A View of Parsing 
Ronald M. Kaplan 
Xerox Pale Alto Research Center 
The questions before this panel presuppose a distinction between parsing and 
interpretation. There are two other simple and obvious distinctions that I 
think are necessary for a reasonable discussion of the issues. First, we must 
clearly distinguish between the static specification of a process and its 
dynamic execution. Second, we must clearly distinguish two purposes that a 
natural language processing system might serve: one legitimate goal of a 
system is to perform some practical ~sk efficiently and well. while a second 
goal is to assist in developing a scientific understanding of the cognitive 
operations that underlie human language processing. 1 will refer to pa~rs 
primarily oriented towards the former goal as Practical Parsers (PP) and refer 
to the others as Performance Model Parsers (PMP). With these distinctions 
in mind. let me now turn to the questions at hand. 
1. The Computational Perspective. 
From a computadonal point of view. there are obvious reasons for 
distinguishing parsing from interpretation. Parsing is the process whereby 
linearly ordered scquences of character strings annotated with information 
found in a stored lexicon are transduced into labelled hierarchical structures. 
Interpretation maps such structures either into structures with different 
formal properties, such as logical formulas, or into sequences of actions to be 
performed on a logical model or database. On the face of it, unless we 
ignore the obvious formal differences between string--to--structure and 
structure--to--structure mappings, parsing is thus formally and conceptually 
distinct from interpretation. The specifications of thc two processes 
necessarily mention different kinds of operations that are sensitive to 
different- features of the input and express quite different generalizations 
about the correspondences betwecn form and meaning. 
As far as I can see. these are simply factual assertions about which there 
can be little or no debate. Beyond this level, however, there are a number of 
controversial issues. Even though parsing and interpretation operations are 
recognizably distinct, they can be combined in a variety of ways to construct 
a natural language understanding system. For example, the static 
specification of a s~stem could freely intermix parsing and interpretation 
operations, so that there is no part of the program text that is clearly 
identifiable as the parser or interpreter, and perhaps no part that can even be 
thought of as more pa~er-like or interpreter-like than any other. Although 
the microscopic operations fall into two classes, there is no notion in such a 
system of separate parsing and interpretation components at a macroscopic 
te~cl. .Macroscopiealty. it might be argued` a ,~yslcm specified in this way 
does not embody a parsmg/interprcmtitm distinctmn. 
On the other hand. we can imagine a system whose static specification is 
carefully divided into two parts, one that only specifies parsing operations 
and expresses parsing generalizations and one that involves only 
interpretation specifications. And there arc clearly untold numbers of system 
configurations that fall somewhere between these extremes. 
I take it to be uncontrovcrsial that. other things being equal, a 
homogenized system is less preferable on both practical and scientific 
grounds to one that naturally decomposes. Practically. such a system is 
easier to build and maintain, since the parts can be designed, developed, and 
understood to a certain extent in isolation, perhaps even by people working 
independently. Scientifically. a decomposable system is much more likely to 
provide insight into the process of natural language eomprehe~ion, whether 
by machines or people. The reasons for this can be found in Simon's classic 
essay on the Architecture of Complexity. and in other places as well. 
The debate arises from the contention that there are important "other 
things" that cannot be made equal, given a completely decomposed static 
specification. In particular, it is suggested that parsing and interpretation 
operations must be partially or totally interleaved during the execuuon of a 
comprehension process. For practical systems, arguments are advanced that 
a "habitable" system, one that human clients fecl comfortable using, must be 
able to interpret inputs before enough information is available for a complete 
syntactic structure or when the syntactic information that is available does 
not lead to a consistent parse. It is also argued that interpretation must be 
performed in the middle of parsing in the interests of reasonable efficiency: 
the interpreter can reject sub-constituents that are semantically or 
pragmatically unacceptable and thereby permit early truncation of long paths 
of syntactic computation. From the performance model perspective, it is 
suggested that humans seem able to make syntactic, semantic, and pragmatic 
decisions in parallel, and the ability to simulate this capability is thus a 
condition of adequacy for any psycholinguistic model. 
All these arguments favor a system where the operations of parsing and 
interpretation are interleaved during dynamic execution, and perhaps even 
executed on parallel hardware (or wetware, from the PMP perspective), If 
parsing and interpretation are run-time indistinguishable, it is claimed, then 
parsing and interpretation must be part and parcel of the same monolithic 
process. 
Of course, whether or not there is dynamic fusit)n of parsing and 
interpetation is an empirical question which might be answered differently 
for practical systems than for perlbrmance models, and might even be 
answered differently ior different practical implementations. Depending on 
the relative computational efficiency of parsing versus interpretation 
operations, dynamic intcrlc:ning might increase or decrease ovendl system 
efli:'ctivcness. For example, in our work t.n the I.UNAR system /Woods. 
