Parsing Without (Much) Phrase Structure 
Michael B. Kac 
Department of Linguistics 
University of Minnesota 
Minneapolis, MN 55455 
USA 
Alexis Manaster-Ramer 
Program in Linguistics 
University of Michigan 
Ann Arbor, M148109 
USA 
Approaches to NL syntax conform in varying degrees to the older 
relational/dependency model, (essentially that assumed in 
traditional grammar), which treats a sentence as a group of words 
united by various relations, and the newer constituent model. 
Some modern approaches have nonetheless involved shifts away 
from essentially constituent-based models of the sort associated 
with Bloomfield and Chomsky to more relation-based ones (e.g. 
case grammar, relational grammar, daughter-dependency and 
word grammar, corepresentational grammar) while some others, 
notably lexical-functional grammar, have nonetheless continued to 
rely crucially on certain techniques inherited from 
constituency-based grammar, particularly context-free grammar. 
In computational linguistics there is a strong (if not universal) 
reliance on phrase structure as the medium via which to represent 
syntactic structure; call this the CONSENSUS VIEW. A significant 
amount of effort has accordingly been invested in techniques by 
which to build such a representation efficiently, which has in turn 
led to considerable work on the formal and computational 
properties of context-free gramamrs (or natural extensions of 
them) and of the associated languages. In its strongest form, the 
consensus view says that the recovery of a fully specified parse 
tree is an essential step in computational language processing, and 
would, if correct, provide important support for the constituent 
model. In this paper, we shall critically examine the rationale for 
this view, and will sketch (informally) an alternative view which 
we find more defensible. The actual position we shall take for this 
discussion, however, is conservative in that we will not argue that 
there is no place whatever for constituent analysis in parsing or in 
syntactic analysis generally. What we WILL argue is that phrase 
structure is at least partly redundant in that a direct leap to the 
composition of some semantic units is possible from a relatively 
underspecified syntactic representation (as opposed to a complete 
parse tree). However, see Rindflesch forthcoming for an 
approach to.parsing which entails a much stronger denial of the 
consensus view. 
The rationale for the consensus view consists of four main 
propositions: (i) phrase structure analysis is well entrenched in 
both 'pure' and computational linguistics; (ii) phrase structure 
grammars are well understood mathematically; (iii) context-free 
languages are provably computationally tractable; (iv) semantic 
processing is either impossible, or at best highly nonoptimal, 
without a complete parse tree to work with (with the possible 
qualification that syntactic and semantic processing might be 
interleaved). We will focus on (ii-iv), (i) amounting to nothing 
more than the identification as such of the consensus view. 
Argument (ii) is compelling if, but only if, one is willing to grant 
certain other assumptions. Since these include the points at issue, 
namely that phrase structure analysis is in principle adequate to the 
task at hand, the argument is circular taken by itself. With regard 
to (iii), note that even if NL's (or large tracts of them) are 
context-free, that is SUFFICIENT tO assure that they are 
computationally tractable, but not NECESSARY. That is, the 
tractability of a language or sublanguage implies nothing with 
regard to context-freeness. 1 
Argument (iv) amounts to saying that the composition of a given 
semantic unit can be identified only after the corresponding 
syntactic constituent has been parsed, but this is false. It is 
156 
possible, both in principle and in fact, to recognize at least some 
semantic units by operating on an 'impoverished' syntactic 
representation, i.e. one which does not yet incorporate any 
information about the syntactic constituents corresponding to the 
units in question. The following sentences are offered by way of 
illustration: \[1. John likes Mary 2. Mary, John likes 3. I think 
John likes Mary 4. Mary, 1 think John likes \] In these examples, 
where all the NP's are single words, it is a trivial matter to assign 
each to one of the following schemata: \[1'. NP 1 P NP 2 2'. 
NP 1NP 2P 3'. NP 1P1 NP2P2NP3 4'. NP 1 NP 2P1NP3 
P2\] The goal in all four eases is to identify a nonlexical predicate 
consisting of likes and Mary and a predication consisting of John 
and the afore-mentioned nonlexical predicate. In 3-4, this 
predication must also be analyzed as a component of a larger one. 
Under the consensus view, this would require identification of 
constituents of the categories VP or VP/NP prior to recognition of 
nonlexical predicates, and the identification of constituents of the 
categories S or S/NP prior to the recognition of predications. But 
given just the amount of structure in the schemata shown in 1'-4', 
we can proceed directly to the semantic units as follows. 
