DETERMINISTIC PARSING 
AND 
UNBOUNDED DEPENDENCIES 
Ted Briscoe 
Dept of Linguistics, Lancaster University 
Bailrigg, Lancashire 
LA1 4YT, UK 
ABSTRACT 
This paper assesses two new approaches to 
deterministic parsing with respect to the analysis of 
unbounded dependencies (UDs). UDs in English are highly 
locally (and often globally) ambiguous. Several researchers 
have argued that the difficulty of UDs undermines the 
programme of deterministic parsing. However, their 
conclusion is based on critiques of various versions of the 
Marcus parser which represents only one of many possible 
approaches to deterministic parsing. We examine the 
predictions made by a LR(1) deterministic parser and the 
Lexicat deterministic parser concerning the analysis of 
UDs. The LR(1) technique is powerful enough to resolve 
the local ambiguities we examine. However, the Lexicat 
model provides a more psychologically plausible account 
of the parsing of UDs, which also offers a unified account 
of the resolution of local and global ambiguities in these 
constructions. 
INTRODUCTION 
Church (1980:117) and Johnson-Laird (1983:313) have 
argued that the high degree of ambiguity in unbounded 
dependencies undermines the programme of deterministic 
parsing. Their conclusion is based on critiques of various 
versions of the Marcus parser (Marcus, 1980; Berwick & 
Weinberg, 1984). This parser represents only one of many 
possible approaches to deterministic parsing.. Therefore, the 
conclusion that deterministic parsing, m general, is 
impractical or psychologically implausible may be 
premature. 
In the next section, we outline the problems for the 
deterministic analysis of unbounded dependencies. In the 
succeeding sections, we present two alternative parsing 
techniques (and associated grammars) which make 
differing predictions concerning the onset and location of 
indeterminacy in the analysis of unbounded dependencies. 
We argue that the LR(1) parser is capable of 
deterministically resolving the local ambiguities which 
occur in these constructions, whilst the Lexicat parser is 
not. In the final section, we evaluate these predictions in 
the light of the Determinism Hypothesis (Marcus, 1980) 
and the Interactive Determinism Hypothesis (Briscoe & 
Boguraev, 1984; Briscoe, in press) and argue that the 
Lexicat parser in conjunction with the Interactive 
Determinism Hypothesis provides the most psychologically 
plausible and unified account of the parsing of unbounded 
dependencies. 
UNBOUNDED DEPENDENCY AMBIGUITIES 
Unbounded dependencies are found in English 
constituent questions, relative clauses and topicalised 
constructions. The dependency is between the preposed 
constituent and its point of attachment. For example, in 
(1) Who is preposed and functioning as direct object of 
the transitive verb like. 
(I) Who do you like _e 
Most current theories of grammar represent the 
grammatical role of the prcposcd constituent by 
associating it with the normal position of a constituent 
having that grammatical role. In several theories, this 
position is occupied by a phonologically null category or 
trace which is grammatically linked to the prcposed 
constituent. Wc will use _e to mark this position because 
each of the grammars associated with the parsers we 
consider adopts a 'positional' account of the recovery of 
the grammatical role of the preposed constituent. 
However, we use _c to mark an unambiguous point of 
attachment without any commitment to the presence of 
phonologically null categories or su'ucmrc in the syntactic 
representation of unbounded dependencies. 
The dependency between preposed constituent and 
point of attachment is unbounded because an unlimited 
amount of lexical material can occur between these two 
points in grammatical English constructions of this type. 
For example, it is possible to construct more and more 
embedded examples like those in (2) which exhibit the 
same dependency as (I). 
(2) 
Who do you think Kim likes c_ 
Who do you expect that Kim hopes Sandy likes e 
A parser for English capable of producing a syntactic 
representation adequate to guide semantic interpretation 
must recover the grammatical role of thc preposed 
constituent. However, whenever these constructions 
contain verbs of ambiguous valency thc correct point of 
attachment for the preposcd constituent also becorncs 
ambiguous. For example, in (3) there are two potential 
attachment points, or doubtful gaps (Fodor, 1979), written 
e?. 
