This paper introduces a parser system for the meta grammar formalism of Extensible Dependency Grammar (XDG). XDG is a generalisation of Topological Dependency Grammar (TDG) (Duchier-Debusmann01).  The XDG parser system comprises a constraint-based parser for all possible instances of XDG, a statically typed grammar input language, and a flexible backend for handling parser output. A powerful graphical user interface provides for easy accessibility of all the functionality of the system. In the future, we will use the XDG parser system to
accomodate new dependency grammar formalisms such as Semantic Topological Dependency Grammar (STDG), and to experiment with other interesting XDG instances

Debusmann, Ralph

Acronym

A Parser System for Extensible Dependency GrammarRalph DebusmannProgramming Systems LabUniversita¨t des Saarlandes, Geb. 45Postfach 15 11 5066041 Saarbru¨cken, Germanyrade@ps.uni-sb.deAbstractThis paper introduces a parser systemfor the meta grammar formalism of Ex-tensible Dependency Grammar (XDG).XDG is a generalisation of TopologicalDependency Grammar (TDG) (Duchierand Debusmann, 2001). The XDGparser system comprises a constraint-based parser for all possible instancesof XDG, a statically typed grammar in-put language, and a flexible backendfor handling parser output. A power-ful graphical user interface provides foreasy accessibility of all the functional-ity of the system. In the future, wewill use the XDG parser system to acco-modate new dependency grammar for-malisms such as Semantic TopologicalDependency Grammar (STDG), and toexperiment with other interesting XDGinstances.1 IntroductionExtensible Dependency Grammar (XDG) is a newmeta grammar formalism for dependency gram-mar (Tesnie`re, 1959). XDG is based on the gram-mar formalism of Topological Dependency Gram-mar (TDG) (Duchier and Debusmann, 2001).XDG can be likened to Head-driven Phrase Struc-ture Grammar (HPSG) (Pollard and Sag, 1994):both XDG and HPSG are meta grammar for-malisms which need to be instantiated with par-ticular principles and parameters to obtain specificgrammar formalisms. The fundamental differenceis that whereas HPSG is based on typed featurestructures, XDG is based on graph descriptions.A parser system for TDG can be found in(Duchier and Debusmann, 2002). The parseruses constraint-based techniques developed in(Duchier, 1999) and (Duchier, 2002) to achievepolynomial parse time in the average-case, al-though the TDG parsing problem has proven to beNP-complete (Koller and Striegnitz, 2002) in theworst-case. The TDG parser system is written us-ing the Mozart-Oz programming language (Moz,1998).In this paper, we present a parser system forXDG. The system includes a constraint-basedparser for all possible instances of XDG, a stati-cally typed grammar input language, and a flexi-ble backend for handling parser output. The XDGparser is a generalisation of the TDG parser, andachieves the same polynomial parse time in the av-erage case for the TDG instance. The XDG parsersystem is also written in Mozart-Oz.The XDG parser system has at least two pur-poses. On the one hand, we use it for parsinggrammars written in Semantic Topological Depen-dency Grammar (STDG), an extension of TDGwith a syntax-semantics interface to underspec-ified semantics in the Constraint Language forLambda Structures (CLLS) (Egg et al., 1998). Onthe other, we provide the parser system as a toolfor research on various interesting dependencygrammar formalisms which are instantiations ofXDG.The structure of the paper is as follows: webriefly introduce XDG in section 2. In section 3,we present the XDG parser, and proceed to presentthe frontend and backend of the system in sections4 and 5. We introduce the graphical user interface(GUI) of the system in section 6, before we con-clude in section 7.2 Extensible Dependency Grammar(XDG)XDG is a graph description language for a set ofgraph dimensions, where each graph dimensioncorresponds to a graph. All graphs share the sameset of nodes, but have different edges and differ-ent edge labels. Simple feature structures can beattached to each node, and by lexicalisation, XDGcan be turned into a powerful dependency-basedmeta grammar formalism.The well-formedness conditions of an XDGanalysis are stipulated by parametrised principles.Each dimension uses a set of principles, stipulat-ing restrictions on the licensed graphs on this di-mension. Principles can also pose restrictions onany number of dimensions simultaneously. AnXDG analysis is well-formed only if all used prin-ciples are satisfied on all dimensions. Principlesare taken from a shared and extensible principlelibrary.The