Kaolan. & Nash-Webbcr. 1q72), we fl)tmd it more ellicient to detbr semantic 
prt~.cssmg until after a complete, well-l~.,nncd parse had been discovered. 
The consistency checks embedded in the grammar could rule out 
syntactically unacceptable structures much more quickly than our particular 
interpretation component was able to do. More recendy. Martin. Church. 
and Ramesh (1981) have claimed that overall efficiency is greatest if all 
syntactic analyses are computed in breadth-fi~t fashion before any semantic 
operations are executed. These results might be taken to indicate that the 
particular semantic components were poorly conceived and implemented, 
with little bearing on systems where interpretation is done "properly" (or 
parsing is done improperly). But they do make the point that a practical 
decision on the dynamic fusion of parsing and interpretation cannot be made 
a priori, without a detailed study of the many other factors that can influence 
a system's computational resource demands. 
Whatever conclusion we arrive at from practical considerations, there is 
no reason to believe that it will carry over to performance modelling. The 
human language faculty is an evolutiol, try compromise between the 
requirements that language be easy to learn, easy to produce, and easy to 
comprehend. Because of this. our cognitive mechanisms for comprehension 
may exhibit acceptable but not optimal efficiency, and we would therefore 
expect a successful PMP to operate with psychologically appropriate 
inefficiencies. Thus. for performance modelling, the question can be 
answered only by finding eases where the various hypotheses make crucially 
distinct predictions concerning human capabilities, errors, or profiles of 
cognitive load. and then testing these predictions in a careful series of 
psycholinguisttc experiments. It is often debated, usually by non-linguists, 
whether the recta-linguistic intuitions that form the empirical foundation for 
much of current linguistic theory are reliable indicators of the naUve 
speaker's underlying competence. When it comes to questions about internal 
processing as opposed to structural relations, the psychological literature has 
demonstrated many times that intuitions are deserving of even much less 
trust. Thus, though we may have strong beliefs to the effect that parsing and 
interpretation are psychologically inseparable, our theoretical commitments 
should rather be based on a solid experimental footing. At this point in 
time. the experimental evidence is mixed: semantic and syntactic processes 
are interleaved on-line in many situations, but there is also evidence that 
these processes have a separate, relatively non-interacting run-time coup. 
103 
However, no matter how the question of. dynamic fusion is ultimately 
resolved, it should bc clear t, ha\[ dynamic interleaving or parallelism carries 
no implicauon of" static homogeneity. A system whose run-rune behavior has 
no distinguishable components may neverthelc~ have a totally dccompo~d 
static description. Given this possibilty, and given me evident scientific 
advantages that a dccornposed static spccifgation aflords. I have adopted in 
my own rescareh on these matters the strong working hypothesis that a 
statically deeomposahle sys~n co~ be constructed to provide the necessary 
efficiencics for practical purposes and ycL perhaps with minor modirr.ations 
and l'twther ~ipulations. Still supp(~n signilicant explanauons of. 
p~ycholingmstic phenomena. 
In short, I maintain the position that the "true" comprehension system 
will also meet our pre-theorctic notions of. scientific elegance and "beauty'. 
This hypothesis, that truth and beauty are highly correlated in this domain, is 
perhaps implausible, but it presents a challenge for theory and 
implementation that has held my interest and fascination for many years. 
2. The Linguistic Perspective. 
While k is certainly Irue that our tools (computers and formal grammars) 
have shoged our views of" what human languages and human language 
preceding may be like, it seems a little bit strange to think that our views 
have been warped by those tools. Warping suggcsts, that there is rome other, 
more accurate view that we would have comc m either without mathematical 
or computational tools or with a set of formal tools with a substantially 
different character. There is no way in principle to exclude such a 
possibility, but it could hc tatar we have the tools wc have because they 
harmonize with the capabilities of the human mind for scientific 
understanding. That is. athough substantially different tools might be better 
suited to the phenomena under investigation, the results cleaved with \[hose 
tools might not be humanly appreciable. "\]'he views that have emerged from 
using our present tools might be far off the mark, but they might be the only 
views \[hat we are c~hle OC 
Perhaps a more interesting statement can be made if the question is 
interpreted as posing a conflict between the views that we as computational 
linguists have come to. guided by our present practical and formal 
understanding of what constitutes a reasonable computation, and the views 
that \[henretical linguisXs, philosophers, and others similarly unconstrained by 
concrete computation, might hold. Historically. computational Brammm~ 
have represented a mixture of intuitions about the significant gntctural 
generalizations of language and intuitions about what can be p~ 
efT~:ientiy, given a pani-'ular implementation that the grammar writer had in 
the back of his or her mind. 