(3) Who do you want e? to succeed e? 
The correct attachment of Who is ambiguous because 
both want and succeed can take, but do not require, NP 
objects. 
211 
The ambiguity in (3) is global with respect to the 
sentence; however, identical local ambiguities exist for a 
parser operating incrementally from left to fight. For 
example, in (4) the attachment of Who as object of want, 
although correct, remains doubtful until the end of the 
sentence. 
(4) Who do you want e? to succeed Bill 
Thus, at the point when the parser reaches a potential hut 
ambiguous attachment point in the left-to-fight analysis of 
the input, it cannot be sure that this is the correct 
attachment point because there may be another further 
downstream in the input, as in (3). Moreover, the point of 
attachment further downstream may be unambiguous and 
obligatory, resolving the local ambiguity in the other 
direction, as in (5). 
(5) Who do you want e? to replace fi 
To resolve the local ambiguities in unbounded 
dependencies the parser requires access to an unbounded 
amount of left and fight context, measured in terms of 
lexical material. Firsdy, when a potential attachment point 
is found, the parser must know whether or not a preposed 
constituent exists to be attached. This requires potentially 
unbounded access to the left context of the analysis since 
the preposed constituent could have occurred an 
unbounded distance hack from its point of attachment. 
Secondly, when a potential but ambiguous attachment 
point is found, the parser must decide whether it is the 
correct point of attachment. However, since this decision 
cannot be made determinately when the potential 
attachment point occurs, the parser requires access to the 
right context of forthcoming material downstream from 
the current position. The examples in (6) illustrate that 
this would require unbounded lookahead. 
(6) 
Who does Kim want e? to think that the boss will 
replace Sandy (with e_.) 
Who does Kim want e? to think that the boss 
expects the directors to replace Sandy (with e_e) 
In (6) the ~t point of attachment cannot be 
determined until the end of the sentence which can be 
arbitrarily far away (in terms of lexical material) in the 
fight context. 
Berwick & Weinberg (1984:153f) argue that the 
Marcus parser can adequately represent an unbounded left 
context with fufite resources if a successive cyclic and 
trace theoretic analysis (eg. Chomsky, 1981) of 
unbounded dependencies is adopted. However, both 
Church (1980) and Fodor (1985) demonstrate that the 
three cell lookahead buffer in the Marcus parser is not 
powerful enough to provide the required access to the 
right context in order to choose the correct point of 
attachment deterministicaUy in many unbounded 
dependency constructions. 
Marcus' (1980) Determinism Hypothesis claims that 
local ambiguities which are not resolvable in terms of the 
lookahead buffer are resolved by parsing strategy and that 
therefore, many unbounded dependency constructions 
should be psychologically complex, 'garden paths' 
requiring extensive reanalysis. There is some evidence for 
syntactic preferences in the analysis of unbounded 
dependencies; the oddity of the examples in (7), which all 
require attachment of the preposed phrase in a doubtful 
position, suggests that the human parser prefers to ignore 
doubtful points of attachment and wait for a later one. 
(7) 
a) Who did you want to give the present to Sue.'? 
b) I gave the boy who you wanted to give the 
books to three books. 
c) Sue wouldn't give the man who was reading the 
book 
However, as Fodor (1985) points out, the great majority 
of unbounded dependency constructions are not complex: 
Furthermore, the Marcus parser predicts a sharp 
distinction between 'short' unbounded dependencies 
which fall within the buffer and the remainder. No such 
distinction seems to be supported by the psychological 
data. Finally, unbounded dependencies exhibit an identical 
kind of ambiguity which can be either local or global. 
Therefore, we would expect a unified account of their 
resolution, hut the Determinism Hypothesis offers no 
account of the resolution of global ambiguities (see eg. 
Briscoe, in press). 
