A PARLOG Implementation of Government-Binding Theory 
Robert J. Kuhns 
205 Walnut Street 
Brookline, MA 02146 
USA 
ABSTRACT 
Tile purpose of this paper is to report on research on a parallel 
parser based on the principles and constraints of Government- 
Binding Theory. The parser outputs a set of licensing relations, 
notably thematic or 0-role assignments and antecedent/anaphor 
and other binding relationships, The primary goal of the system 
ix as a liaguistics tool for exploring concurrency and autonomy 
among ir~odules of tile theory. 
BACKGROUND 
Parse,'s based oil Government-Binding (GB) Theory have been 
tile subject of a nunlber of research progranlmes. ~ While they all 
share a colllnlon linguistic framework, the design goals and 
underlying parsing strategies are varied. Some are tools for 
ewduating competence and/or performance models of language 
and others remain neutral with respect to psychological reality. 
The parsing engines range from bottom-u p to top-dow n to in ix - 
nlode methods and fronl backtracking to deterministic algo- 
rithms. Howe\er, they all approach parsing as a sequential 
process, and allhough there have been a number o\[' efforts oil 
parallel parsing (of. /I.\[uang and Guthrie 1986/, /Matsmnoto 
1986/, and/Waltz and Pollack 1985/), few are linguistically- 
oriented and none arc GB-based. Since GB is a highly modular 
system of components, development of parallel parsers incorpo- 
rating the theory allows for a t'cmnal examination into the com- 
putational aspects of GB-modules and their interfaces. 
SCOPE AND OBJECTIVES 
In oMer to explore ttle relationsllips between GB-subsystems 
and their realization in a parallel parser, a development goal is 
one of modularity ill that each GB.-module should be transpar- 
ently encoded. 2 This perlnits investigation of processes within 
and across GI:~ components to be made explicit, 
Since the focus of tile parscr is on GB principles and since GB 
itself is a theory of core gralnmar, file coverage of the parser is 
restricted to a subset of English that reflects syntactic processes 
explained by the theory. 
While it may be premature to speak of psychologically-real 
parallel parsers, many of the cognitive presuppositions that are 
the basis of the work of/Berwick and Weinberg 1984/./Marcus 
1980/, and/Milne 1983/have been the motivation for several of 
the design decisions that have been incorporated in the system. 
More specifically, the parser is deterministic and has the capa- 
bility of delaying certain actions, e.g., projection of nodes and 
resolution of lexical ambiguity, until more information is avail- 
able, rather than computing competing parses or backtracking 
uncontrollably. These features together with the goal of trans- 
parent representation of GB principles should provide a founda- 
tion for future research into tile cognitive plausibility of parallel 
parsers. 
AN IMPLEMENTATION NOTE 
The parser is being developed in PC-PARLOG: requiring an 
IBM PC, XT, AT or compatible machine with tit least 512Kb of 
memory and two floppy disk drivcs or a hard disk. 
Although tile inlplenlentation sinmlates para!lelism by atin~eshar- 
ing scheduler, tile parser run~ on a single processor nlachinc and, 
therefore, lacks true parallelism, ttowever, insofar as the pur- 
pose of the parser is to specify concurrency of G13-subsystems 
at a high level of abstraction, tile analyses are in ternls of 
compulalional processes rather than processors. As parallel 
haMware becomes tnore readily available, a valuable By-prod- 
uct of this research is that tile system coukl be ported to parallel 
logic machines with little or no additional elTort. 
PARLO(i 
PARLOG t'q~arallel Logic '+) is a parallel declarative program- 
mmg language based on logic (/Con lon 1989/, /Gregory 1987/, 
and/Shapiro 1988/). "File basic fornl of a clause is 
(I) head <-- guard: body 
where tile head is a goal r(tl ..... tn) and both guard (optiomfl) 
and hod)(obligatory) are conjunctions of goals. Each procedure 
nlust be preceded by a mode declaration specifying input ('?) and 
output (A) arguments. Sequential a xl para lel co lit lctio a (AND) 
and sequential and parallel disjunction (OR) are represented by 
"'&", ",", "';", "'.", respectively. 
