This document compiles information from material published in the WWW by Hans Uskoreit, Annie Zaenen and Juan Carlos Ruiz Antón.

GFU:
A unification based grammar development system

Unification Grammars

• A very advanced and wide-spread class of linguistic formalisms are the so-called constraint-based grammar formalisms.
• Also often subsumed under the term unification grammars
• Constraint-baed grammars should not be confused with constraint grammars
• Unification grammars go beyond many earlier representation languages in that they have a clean denotational semantics
• Permit the encoding of grammatical knowledge independent from any specific processing algorithm
• These formalisms are currently used in a large number of systems
• Among the most used, constraint-based grammar models are:
  • Functional Unification Grammar (FUG) [Kay84]
  • Head-Driven Phrase-Structure Grammar (HPSG) [PS94]
  • Lexical Functional Grammar (LFG) [Bre82]
  • Categorial Unification Grammar (CUG) [HKM87,Kar89,Usz86]
  • Categorial Unification Grammar (CUG) [HKM87,Kar89,Usz86]
• For these or similar grammar models, powerful formalisms have been designed and implemented that are usually employed for both grammar development and linguistic processing, e.g, LFG [Bre82], PATR [SURT83], ALE [Car92a], STUF [BKU88], ALEP [AAB91], GFU [Ruiz91], CLE [Als92] TDL [KS94] TFS [EZ90]
• One essential ingredient of all these formalisms is complex formal descriptions of grammatical units (words, phrases, sentences) by means of sets of attribute-value pairs, so called feature terms
• Feature terms can be nested, i.e., values can be atomic symbols or feature terms.
• Feature terms can be underspecified.
• Feature terms may contain equality statements expressed by variables or coreference markers
• The formalisms share a uniform operation for the merging and checking of grammatical information, which is commonly referred to as unification
• The formalisms differ in other aspects:
  • Some of them are restricted to feature terms with simple unification (PATR)
  • Others employ more powerful data types such as disjunctive terms, functional constraints, or sets
  • Most formalisms combine phrase-structure rules or other mechanisms for building trees with the feature-term component of the language (LFG, TAG, TDL)
  • A few formalisms incorporate the phrase-structure information into the feature terms (HPSG, TFS)
• Some frameworks use inheritance type systems (HPSG, TFS, TDL, ALE). Classes of feature terms belong to types.
  • The types are partially ordered in a tree or in a (semi) lattice.
  • The type hierarchy determines for every type from which other types attributes and values are inherited,
  • which attributes are allowed and needed for a well-formed feature term of the type,
  • which types of values these attributes need,
  • and with which other types the type can be conjoined by means of unification.
• If the feature system allows complex features, attribute-value pairs in which values may again be feature-terms, this recursion can be constrained by recursive type definitions. In fact, all of grammatical recursion can be elegantly captured by such recursive types. In the extreme, the entire linguistic derivation (parsing, generation) can be construed as type deduction (HPSG, TFS).
• The strength of unification grammar formalisms lies in the advantages they offer for grammar engineering.
• Another important advabtage is the reusability of grammars

Advantages for grammar engineering

Experience has proven that large grammars can be specified, but that their development is extremely labour-extensive. Currently no methods exist for efficient distributed grammar engineering. This constitutes a serious bottleneck in the development of language technology products. The hope is that the new class of declarative formalisms will greatly facilitate linguistic engineering and thus speed up the entire development cycle. There are indications that seem to support this expectation. For some sizable grammars written in unification grammar formalisms, the development time was four years or less (TUG, CLE, TDL), whereas the development of large annotated phrase structure grammars had taken 8--12 years.

Reusability of Grammars

Another important issue in grammar engineering is the reusability of grammars. The more a grammar is committed to a certain processing model, the less are the chances that it can be adapted to other processing models or new application areas. Although scientists are still far from converging on a uniform representation format, the declarative formulation of grammar greatly facilitates porting of such grammars from one formalism to the other. Recent experiments in grammar porting seem to bear out these expectations.

It is mainly because of their expected advantages for grammar engineering that several unification formalisms have been developed or are currently used in industrial laboratories. Almost all ongoing European Union-funded language technology projects involving grammar development have adopted unification grammar formalisms.

Constraint Grammar

The aim of the Constraint Grammar (CG) formalism, developed by Fred Karlsson (from the Research Unit for Computational Linguistics at the University of Helsinki), is to analyse real text, that is, to be used with row text. On the other hand, CG supports a parsing based on morphology. A very important part of the analysis through CG is morphological disambiguation, namely the treatment of ambiguous output from morphological analysis using constraints based on linguistic knowledge.

This formalism has been applied to the grammar of Basque by the IXA Grout at the University of the Basque Country.

References

• Surveyof the State of the Art in Human Language Technology. This "online" book, available through Internet, surveys the state of the art of human language technology. The book consists of thirteen chapters written by 97 different authors. Editorial Board: Ronald A. Cole, Editor in Chief; Joseph Mariani; Hans Uszkoreit; Annie Zaenen; Victor Zue. Contents: Spoken Language Input, Written Language Input, Language Analysis and Understanding, Language Generation, Spoken Output Technologies, Discourse and Dialogue, Document Processing, Multilinguality, Multimodality, Transmission and Storage, Mathematical Methods, Language Resources, Evaluation.
• La Lingüística Computacional, página de Juan Carlos Ruiz Antón, Universitat Jaume I.