ALTERNATIVE DETERMINISTIC PARSERS 
There are a number of alternative deterministic parsing 
techniques many of which are in common use in 
compilers for high-level computer programming 
languages. Berwick (1985:313f) compares the Marcus 
parser to the most general of these techniques, LR(1) 
parsing (eg. Aho & Ullman, 1972), and argues that the 
Marcus parser can be seen as both an extension and 
restriction of the LR(1) technique. In fact, he argues that 
it is equivalent to a bounded context parser (eg. Aho & 
Ullman, 1972) which only allows literal access to 
grammatical symbols in the c-command domain in the 
left context and to two grammatical symbols in the right 
context. 
To date, little attention has been given to alternative 
deterministic techniques as models of natural language 
parsing in their own right, though. One exception is the 
work of Shieber (1983) and Pereira (1985), who have 
proposed that a simple extension of the LALR(1) 
technique can be used to model human natural language 
parsing strategies. The LALR(1) technique is a more 
efficient variant of the LR(1) technique. Since our 
implementation of the Shieber/Pereira model uses the 
latter technique, we will refer throughout to LR(1). With 
the grammar discussed below, the behaviour of a parser 
using either technique should be identical (see Aho & 
Ullrnan, 1972). In addition, Briscoe & Boguraev (1984) 
and Briscoe (in press) propose that a bounded context, 
deterministic parser in conjunction with an extended 
categorial grammar will also model these strategies. 
Below these two alternative approaches are compared 
with respect to unbounded dependency constructions. 
212 
The Shieber/Pereira Parser 
The LR(1) technique involves extensive preprocessing 
of the grammar used by the parser to compute all the 
distinct analysis 'paths' licensed by the grammar. This 
preprocessing results in a parse table which will 
deterministically specify the operations of a shift-reduce 
parser provided that the input grammar is an 'LR(1) 
grammar'; that is, provided that it is drawn from a subset 
of the unambiguous context-free grammars (see Aho & 
UUman, 1972; Briscoe, in press). The parse table encodes 
the set of possible left contexts for an LR(1) grammar as 
a deterministic finite-state machine. In intuitive terms, the 
LR(1) technique is able to resolve deterministically a 
subset of the possible local ambiguities which can be 
represented in a context-free grammar, and none of the 
global ambiguities. If an LR(1) parsing table is 
constructed from a grammar covering a realistic, 
ambiguous fragment of English, the resulting non- 
deterministic parsing table will contain 'clashes' between 
shift and reduce operations and between different reduce 
operations. Shieber and Pereira demonstrate that if 
shift/reduce clashes are resolved in favour of shifting and 
reduce/reduce clashes in favour of reducing with the rule 
containing the most daughters, then the parser will model 
several psychologically plausible parsing strategies, such 
as right association (eg. Frazier, 1979). 
Shieber (1983) and Pereira (1985) both provide 
grammars with a GPSG-style (Gazdar et al., 1985) 
SLASH feature analysis of unbounded dependencies. (8) 
presents a- grammar fragment written in the same style to 
mimic the GPSG account of unbounded dependencies in 
a context-free notation. 
(8) 
Terminals 
Det N Vtr V Aux want to wh $ 
Non-terminals 
SENT S VP VPinf VP/NP VPinf/NP NPwh NP 
0) SENT-> S $ 
1) S-> NP VP 
2) NP->DetN 
3) VP -> Vtr NP 4) 
VP/NP-> Vtr 
5) NP->N 
6) NPwh -> wh 
7) VP/NP -> want VPinf/NP 
8) VP/NP -> want VPinf 
9) VPinf-> to VP 
10) VP -> V NP 
11) VP -> V 
12) VPinf/NP-> to VP/NP 
13) VP/NP-> V 
14) S -> NPwh Aux VP 
15) S -> NPwh Aux NP VP/NP 
16) VP -> want VPinf 
17) VP -> want NP VPinf 
18) VP/NP -> want NP VPInf/NP 
The LR(1) technique applied to this grammar is very 
successful at resolving local ambiguities in these 
constructions; neither of the sentences in (9) result in any 
indeterminacy during parsing, despite the potential local 
ambi.guity over the attachment of the preposed 
constituent. 