Assuming that processing starts at the left: (a) in a sequence of 
the form NP 1 NP 2 P, leave NP 2 unlabelled; (b) in a sequence 
of the form NP P, label the NP as Subject of the P; (c) if no NP 
appears to the right of a P requiring an NP Object, associate this 
function with the nearest unlabelled NP to the left. 
We illustrate with 4. In either case, at the conclusion of the first 
pass, the predication corresponding to the subordinate clause is 
fully specified and at least the Subject of the predication 
corresponding to the main clause is identified. On the second 
pass, it suffices to search for P's requiring Object complements 
and to assign this function to any predication whose own P lies to 
the right of such a predicate. (Discontinuity poses no difficulties, 
nor is it necessary to make use of auxiliary devices such as empty 
categories to mark the positions of syntactic gaps.) Further, once 
a transitive P and its Object have been identified, these may be 
composed into a larger intransitive predicate. 
A second instructive example is provided by the problematical 
Dutch constructions discussed in Bresnan et al. 1982. The 
problem, briefly put, is that there is a class of VP's in Dutch 
which take the form NP n-1 V n but which cannot, apparently, be 
assigned a center-embedding constituent structure. Using a 
lexical-functional framework, the authors show that constraints on 
f-structure can be used as a filter on c-structure which are 
generable by the (context-free) phrase structure component of the 
grammar. If one applies this conception seriously to parsing, then 
it follows that what the parser must construct is functionally 
annotated parse trees, and yet it is not difficult to see how the 
functional information could be used, much as it was in the earlier 
example, to bypass at least some of the steps involved in 
conslxucting a c-structure. As an example, consider ... dat Jan 
Pier Marie zag helpen zwemmen 'that John saw Piet help Marie to 
swim'. One way to look at the problem would be this: imagine 
that there is a recursive way of constructing complex verbs out of 
simple verbs such that the complex inherits the arguments of the 
simplexes, and that the arguments of the complex must appear in a 
linear order corresponding to the order of the simplexes with 
which they are associated. Imagine ful'ther that it is possible to 
have rules like \[ 5. VP -> V" V; 6. V" -> NP^n V' (UP n OBJ 
DOWN)\] Given a stxing of Object NP's, we would have each of 
them beat" a different relation to the complex verb: the leftmost 
would be lOB J, the next leftmost 2OBJ, etc. There is now no 
difficulty coming up with a way to capture the generalization that 
1OBJ is the OBJ of the first simplex verb, 2OBJ the OBJ oi! the 
second and so on. In regard to parsing, we can now see that as 
long as there is a way to build up a complex V (we maintain a 
neutral stance as to how that might be done), then tile compos:,~tion 
of the semantic unit corresponding to the VP referred to in rule 
5--and the relations which obtain within it.--can be recovered 
without actually building the VP constituent of the c-structure. As 
long as there is a way, somehow, to build up as much structure as 
is represented in the schema NP NP NP \[V' V V\] V then the 
following will yield the desired results: (a) leave the initial NP 
unlabelled on the first pass; (b) for all n _> 2, label the n th NP n 
- 1OBJ of V n, 
In the example under discussion, this will make Piet and Marie 
respectively IOBJ and 2OBJ of the V' zag..helpen. The entire 
predicate can then be identified by composing the fightmost V with 
the expression consisting of the V' and its arguments; by the same 
token, the pairings of arguments of the V' with the appropriate 
daughter V's is easily accomplished. The end result is the 
recognition of all the f-structures which have to be extracted f?om 
the string without prior recognition of either the V" or VP 
constituents referred to in the rules (5-6). 
Our examples are simplified in one respect, namely that they 
involve no NP's longer than a single word. It is possible that 
something mole like phrase structure analysis is required to handle 
such units (as well as the V' referred to in the analysis of the 
Dutch example), though Rindflesch (forthcoming) argues that this 
is not the case. (See also Hudson 1976, 1984.) 
Up to this point, we have been concerned with showing that: the 
case FOR the consensus view is not especially compelling; we 
now proceed to the arguments AGAINST it. The illustration just 
given actually amounts to an m'gument against since it shows that 
tile S- or S/NP - mid VP or VP/-NP constituents of a parse trex: am 
inessential to cue the recognition of predications and nonlexical 
predicates. 
The arguments up to this point have been concerned with the 
output of a syntactic parser; it needs to be noted as well that there 
are some difficulties associated with the idea that a parser operates 
with a set of phrase structure rules, or formally similar objects. 