This is certainly \[rue of my own work on some of the catty ATN 
grammars. Along with many others, I felt an often unconscious pressure to 
move forward along • given computational path as long as possible before 
throwing my gramnmtical fate to the purser's general nondeterntioLs~ c~oice 
mechanisms, even though \[his usually meant that feaster contents had to be 
manipulated in linguistically unjustified ways. For example, the standard 
ATN account of" passive sentcnces used register operations to •void 
backtracking that would re.analyze the NP that was initially parsed as an 
active subject. However. in so doing, the grammar confused the notions of 
surfare and deep suh)eets, and lost the ability to express gcnendizations 
concerning, for examplc, passive tag questions. 
In hindsighL I con~der that my early views were "warped" by both the 
ATN formalism, with its powerful register operations, and my understanding 
of the particular top-down, le right underlying pa~ing algorithm. As \[ 
developed the more sophisticated model of parsing embodied in my General 
Syntactic Processor, l realized that \[here was a systematic, non-fpamrr~*_~*~J 
way at" holding on to funcXionally mis-assigned constituent structures. Freed 
from worrying about exponential constituent su'ucture nondetermism, it 
became possible to restrict and simplify \[he ATN's register oparaUons and, 
ultimately, to give them a non-proceduraL algebraic interpretation. The 
result is a new grammatical formalism, Lexical-Functiona\] Grammar CKaplan 
& Bresnan, in press), a forrnalisan that admits a wider class of eff¢ient 
computational implementations than the ATN formalism just becat~ she 
grammar itself" makes fewer computational commi~nen~ Moreover, it is a 
104 
formalism that provides for the natural statement of" many language 
particular and universal gencralizations, h also seems to bc a formalism d'mt 
fatal/tales cooperaoon between linguists and computational linguists, despite 
the.~" diffcnng theoretical and me\[hodologeaI bmses. 
Just as we have been warped by our computational mechanisms, 
linguists have been warped by their formal tools, particularly the 
r~ansformational formalism. The convergence represented by Lexical- 
Functional Grammar is heartening in that it suggests that imperfect tools and 
understanding can and will evolve into better tools and deeper insights. 
3. The Interactions. 
As indicated •hove, I think computational grammars have been influenced by 
the algorithms that we expect to appb them with. While difficult w weed 
out, that influence is not a thcoretica\] or practical oeces~ty. By reducing and 
eliminaong the computational commitments of Our grammaocal forn~ism, as 
we have done with Lexical-Functional Grammar, it is possible to devise a 
variety or different parsing schemes. By comparing and coou'asUng their 
behavior with different grammars and sentences, we can begin to develop a 
deeper understanding of \[he way compulationa\] resources depend on 
properties of grammars, smngs, and algorithms. This unders~nding is 
essenUal both to practic~ implementations and also to psycholinguistic 
modelling. Furthermore, if a formalism allows grammars to be written as an 
abstract characterization of string--structure correspondences, the Jp~nunm" 
should be indifferent as to recognition or generation. We should be •hie to 
implement fcasible generators as well as parsers, and again, shed light on the 
interdependencies of grammars and grammaucal prrx:cssmg, 
. Lc( me conclude with a few comments about the psychol,ogeaI validity 
or grammars and parsing algorithms. To the extent that a grammar cor~j.ly 
models a native speaker's lingtusuc compelcnce, or, less tend~Uously, the set 
of meta-linguistic judgments he is able to make. then ti'mt srammar has a 
certain psyehok~gical "validity'. h becomes much more interepang, however, 
if" it can •l~.J be cmpeddcd in a psychologeally accurate motel of speaking 
and comprehending, h.~ all cumpct¢,nce grammars will mcc~ \[his additional 
requL,~ment, but I have the optLmis~c belief that such a grammar will 
~y be found. 
It is also possible to find psychological validation for a parsing algorithm 
in the •bsence of a particular Ipmnn~. One could in principle adduce 
evidence to \[he effect that \[he architecture of \[he parser, the structuring of its 
memory and operations, corresponds point by point to well-e,.,.,.,.,.,.,.,.,~mhl~hed 
cognitive mectmnisms. As • research strategy for •fraying at a psychologically 
valid model of comprehension, it is much more reasonable to develop 
linguisr.ically justified 8rammars and computationaUy motivated pmT, ing 
algorithms in a collaborative effort. A model with such independently 
motivated yet mutually compatible knowledBe and process components is 
much more likely to resuh in an explanatory account of \[he mechanisms 
underlying human linguisl~ abilil~=. 
References 
Kaplan, R. & Bres.oan, J. Lexical-functional grammar:. A fen'hal system for 
grammatical representation" In J. Bresnan ted.), The me;m~l 
repvecentalion of ~mmal~.ol rela,on~ Cambridse: MIT Press. in 
prem. 
Martin. W~ Church, K.. & P, ame~, P. Paper presented to the Symposium 
on Modelling Human Parsing Strategies, Unive~ty of Texas at Austin, ~z. 
Woods. W. Kaplan, R. & Nash-Wehber. B. The Lunar sr/ences nalum/ 
language information .Wslem. Cmnbridsc: Belt "Ikranek and Newnlan` 
Report 2378, 1972. 