(9) 
a) Who do you want Bill to succeed? 
b) Who do you want to succeed Bill? 
That is, these local ambiguities fall within the subset of 
possible local ambiguities representable in a context-free 
grammar which are resolvable by this technique. On the 
other hand, parsing the globally ambiguous example in 
(3) using the same parse table derived from this grammar 
results in a reduce/reduce conflict, because the LR(1) 
technique cannot resolve global ambiguity (by definition). 
The conflict arises when the parser is in the configuration 
shown in Figure 1. At this point, the parser must choose 
between reducing succeed to VP or to VP/NP. When this 
indeterminacy arises the entire sentence has been shifted 
onto the parse stack. 
Stack 
NPwh Aux NP 
Who do you 
Input Buffer 
want to V $ 
want to succeed 
Figure I - Configuration of LR(1) Parser 
In general, because of the LR technique of preprocessing 
the grammar and despite the unbounded nature of the 
ambiguity, the decision point will always be at the end of 
the sentence. Therefore, local ambiguities involving the 
point of attachment of preposed constituents will not 
involve parsing indeterminacy using this technique. In 
this instance, the suspicion arises that the power of 
technique may be too great for a model of human parsing 
because examples such as those in (7) above do appear to 
be garden paths. However, normally such effects are only 
predicted when a parsing conflict is resolved incorrectly 
by the rules of resolution (eg. Shieber, 1983) and no 
conflict will arise parsing these examples with a grammar 
like that in (8). 
At first sight it is surprising that these local 
ambiguities cause no problems since an LR(1) parser 
appears to have less access to the right context than the 
Marcus parser. However, the LR(1) parser makes greater 
use of the left context and also delays many syntactic 
decisions until most of the input is in the parse stack; in 
the configuration in Figure 1 no clause level attachments 
have been made, despite the fact that the complete 
sentence has been shifted into the parse stack. 
The reduce/reduce conflict in the globally ambiguous 
case occurs much later than the position of the initial 
doubtful attachment point. Moreover, the conflict cannot 
be resolved using the Shieber/Pereira resolution rules as 
they stand, since both possible reductions (VP -> V; 
VP/NP -> V) only involve one daughter. 
213 
The Lexicat Parser 
The LEXlcal-CATegorial parser is a deterministic, 
shift-reduce parser developed for extended categoriai 
grammars which include a rule of syntactic composition, 
as well as the more usual rule of application. An earlier 
version of the parser is briefly described in Briscoe & 
Boguraev (1984). Briscoe (in press) provides a complete 
description of Lcxicat. Ades & Stcedman (1982), 
Steedman (1985) and Briscoe (in press) discuss 
composition in further detail from the perspectives of 
syntax, semantics and parsing. 
In a categofiai grammar most syntactic information is 
located in the assignment of categories to lexical items. 
The rules of composition and application and a lexicon 
which suffices for the fragment under consideration are 
given in (I0). 
(10) 
Function-Argument Application 
X YIX => Y 
Function-Function Composition 
XIY YIZ => XIZ 
Bill : NP to : VPinf/VP 
you : NP do : S/VP/NP 
who : NP succeed : VPINP, VP 
want : VP/VPinflNP, VP/VPinf 
grammar assigns the two analyses shown in Figure 2 This 
to the ambiguous example (3). 
who do you want to succeed 
NP S/VP/NP NP VP/VPinf VPinf/VP VPINP 
App 
S/VP 
Comp 
S/VPinf 
S/VP 
-Comp 
Comp 
SINP 
.... -App 
S 
who do you want to suc. 
NP S/VP/NP NP VP/VPinflNP VPinffVP VP 
-App 
S/VP 
- -7- -Comp 
S/VPinflN? 