recipe to get an instance of XDG is easy:1) define the set of used graph dimensions, and 2)define the set of used principles (and their param-eters). The TDG instance uses two dimensions,ID (Immediate Dominance) and LP (Linear Prece-dence). On the ID dimension, we use the tree, in(accepted edges) and out (valency) principles. Onthe LP dimension, we use the tree, in, out, pro-jectivity, climbing and barriers principles. A de-tailed description of these principles can be foundin (Duchier, 2002).3 XDG parserThe XDG parser is at the heart of the XDG parsersystem. It can be instantiated to yield parsers forany instance of XDG, including TDG. The parseris a generalisation of the TDG parser (Duchier andDebusmann, 2002), and makes use of the sameprogramming techniques, originally developed in(Duchier, 1999) and (Duchier, 2002).The XDG parser can handle any number of di-mensions, and any number of principles used onthem. It provides a predefined principle library,including implementations of all the principles re-quired for the TDG instance of XDG. The princi-ple library can be freely extended with new prin-ciples written in Mozart-Oz.Each principle in the principle library consistsof a number of Oz functors which implement theprinciple. Therefore, it is not only possible to re-place whole principles with new ones, but also toonly partially substitute parts of principles withothers.1The parser retains the good average-case perfor-mance of the TDG parser for the TDG instanceand small test grammars. As with the TDG parser,the lack of bigger grammars has as of yet prohib-ited proper evaluation. However, we are currentlyworking on the creation of bigger grammars forCzech, English and German.4 FrontendThe XDG parser system frontend provides a in-put language for grammars called IL (Intermedi-ate Language). A grammar defines 1) an instanceof XDG (i.e. a set of used dimensions and a set ofused principles and parameters), and 2) a lexicon.The IL is statically typed to ease the discovery oferrors.Grammars written in the IL are hard to read forhumans. This is why the parser system arrangesfor the definition of input lanuguages on top of theIL. At the moment, the system provides a languagecalled UL (User Language) which is much bettersuited to write grammars in. We also provide anemacs mode for the UL. In the near future, we willprovide an XML-based input language to improveinteroperability.Grammars written in any language are firstcompiled into the IL, and then into the PL (ParserLanguage), which is the language used in the XDGparser itself.2Also new to the XDG parser system is the in-clusion of features originating from Metagrammar(Candito, 1996). Most notably, it is possible to usedisjunction almost everywhere in the lexicon tospecify a non-deterministic choice. Together withconjunction (which amounts to lexical inheritance1This is particularly useful for non-deterministic search:the existing functor implementing naive search can easily bereplaced by a functor implementing oracle-guided best-firstsearch.2At this stage, one big advantage of the new XDG parsersystem over the old TDG parser system is the ability to saveprecompiled grammars to disk, such that there is no need tocompile the grammars again and again before use.of lexical types), the frontend provides for the suc-cinct description of a number of lexical entriesby just one metagrammatical expression. Lexicaleconomy is further improved by parametrised lex-ical types. Below is an example metagrammaticalexpression written in the User Language UL:finite($ third sg) &(canonical | noncanonical)(1)The expression uses three lexical types (finite,canonical and noncanonical). finite isparametrised by an agreement expression (here:$ third sg for third person singular), and standsfor a finite verb. canonical and noncanonicalstand for canonical position and non-canonicalposition respectively. The expression defines a setof lexical entries which are finite verbs in third per-son singular inflection, and which are in canonicalposition or in non-canonical position. After com-pilation, this yields at least two distinct lexical en-tries, one for the finite verb in canonical position,and one in non-canonical position. Both entrieswill have third person singular inflection.5 BackendAfter parsing, a parser system needs to providea means to conveniently display the parsing re-sult. This is the role of the backend of the XDGparser system. We provide an output library ofoutput functors to display the result of a parse ei-ther graphically or textually. It is also possible toget output which is directly usable in LATEX docu-ments. Below in (2), we