THE PARSER 
The parser combines top-dov, n and bottom-up strategies and 
recovers a set of licensing relations directly; phrase structure is 
considered derivative and is not cornputed.+ The basic opera- 
tions rely on the current governing category as the left bounded 
context and are deterministic in the sense that once a node is 
typed or licensed it cannot be altered. The output of the parser is 
a constructed list of structures 
(2) CP = lcp(C(mlplementizer),ext arg(Subject), 
infl(lnfl elenlent),predicate(Verb), 
int arg 1 (Object 1 ),int arg 2(Object 2), 
indirecL.arg(PP) l, 
where cp represents the complementizer phrase, Ihe head of the 
CP or S-bar, inll denotes the inflectional elemenl, tile head of S, 
and predicate is tile verb. Tile struclures ext_arg, int_arg_l, 
and int__arg 2 are tile arguments of the verb, the lirst being the 
external argument (subject) and the last two being tile (direct) 
internal arguments (objects). The indireet_arg is a preposi- 
tional phrase (PP) that a verb may license and there may be 
several of these structures depending on the lexical specifica- 
tions of the verb. While the cp, ext_arg, infl, and predicate are 
obligatory, the internal and indirect arguments Irre contingent 
upon the 0-grid of the verb and, therefore, are optional. 
The basic objects of the system are nodes where a node is a 
structure of the form xp(Word,x_bar(Features),lndex). Word 
is a lexical item, an empty category PRO, trace, or variable, or 
an empty complementizer or inflectional element, and Featm'es 
represents the type of the node in terms of x-bar features_+N,_+V. 
Every node receives a unique index unless it is bound (co- 
indexed) to another via Binding or Control Theory. Each of the 
terms of (2) are nodes except for the arguments which are a list 
of nodes that represent the specifier, head, and complement 
structures. Specifiers may be determiner or adjectival phrases, 
and complemeuts may be PPs or, in the case of retative clauses, 
a CP, which would be reflected in another list whose structure is 
analagous to (2). 
THE LEXICON 
In accordance with GB Theory, the lexicon plays :t cent,'al role. 
Each lexical item contains the idiosyncratic features of the 
lexeme and they direct many actions of the parser. The lexicon 
is a database of PARLOG assertions which may be searched in 
parallel. A small subset of the lexicon is 
(3) mode lexeme(item?,x_bar ^, feature^). 
a. lexeme(man, \[x_bar(nl,vo) l,\[pl(men)\]). 
b. lexeme(put, \[x_bar(no,v l)\],\[...,s(puts), 
ing(putting), 0-grid(int_arg_ 1, 
locative_PP),, i.\] ). 
c. lexeme(plan, \[x._bar(no,v l),x bar(n l,vo)\], 
\[ .... 0-grid (proposition), 
tenseless,subject control,... \] ). 
The first te,'m of each lexeme is the lexical entry and the second 
term is a list of structures of the form x bar(F I ,F2), where F 1 and 
F2 are x-bar primitives, _+N, +_V, with "l" and "0" denoting "+" 
and "-", respectively. Lexically ambiguous items, e.g., (3)c., 
have more tlmn one x-bar structure. The third term is a list of 
symbols and structures that depend on the lexical item and its 
type, i.e., x-bar features. For instance, as (3)a. illustrates, nouns 
(x_bar(nl,vo)) have a plural form as It member in their list of 
features while verbs (x bar(no,vl )) have morphological deriva- 
tives, e.g., participial forms or ing, in their features list as (3)b. 
shows. Verbs also have 0-grids which characterize argument 
structures and other features indicating specifics of complement 
structure such as tenseless (infinitival) subcatcgorizations or 
control criteria, e.g., subject_control ((3)c.)." 
In the currant version of the parser, derivational morphology is 
minimal. 
THE PARSING ENGINE 
The basic actions of tile parser lrre to identify gaps, to construct 
maximal projections, and to license these projections, and these 
are encoded by tile respective procedures, detect gap, project, 
and license. The parsing loop is called recursively until tile 
sentemial input, a list, is exhausted. During a parse, governing 
categories are produced and used as left-bounded context for" 
certain procedures before being shunted to the list that will 
ultimately represent the output. 
GB MODULES 
The GB-modules of Trace, Binding, Control, and Bounding 
Theories are incorporated in the parser, especially in goals 
detect_gap and license. Detect_gap identifies the presence, if 
any, of an implicit elemeut. This may be an argument, viz., trace, 
PRO, or variable, or a non-lexical item, viz., an empty comple- 
mentizer or inflectional element. A fi'agment ~' of the PARLOG 
code for detect_gap is 
(4) mode detect gap(sentence?,gov__cat^,empty_cat^). 
a. detect_gap(\[WordlWords\],Gov cat,Empty cat)<-- 
passive_morphology(Word,Gov cat): 
Empty_cat = trltce. 
b. detectgap(\[ WordlWords\],Gov_cat,Empty cat )<--- 
Word = to,subjectless inf comp(Gov cat): 
Empty_cat = pro. 