..... App 
S/VPinf 
............ -Comp 
S/VP 
.................... App 
S 
Figure 2 - Analysis of Unbounded Dependencies 
The grammar represents the grammatical role of the 
preposed constituent by relating it directly to the verbal 
category. The material intervening between the preposed 
constituent and its point of attachment is composed into 
one (partial) constituent. Steedman (1985) provides 
linguistic motivation for a very similar analysis. 
Lcxicat employs one push down stack for analysis and 
storage of partially analysed material in thc left and fight 
context. Parsing proceeds on the basis of a three cell 
window into the stack. The item in the first cell (at the 
fight hand end) of the stack represents a one word 
lookahcad into the fight context. This cell can only 
contain the lexical entry for the next word in the input. 
So, in common with LR(1) parsers but unlike the Marcus 
parser, Lexicat is restricted to lookahead of one Icxical 
item. The second cell contains the syntactic category or 
categories associated with the current (partial) constituent 
under analysis. The third cell provides the left context for 
the parse and can contain the syntactic category or 
categories associated with the adjacent (partial) 
constituent to the left of the current constituent. Cells 
further down the stack contain (partial) constituents 
awaiting further integration into thc analysis, but do not 
form part of the left context for parsing decisions. 
Lexicat is a (1,1)-bounded context parser because it 
only has access to one set of grammatical symbols to the 
left and one set of grammatical symbols to the right of 
the current constituent (see Bfiscoe, in press). As such it 
is demonstrably less powerful than the LR(1) technique, 
which ailows access to any aspect of the left context 
which can be represented as a regular expression, and the 
Marcus parser, which allows access to grammatical 
symbols in the c-command domain in the left context and 
two (not neccssafily terminal) symbols in the right 
context (eg. Berwick, 1985:313f). However, it is unclear, 
at present, what precisely can be concluded on the basis 
of these differences in the parsing techniques because of 
the differing properties of the grammatical theories 
employed in each model. 
The Lexicat parser does not employ a parse table or 
use parsing states to maintain information about the 
nature of the left context. Rules of application and 
composition (with various resa'ictions on the directionaiity 
of reduction and the range of categories to which each 
rule applies) are used directly by the parser to perform 
reductions. There are two stages to each step in the 
parsing algorithm; a checking phase and a reduction 
phase. A rule of forward application and a rule of 
forward composition are used to check for the possibility 
of reduction between the categories in Cell1 and Cell2. If 
reduction is possible, Lexicat shifts the next item from 
the input buffer onto the stack. If reduction is impossible, 
Lexicat moves to the reduction phase and attempts a 
reduction between Ceil2 and Cell3 using rules of 
backward and forward application and a more constrained 
rule of forward composition. If this fails, then Lexicat 
shifts. This completes one step of the parsing algorithm, 
so Lexicat returns to the checking phase. This process 
continues until the parse is complete or the input is 
214 
exhausted, at which point the parser halts under the usual 
conditions. 
Quite often, a shift/reduce conflict arises during the 
checking phase in which case Lexicat opts, by default, to 
shift. In most constructions this resolution rule results in 
analyses which conform to the parsing strategies of late 
closure and right association (eg. Frazier, 1979). 
However, in unbounded dependencies it results in late 
attachment of the preposed constituent. For example, if 
Lexicat is in the configuration shown in Figure 3, then a 
shift/reduce conflict will occur in the checking phase. 
Stack Input Buffer 
3 2 1 
NP S/VPinflNP VPinf/VP succeed 
S/VPinf 
Who (do you want) to 
Figure 3 - Configuration of Lexicat Parser 
The ambiguity of the partial constituent do you want 
results from the ambiguous valency of want. Depending 
on which category in Cell2 is chosen, reduction by 
forward composition will or will not be possible. By 
default, Lexicat will shift in the face of this conflict; thus 
the potential for reduction by backwards application 
between Cell2 and Cell3 will not be considered during 
this step. In the next configuration, the preposed 
constituent will be in Cell4 outside the parser's 'window' 
into the stack. Therefore, the possibility of attaching the 
preposed constituent does not arise again until do you 
want to succeed has been composed into one (partial) 
constituent. At this point, the only possible attachment 
which remains is as object of succeed. 