show an example graphoutput:(2)The output library is also extensible. We pro-vide means to easily write own output functors inMozart-Oz, to display both partial and full parses.6 GUIThe GUI of the XDG parser system makesall functionality easily accessible and simplifiesgrammar debugging. It allows to precompilegrammars, load them and create parsing log files.Grammars can be loaded in any of the available in-put languages, and all available output functors ofthe backend can be used. We display an examplescreenshot in (3) below:(3)7 ConclusionsWe introduced a new parser system for ExtensibleDependency Grammar (XDG), as a generalisationof the existing TDG parser system (Duchier andDebusmann, 2002). The XDG parser system canbe found in (Debusmann and Duchier, 2003). Itimproves in almost all respects on the TDG parsersystem: the XDG parser per se is much more flex-ible and extensible, and can cater for all instancesof the XDG meta grammar formalism. The fron-tend is also much more flexible and includes fea-tures from Metagrammar to significantly improvelexical economy. The backend is much more flex-ible too, and also easily extensible.We hope that the XDG parser system encour-ages many people to join working on XDG and itsvarious instances.ReferencesM.H. Candito. 1996. A principle-based hierarchicalrepresentation of LTAG. In COLING 1996 Pro-ceedings, Kopenhagen/DEN.Ralph Debusmann and Denys Duchier. 2003. eX-tensible dependency grammar. http://www.mozart-oz.org/mogul/info/debusmann/xdg.html.Denys Duchier and Ralph Debusmann. 2001. Topo-logical dependency trees: A constraint-based ac-count of linear precedence. In ACL 2001 Proceed-ings.Denys Duchier and Ralph Debus-mann. 2002. Topological Depen-dency Grammar 1.2. http://www.mozart-oz.org/mogul/info/duchier/coli/dg.html.Denys Duchier. 1999. Axiomatizing dependency pars-ing using set constraints. In Sixth Meeting on theMathematics of Language, Orlando/FL.Denys Duchier. 2002. Configuration of labeled treesunder lexicalized constraints and principles. To ap-pear in the Journal of Language and Computation.Markus Egg, Joachim Niehren, Peter Ruhrberg, andFeiyu Xu. 1998. Constraints over lambda-structures in semantic underspecification. In Pro-ceedings of COLING/ACL 1998, pages 353–359,Montreal/CAN.Alexander Koller and Kristina Striegnitz. 2002. Gen-eration as dependency parsing. In Proceedings ofACL 2002.1998. Mozart. http://www.mozart-oz.org/.Carl Pollard and Ivan Sag. 1994. Head-Driven PhraseStructure Grammar. University of Chicago Press,Chicago.Lucien Tesnie`re. 1959. Ele´ments de Syntaxe Struc-turale.

A parser system for extensible dependency grammar

Scientific documents from the Saarland University,

A Parser System for Extensible Dependency Grammar
Ralph Debusmann
Programming Systems Lab
Universita¨t des Saarlandes, Geb. 45
Postfach 15 11 50
66041 Saarbru¨cken, Germany
rade@ps.uni-sb.de
Abstract
This paper introduces a parser system
for the meta grammar formalism of Ex-
tensible Dependency Grammar (XDG).
XDG is a generalisation of Topological
Dependency Grammar (TDG) (Duchier
and Debusmann, 2001). The XDG
parser system comprises a constraint-
based parser for all possible instances
of XDG, a statically typed grammar in-
put language, and a flexible backend
for handling parser output. A power-
ful graphical user interface provides for
easy accessibility of all the functional-
ity of the system. In the future, we
will use the XDG parser system to acco-
modate new dependency grammar for-
malisms such as Semantic Topological
Dependency Grammar (STDG), and to
experiment with other interesting XDG
instances.
1 Introduction
Extensible Dependency Grammar (XDG) is a new
meta grammar formalism for dependency gram-
mar (Tesnie`re, 1959). XDG is based on the gram-
mar formalism of Topological Dependency Gram-
mar (TDG) (Duchier and Debusmann, 2001).
XDG can be likened to Head-driven Phrase Struc-
ture Grammar (HPSG) (Pollard and Sag, 1994):
both XDG and HPSG are meta grammar for-
malisms which need to be instantiated with par-
ticular principles and parameters to obtain specific
grammar formalisms. The fundamental difference
is that whereas HPSG is based on typed feature
structures, XDG is based on graph descriptions.
A parser system for TDG can be found in
(Duchier and Debusmann, 2002). The parser
uses constraint-based techniques developed in
(Duchier, 1999) and (Duchier, 2002) to achieve
polynomial parse time in the average-case, al-
though the TDG parsing problem has proven to be
NP-complete (Koller and Striegnitz, 2002) in the
worst-case. The TDG parser system is written us-
ing the Mozart-Oz programming language (Moz,
1998).