In Kac et al. 1986, it is argued that there are advantages to parsing 
in a series of graded stages such that at each stage onlay a 
particular, restricted type of structural infornlation (e.g. 
information about relations of subordination among verb!;) is 
being sought. A variety of different types of information are 
'compacted' into phrase structure grammars in a way which makes 
'it difficult to isolate a given type and operate with it independently 
of the others. While there is nothing in principle to prevent this 
information from being extracted from a set of PS-rules, the 
overhead imposed by the interpretation process makes this an 
unatn'aetive option. A preferable stragegy would be to have a 
highly structured grammar for the parser to refer to, with a 
hierarchy of different types of information corresponding to the 
various phases via which the entire structural representation is 
built up. 
We offer one last example which suggests strongly that phrase 
structure analysis is problematical in some cases. Consider the 
coordinate sentence John likes and Bill hates beans. One 
immediate observation that we can make is that the sequence Bill 
hates beans would, in isolation, be a sentence, which might in 
turn lead us to an analysis which, whatever else it might entail, 
would treat the material to the right of and as an S, coordinated by 
the and to some constituent occurring to the left of the 
conjunction. An obvious difficulty which stands in the way of 
this conclusion is that there does not appear prima facie to be any 
way to treat anything to the left of the and as an S, thereby 
violating the widely assumed principle that only like constituents 
can be coordinated (the principle of 'categorial harmony'). Four 
alternatives thus present themselves: abandon the analysis in favor 
of one in which the right-conjunct belongs to a category other than 
S; abandon the principle of categorial harmony; modify the 
principle of categorial harmony; find some way of analyzing the 
material to tile left of and as an S. 
The first alternative looks initially most attractive, especially when 
seen in the light of the approach to categories originally proposed 
by Gazdar (1982) and other expositions of GPSG. We could thus 
analyze the example as having the smlcture \[S\[S/NP \[S/NP John 
likes\] and \[S/NP Bill hates\] bean@ Part of the justification for 
this analysis is tile presence of an intonation break directly after 
hates that is not present when Bill hates beans is present in 
isolation. This move, however, creates two new problems. First 
of all, it involves a possibly unwarranted intrusion of phonology 
into syntax. It is one thing to argue that a phrase structure analysis 
with purely syntactic motivation serves as an accurate predictor of 
where intonation breaks will fall, quite another to let the phrase 
structure analysis he dictated by phonological considerations (in 
which case the predictions are self-fulfilling). There is a more 
serious difficulty, however, namely that while there is indeed a 
break after hates, it is not the major break (which comes directly 
after likes) despite die fact that the analysis places the major 
syntactic boundary at this point. Full consistency with the 
phonological facts would require a syntactic analysis like \[S\[S/NP 
John likes\] and \[S \[Bill hates\] beans\]\] We would then run into 
problems with the categories, however, since we would again 
have coordination of unlike constituents. Note, moreover, that it 
would not be possible to subsume S and S/NP by an 
'archicategory' (Sag et al. 1985) since the GPSG treatment of 
coordinability depends crucially on the categorical impossibility of 
coordinating X with X/Y (Gazdar 1981). 
What we have said so far should be enough to make it clear that 
finding a way to analyze an example like the one under discussion 
in phrase structure terms is not as straightforward a matter as it 
might first have appeared to be. It is conceivable that ways can be 
found around the difficulties we have mentioned, though one 
might reaonably ask whether the effort would be of genuine 
interest or whether it would be more in the nature of a holding 
action. It is, in any case, possible to handle examples like the 
ones under discussion in a straightforward manner without 
attempting a phrase slructure analysis (Kac 1985). 
Summary: 
1. The rationale for phrase structure analysis is 
uncompelling on both computational and linguistic grounds. 
2. A fully specified parse tree is partially redundant 
insofar as structural cues for the recovery of semantic 
information ate concerned. 
3. Phrase structure rules and allied formalisms do not 
provide the optimal way of representing the grammatical 
information on which a parser depends. 
4. Phrase structure analysis is problematical in certain 
cases. 
'\['hese facts imply that alternatives to the consensus view deserve to 
be investigated. 
157 
Note 
i. There is a deeper difficulty here, namely the presumption that 
NL's must be eomputationally tractable. There is, to our 
knowledge, no evidence that this is the case. While it is undeniable 
that humans parse rapidly and effortlessly most of the time, nothing 
follows from this fact regarding the computational properties of any 
NL taken as a whole. At most, it shows an understandable 
predisposition to communicate via easily parsed structures. 

References 

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Gazdar, G. 1981. Unbounded dependencies and coordinate 
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Hudson, R.A. 1976. Arguments for a Nontransformational 
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Hudson, R.A. Word Grammar. Oxford: Basil Blaekwell. 

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