If the parser is analysing (9a), and Bill rather than to 
is the item in Celll in the configuration in Figure 3, then 
essentially the same situation obtains; there will be a 
shift/reduce conflict, shift will be chosen by default and 
the parser will go on to build the late attachment 
analysis. If, on the other hand, the parser is analysing 
(9b) and Bill is in the input buffer at the end of the 
sentence, the parse configuration at the moment of 
indeterminacy will still be as in Figure 3 and the same 
default analysis will be chosen since the parser has no 
access to the contents of the input buffer to guide its 
decisions. However, in this case the parse will fail 
because Bill will be attached as object of succeed and 
Who will be left dangling. 
Unlike the LR(1) model, Lexicat faces parsing 
indeterminacy at the point when the first potential point 
of attachment occurs. The resolution rule in favour of 
shifting predicts that late attachment of preposed 
constituents is preferred and this prediction is compatible 
with the garden path data in (7). The Lexicat parser 
employs the grammar directly without preprocessing and 
therefore conforms to Berwick & Weinberg's (1984) 
transparency condition. 
INTERACTIVE DETERMINISM 
Marcus' (1980) Determinism Hypothesis claims that 
local ambiguity is resolved autonomously either by 
lookahead or, if this fails, by parsing strategy. This 
predicts that strategy-violating local ambiguities which 
fall outside the span of the Marcus parser's lookahead 
buffer will be garden paths. The theory tells us little 
about the resolution of global ambiguities, but implies 
that the mechanism employed must be similar to that 
used to recover from garden paths, involving interaction 
with other components of the language comprehension 
system. 
Using the Determinism Hypothesis, it is difficult to 
select between the two models outlined above (or indeed 
to conclusively rule out the Marcus parser) because the 
differing predictions concerning the onset of 
indeterminacy in the face of identical ambiguities are 
unimportant. The Determinism Hypothesis concerns only 
judgements of psychological complexity. These 
judgements marginally favour the Lexicat parser, but the 
data relating to garden paths with unbounded 
dependencies is hardly overwhelming. Moreover, the 
Determinism Hypothesis seems highly suspect in the light 
of the unbounded dependency examples because it 
predicts that local and global ambiguities are resolved 
using completely different mechanisms. At the very least, 
this approach is unparsimonious and leaves the resolution 
of global ambiguity largely unexplained. In addition, the 
extreme similarity between local and global ambiguities 
in unbounded dependency constructions suggests that one 
mechanism for the resolution of local and global 
ambiguity is quite feasible. 
Briscoe & Boguraev (1984) and Briscoe (in press) 
propose a different account of the relationship between 
parsing and other components of comprehension than that 
entailed by the Determinism Hypothesis, dubbed the 
Interactive Determinism Hypothesis (IDH). Under this 
account of deterministic parsing, the parser proceeds 
autonomously until it is faced with an indeterminacy and 
then requests help from other components of the 
comprehension system. By default, the parser will apply a 
resolution rule or parsing strategy at such a point, but this 
can be overruled by specific non-syntactic information at 
the onset of the indeterminacy. The IDH implies that both 
local and global ambiguity is resolved at its onset 
(relative to some parsing technique) either by strategy or 
interactive blocking of the strategy. The IDH predicts, in 
addition, that garden paths will arise when a strategy- 
violating, local ambiguity is not resolved interactively, as 
a result of the absence or removal of the relevant non- 
syntactic information. 
Under the IDH, the differing predictions concerning 
the onset of indeterminacy in ambiguous unbounded 
dependency constructions become crucial in any 
comparison of the two parsing models outlined above. 