In this paper, we present a parser system for
XDG. The system includes a constraint-based
parser for all possible instances of XDG, a stati-
cally typed grammar input language, and a flexi-
ble backend for handling parser output. The XDG
parser is a generalisation of the TDG parser, and
achieves the same polynomial parse time in the av-
erage case for the TDG instance. The XDG parser
system is also written in Mozart-Oz.
The XDG parser system has at least two pur-
poses. On the one hand, we use it for parsing
grammars written in Semantic Topological Depen-
dency Grammar (STDG), an extension of TDG
with a syntax-semantics interface to underspec-
ified semantics in the Constraint Language for
Lambda Structures (CLLS) (Egg et al., 1998). On
the other, we provide the parser system as a tool
for research on various interesting dependency
grammar formalisms which are instantiations of
XDG.
The structure of the paper is as follows: we
briefly introduce XDG in section 2. In section 3,
we present the XDG parser, and proceed to present
the frontend and backend of the system in sections
4 and 5. We introduce the graphical user interface
(GUI) of the system in section 6, before we con-
clude in section 7.
2 Extensible Dependency Grammar
(XDG)
XDG is a graph description language for a set of
graph dimensions, where each graph dimension
corresponds to a graph. All graphs share the same
set of nodes, but have different edges and differ-
ent edge labels. Simple feature structures can be
attached to each node, and by lexicalisation, XDG
can be turned into a powerful dependency-based
meta grammar formalism.
The well-formedness conditions of an XDG
analysis are stipulated by parametrised principles.
Each dimension uses a set of principles, stipulat-
ing restrictions on the licensed graphs on this di-
mension. Principles can also pose restrictions on
any number of dimensions simultaneously. An
XDG analysis is well-formed only if all used prin-
ciples are satisfied on all dimensions. Principles
are taken from a shared and extensible principle
library.
The recipe to get an instance of XDG is easy:
1) define the set of used graph dimensions, and 2)
define the set of used principles (and their param-
eters). The TDG instance uses two dimensions,
ID (Immediate Dominance) and LP (Linear Prece-
dence). On the ID dimension, we use the tree, in
(accepted edges) and out (valency) principles. On
the LP dimension, we use the tree, in, out, pro-
jectivity, climbing and barriers principles. A de-
tailed description of these principles can be found
in (Duchier, 2002).
3 XDG parser
The XDG parser is at the heart of the XDG parser
system. It can be instantiated to yield parsers for
any instance of XDG, including TDG. The parser
is a generalisation of the TDG parser (Duchier and
Debusmann, 2002), and makes use of the same
programming techniques, originally developed in
(Duchier, 1999) and (Duchier, 2002).
The XDG parser can handle any number of di-
mensions, and any number of principles used on
them. It provides a predefined principle library,
including implementations of all the principles re-
quired for the TDG instance of XDG. The princi-
ple library can be freely extended with new prin-
ciples written in Mozart-Oz.
Each principle in the principle library consists
of a number of Oz functors which implement the
principle. Therefore, it is not only possible to re-
place whole principles with new ones, but also to
only partially substitute parts of principles with
others.1
The parser retains the good average-case perfor-
mance of the TDG parser for the TDG instance
and small test grammars. As with the TDG parser,
the lack of bigger grammars has as of yet prohib-
ited proper evaluation. However, we are currently
working on the creation of bigger grammars for
Czech, English and German.
4 Frontend
The XDG parser system frontend provides a in-
put language for grammars called IL (Intermedi-
ate Language). A grammar defines 1) an instance
of XDG (i.e. a set of used dimensions and a set of
used principles and parameters), and 2) a lexicon.
The IL is statically typed to ease the discovery of
errors.
Grammars written in the IL are hard to read for
humans. This is why the parser system arranges
for the definition of input lanuguages on top of the
IL. At the moment, the system provides a language
called UL (User Language) which is much better
suited to write grammars in. We also provide an
emacs mode for the UL. In the near future, we will
provide an XML-based input language to improve
interoperability.
Grammars written in any language are first
compiled into the IL, and then into the PL (Parser
Language), which is the language used in the XDG
parser itself.2
Also new to the XDG parser system is the in-
clusion of features originating from Metagrammar
(Candito, 1996). Most notably, it is possible to use
disjunction almost everywhere in the lexicon to
specify a non-deterministic choice. Together with
conjunction (which amounts to lexical inheritance
1This is particularly useful for non-deterministic search:
the existing functor implementing naive search can easily be
replaced by a functor implementing oracle-guided best-first
search.
