantages: simpler implementation, a single lexicon, and no need to keep two separate components synchronized. The technical basis is the PATR-II formalism. In PATR-II, unification of feature structures is order-independent, so the rules are not tied to any particular process. For generation, the author proposes a symmetric scheme. A strategic module decides what to say and outputs a logical form with functional features. A tactical module decides how to say it and turns the specification into a sentence. The main finding is the problem of logical-form equivalence. The strategic module cares only about the content of a formula, while the tactical module cares about its syntax. As a result, logically equivalent formulas may yield different sentences or none at all. The paper states this key obstacle to reversible grammars early and clearly. ## Key points - A bidirectional grammar has two basic requirements. First, it must be declarative. Second, it must define a one-to-one mapping between strings and a meaning representation (logical form plus functional features). For example, an active sentence and a passive sentence share a logical form but differ in their features. DIAGRAM is given as an example of a procedural grammar that cannot be used for generation. - In PATR-II, bidirectionality comes from the associativity and commutativity of unification. Shieber's generator (still work in progress at the time) takes the logical form the parser would produce and reproduces the same sentence. - An example from electrical circuit repair: the formula Resistance-of(R1, ohm(500)) maps poorly onto the constituents of the sentence, whereas a version with an equality predicate (the verb *be*) fits better. Swapping the arguments of the equality yields a different sentence. If this conflicts with the functional features, the generator produces nothing. - The grammars of KAMP, TEXT, MUMBLE and NIGEL/PENMAN have no explicitly represented formal semantics. Their semantics is built into procedures, so these systems avoid the problem but cannot be considered bidirectional. - The author sees a possible way out in allowing the tactical component limited logical transformations. For example, unification could treat equality and the logical connectives as associative and commutative. *Our summary of the paper, in our own words — not its text. The work belongs to its authors; read it at the links above.* Fields: [Linguistics & languages](https://md.research.elamur.ai/free/en/shelves/linguistics/) · [Machine translation](https://md.research.elamur.ai/free/en/shelves/machine-translation/) ## Cite The paper itself, not this page. BibTeX from the library's record of the work. ```bibtex @inproceedings{appelt1987bidirectional, title = {{Bidirectional Grammars and the Design of Natural Language Generation Systems}}, author = {Appelt, Douglas E.}, booktitle = {Theoretical Issues in Natural Language Processing 3}, year = {1987}, doi = {10.3115/980304.980352}, url = {https://doi.org/10.3115/980304.980352} } ``` - [← previous A Compositional Semantics of Temporal Expressions in English](https://md.research.elamur.ai/free/en/papers/hinrichs-1987-compositional-semantics-temporal-expressions-english/) - [next → Machine Translation, Linguistics, and Interlingua](https://md.research.elamur.ai/free/en/papers/sgall-1987-machine-translation-linguistics-interlingua/) - [↑ back to papers of 1987](https://md.research.elamur.ai/free/en/papers/1987/) ## References 11 references, 0 of them with a card of ours. ↗ leads to the work, «find» searches for one we have no id for yet. 1. Douglas E. Appelt. Planning English Sentences. Cambridge University Press, Cambridge, England, 1985. [↗](https://doi.org/10.1017/cbo9780511624575) 2. Laurence Dardos. Conceptual and linguistic decisions in generation. In Proceedings of the Tenth International Conference on Computational Linguistics, pages 501-504, 1984. [find](https://scholar.google.com/scholar?q=Laurence+Dardos%2E+Conceptual+and+linguistic+decisions+in+generation%2E+In+Proceedings+of+the+Tenth+International+Conference+on+Computational+Linguistics%2C+pages+501%2D504%2C+1984%2E) 3. William C. Mann. An overview of the PENMAN text generation system. In Proceedings of the National Conference, pages 261-265, American Association for Artificial Intelligence, 1983. [find](https://scholar.google.com/scholar?q=William+C%2E+Mann%2E+An+overview+of+the+PENMAN+text+generation+system%2E+In+Proceedings+of+the+National+Conference%2C+pages+261%2D265%2C+American+Association+for+Artificial+Intelligence%2C+1983%2E) 4. David D. McDonald. Natural Language Generation: Complexities and Techniques. Counselor Project Tecnical Memo 14, University of Massachusetts, 1986. [find](https://scholar.google.com/scholar?q=David+D%2E+McDonald%2E+Natural+Language+Generation%3A+Complexities+and+Techniques%2E+Counselor+Project+Tecnical+Memo+14%2C+University+of+Massachusetts%2C+1986%2E) 5. David D. McDonald. Natural Language Generation as a Process of Decision Making under Constraint. PhD thesis, Massachusetts Institute of Technology, 1980. [find](https://scholar.google.com/scholar?q=David+D%2E+McDonald%2E+Natural+Language+Generation+as+a+Process+of+Decision+Making+under+Constraint%2E+PhD+thesis%2C+Massachusetts+Institute+of+Technology%2C+1980%2E) 6. Kathleen McKeown. Text Generation. Cambridge University Press, Cambridge, England, 1985. [find](https://scholar.google.com/scholar?q=Kathleen+McKeown%2E+Text+Generation%2E+Cambridge+University+Press%2C+Cambridge%2C+England%2C+1985%2E) 7. Fernando Pereira and Stuart Shieber. The semantics of grammar formalisms seen as computer languages. In Proceedings of the Tenth International Conference on Computational Linguistics, pages 123-129, 1984. [find](https://scholar.google.com/scholar?q=Fernando+Pereira+and+Stuart+Shieber%2E+The+semantics+of+grammar+formalisms+seen+as+computer+languages%2E+In+Proceedings+of+the+Tenth+International+Conference+on+Computational+Linguistics%2C+pages+123%2D129%2C+1) 8. g Jane Robinson. DIAGRAM: a grammar for dialogues. Communications of the A CM, 25(I):27-47, 1982. [find](https://scholar.google.com/scholar?q=g+Jane+Robinson%2E+DIAGRAM%3A+a+grammar+for+dialogues%2E+Communications+of+the+A+CM%2C+25%28I%29%3A27%2D47%2C+1982%2E) 9. Stuart Shieber. An Introduction to Unification-Based Approaches to Grammar. Lecture Note Series Vol. 4, Center For the Study of Language and Information, Stanford University, 1986. [I0] Stuart Shieber. A simple reconstruction of GPSG. In Proceedings of the Eleventh International Conference on Computational Linguistics, pages 211-215, 1986. [find](https://scholar.google.com/scholar?q=Stuart+Shieber%2E+An+Introduction+to+Unification%2DBased+Approaches+to+Grammar%2E+Lecture+Note+Series+Vol%2E+4%2C+Center+For+the+Study+of+Language+and+Information%2C+Stanford+University%2C+1986%2E+%5BI0%5D+Stuart+Shieber) 10. Henry Thompson. Strategy and tactics: a model for language production. In Papers from the Thirteenth Regional Meeting, Chicago Linguistics Society, 1977. [find](https://scholar.google.com/scholar?q=Henry+Thompson%2E+Strategy+and+tactics%3A+a+model+for+language+production%2E+In+Papers+from+the+Thirteenth+Regional+Meeting%2C+Chicago+Linguistics+Society%2C+1977%2E) 11. Hans Uszkoreit. Categorial unification grammars. In Proceedings of the Eleventh International Conference on Computational Linguistics, pages 187-194, 1986. [find](https://scholar.google.com/scholar?q=Hans+Uszkoreit%2E+Categorial+unification+grammars%2E+In+Proceedings+of+the+Eleventh+International+Conference+on+Computational+Linguistics%2C+pages+187%2D194%2C+1986%2E)