Tile first relation has a guard to determine if the current word has 
passive morphology. In formally, the guard passivemorphology 
checks the inflectional element of the CtuTent governing cate- 
gory fro" a form of BE and, in parallel, determines if the current 
word is a verb ot'the passive form. If the guard succeeds, then the 
output substitution is complete with Empty_eat being unified 
with trace. In (4)b., the guards provide checks with the current 
token (Word) and governing category for the features of the 
verb and possible presence of an overt subjec~ in order that 
detect gap may detect a PRO. The rest of the specifications for 
detect gap have similar strategies for finding variables and 
empty inflections and complementizers. Each of these clauses 
are computed in parallel together with one that determines an 
absence of an empty category in tile current position of the 
sentence. 
The next goal in parse is project, which constructs a maximal 
projection, xp(ltem, Type). In the case that Item is lexicalty 
ambiguous, project has calls to lexical disambiguation routines 
which are invoked in parallel to attempt a resolution. 
The goal license indexes and assigns 0-roles to arguments of 
predicates. Binding Theory (/Cllomsky 1981/) has three prin- 
ciples, frequently labeled A, B, and C m the GB literature, that 
specify- co-indexation procedures for irnaphm's, pronominals. 
and referential (R-) expressions, respectively. Since the prin- 
ciples are independent, they are canclidates for parallel execu- 
tion. Thus, Binding is specified in the system as 
(5)mode binding(proj?,gov_cat?,indexed_proj^). 
binding (Proi,Gov cat,lndexed_proj) +- 
i s anaphor( Proj ): 
principle a(Proj,Gov cat, Indexed_proj). 
binding(Proj,Gov cat, Indexed proj) +-- 
i s pronom in a l ( Proj ): 
principle_b(Proj,Gov cat, ludexed_pmj). 
bi nding(Proj,Gov_cat,lnde xed_pmj) +-- 
is r_cxp(Proj): 
principle c(Proj,Gov_caI, Indexed proj). 
The purpose of the guards is to determine tile argument type of 
a particular argument (Proj) which then invokes a call tn a 
specific method of indexing in accordance with Binding Theory. 
(There is a numerical indexing scheme that is embedded in each 
principle.) The result of binding is a node, Indexed_proj, that 
has an index associated with it. 
The other major part of license is the assigmnem of 0-roles. The 
parser assigns 0-roles to the respective arguments of the predi- 
cates based on their 0-grids. Tile binding and ()-assignment 
procedures together comprise the licensing procedure. Their 
communication channels are being explored and stream AND 
parallelism appears promising. 
395 
AN EXAMPLE 
For the sentence 
(6) John was persuaded to leave. 
the parser produces 
(7) \[cp(emp,cp, 1 ), 
ext_arg(john,x_bar(n l,vo),2), 
infl(was,infl,3), 
predicate(persuaded,x_bar(no,v t),4), 
int arg_ 1 (trace,x bar(n I ,vo),2), 
int_arg 2(cp l,cp,5), 
\[cp_l (emp,cp,5), 
ext arg(pro,x bar(n I ,vo),2), 
in fl(to,infl,6), 
predicate(leave,x bar(no,v 1 ),7)\]1, 
which illustrates PP, O and trace detection and binding. 
FUTURE DIRECTIONS 
The primary focus of the experimental parser will be to include 
wider coverage within st GB-framework, including adjuncts, 
and to examine psychological aspects of concurrent language 
processing. 
CONCLUSION 
The investigation into the concurrent aspects of GB Theory is 
still in the embryonic stages. Nevertheless, early results seem to 
indicate that such research may yield significant insights into 
parallel parsing and human sentence processing. 
NOTES 
~Cf. /Abney 1986/, /Berwick 1987/, /Correa 1987/, /Kashket 
1987/,/Kuhns 1986/,/Sharp.1985/,/Stabler 1987/,/Thiersch 
1988/, and/%'ehrli I984/. 
-'/Berwick and Weinberg 1981/and/Wehrli 1988/have excel- 
lent discussions of the notion of transparency or direct encoding 
of GB-modules. 
~PC-PARLOG is conmaercially available from Parallel Logic 
Programming Limited (Twickenham, Enghmd). 
a/Betwick 1987/presents various ot, tput options with respect to 
GB-based parsers./Abney 1986/also has a novel approach to 
parsing. 
50-roles are denoted by the neutral labels ext arg, int arg_ 1, and 
int arg__2 rather than by the terms of agent, tbeme, recipient, etc. 
/Zubizaretta 1987/, for example, argues for this type of represen- 
tation. 
~'For the sake of expository simplicity and for lack of space, low- 
level implementation details, e.g., certain initialization and 
stack manipulation procedures, arc suppressed in the discussion. 

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