2At this stage, one big advantage of the new XDG parser
system over the old TDG parser system is the ability to save
precompiled grammars to disk, such that there is no need to
compile the grammars again and again before use.
of lexical types), the frontend provides for the suc-
cinct description of a number of lexical entries
by just one metagrammatical expression. Lexical
economy is further improved by parametrised lex-
ical types. Below is an example metagrammatical
expression written in the User Language UL:
finite($ third sg) &
(canonical | noncanonical)
(1)
The expression uses three lexical types (finite,
canonical and noncanonical). finite is
parametrised by an agreement expression (here:
$ third sg for third person singular), and stands
for a finite verb. canonical and noncanonical
stand for canonical position and non-canonical
position respectively. The expression defines a set
of lexical entries which are finite verbs in third per-
son singular inflection, and which are in canonical
position or in non-canonical position. After com-
pilation, this yields at least two distinct lexical en-
tries, one for the finite verb in canonical position,
and one in non-canonical position. Both entries
will have third person singular inflection.
5 Backend
After parsing, a parser system needs to provide
a means to conveniently display the parsing re-
sult. This is the role of the backend of the XDG
parser system. We provide an output library of
output functors to display the result of a parse ei-
ther graphically or textually. It is also possible to
get output which is directly usable in LATEX docu-
ments. Below in (2), we show an example graph
output:
(2)
The output library is also extensible. We pro-
vide means to easily write own output functors in
Mozart-Oz, to display both partial and full parses.
6 GUI
The GUI of the XDG parser system makes
all functionality easily accessible and simplifies
grammar debugging. It allows to precompile
grammars, load them and create parsing log files.
Grammars can be loaded in any of the available in-
put languages, and all available output functors of
the backend can be used. We display an example
screenshot in (3) below:
(3)
7 Conclusions
We introduced a new parser system for Extensible
Dependency Grammar (XDG), as a generalisation
of the existing TDG parser system (Duchier and
Debusmann, 2002). The XDG parser system can
be found in (Debusmann and Duchier, 2003). It
improves in almost all respects on the TDG parser
system: the XDG parser per se is much more flex-
ible and extensible, and can cater for all instances
of the XDG meta grammar formalism. The fron-
tend is also much more flexible and includes fea-
tures from Metagrammar to significantly improve
lexical economy. The backend is much more flex-
ible too, and also easily extensible.
We hope that the XDG parser system encour-
ages many people to join working on XDG and its
various instances.
References
M.H. Candito. 1996. A principle-based hierarchical
representation of LTAG. In COLING 1996 Pro-
ceedings, Kopenhagen/DEN.
Ralph Debusmann and Denys Duchier. 2003. eX-
tensible dependency grammar. http://www.mozart-
oz.org/mogul/info/debusmann/xdg.html.
Denys Duchier and Ralph Debusmann. 2001. Topo-
logical dependency trees: A constraint-based ac-
count of linear precedence. In ACL 2001 Proceed-
ings.
Denys Duchier and Ralph Debus-
mann. 2002. Topological Depen-
dency Grammar 1.2. http://www.mozart-
oz.org/mogul/info/duchier/coli/dg.html.
Denys Duchier. 1999. Axiomatizing dependency pars-
ing using set constraints. In Sixth Meeting on the
Mathematics of Language, Orlando/FL.
Denys Duchier. 2002. Configuration of labeled trees
under lexicalized constraints and principles. To ap-
pear in the Journal of Language and Computation.
Markus Egg, Joachim Niehren, Peter Ruhrberg, and
Feiyu Xu. 1998. Constraints over lambda-
structures in semantic underspecification. In Pro-
ceedings of COLING/ACL 1998, pages 353–359,
Montreal/CAN.
Alexander Koller and Kristina Striegnitz. 2002. Gen-
eration as dependency parsing. In Proceedings of
ACL 2002.
1998. Mozart. http://www.mozart-oz.org/.
Carl Pollard and Ivan Sag. 1994. Head-Driven Phrase
Structure Grammar. University of Chicago Press,
Chicago.
Lucien Tesnie`re. 1959. Ele´ments de Syntaxe Struc-
turale.


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A parser system for extensible dependency grammar

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