215 
The Lexicat parser makes far stronger predictions because 
indeterminacy occurs much earlier in the analysis when 
less of the input is available in the left context. Since 
Lexicat prefers late attachment by default, it predicts that 
when a doubtful point of attachment is reached, which is 
the correct point of attachment, non-syntactic information 
in the available left context should block the preference 
to shift and force early reduction with the preposed 
constituent. By contrast, the Shieber/Pereira parser does 
not meet an indeterminacy except in globally ambiguous 
cases and then not until all the input is in the left 
context. It therefore predicts in conjunction with the IDH 
that there should be no garden paths involving unbounded 
dependencies and that there should be some non-syntactic 
information in the entire input which resolves the global 
ambiguity. The former prediction appears to be wrong in 
the light of the examples in (7) and the latter is so weak 
as to be trivial. 
It turns out that there is some evidence supporting the 
far stronger predictions of the Lexicat model in 
conjunction with the IDH. This evidence comes from the 
the distribution of prosodic boundaries in relation to the 
onset of strategy-violating syntactic ambiguities. For 
example in (11) 
(11) Without her, contributions to the fund would be 
inadequate. 
the comma (an orthographic counterpart to certain types 
of prosodic boundary) marks the location of an 
intonational or major tone group boundary which would 
normally occur in the spoken version of this sentence. 
The prosodic boundary prevents the potential 
misinterpretation in which her contributions is reduced as 
one NP. In unbounded dependency constructions, Danly 
(reported in Cooper & Paecia-Cooper, 1980:159t3 has 
demonstrated that the final syllable of the verb precceding 
a correct attachment point is lengthened relative to an 
environment without a potential attachment point, or with 
a potential but incorrect one. Syllabic lengthening is 
indicative of a phrasal or minor tone group boundary. 
Paul Warren and the author have since tested Danly's 
result by acoustically analysing ten readers' productions 
of four examples containing doubtful but correct early 
points of attachment and four similar examples with 
doubtful and incorrect early attachment points. The results 
tend to confirm the original finding since lengthening was 
found consistendy (although the measurements did not 
achieve statistical significance; see Briscoe, in press). A 
final piece of evidence that lengthening occurs before a 
correct point of attachment comes from the acceptability 
of contraction in (12a), but not in (12b). 
(12) 
a) Who do you warma succeed 
b) *Who do you wanna succeed Bill 
Contraction forces late attachment of Who in a), but b) is 
unacceptable because the only possible interpretation 
involves attachment 'into' the contracted form. Fodor 
(1979:277n17) notes that it is only the occurrence of 
contraction which appears to provide determinate 
information about the correct analysis and that, since 
contraction is optional, this information cannot be relied 
on. However, metrical phonologists (eg. Nespor & Vogel, 
1986) argue that such rules arc not blocked syntactically 
by the presence of the trace/gap, but by an intervening 
prosodic boundary and that this explains the coincidence 
of other phonetic effects, such as lengthening, at points 
where contraction is blocked (Cooper & Paccia-Cooper, 
1980:Ch10). In other words, contraction is the tip of a far 
more systematic prosodic iceberg which does reliably cue 
the presence of a correct attachment point. 
When Lcxicat reaches a potential point of attachment, 
it is faced with a shift/reduce ambiguity. By default, 
Lexicat prefers to shift, but this strategy can be blocked 
by a prosodic boundary intervening between the verb and 
item about to be shifted into the parse stack. Therefore, 
the parser opts for early attachment of the preposed 
constituent. In terms of Lexicat's operation, the prosodic 
boundary in the unbounded dependency construction 
plays the same role as thc prosodic boundary in (II); 
they both block the shift operation. By contrast, in the 
Shieber/Pcrcira parser it is difficult to see how a prosodic 
boundary in unbounded dependencies could be used to 
select one of two possible reductions, whilst in an 
example like (11) it would need to force the parser to 
shift rather than reduce. In addition, the relevant non- 
syntactic information occurs at the onset of the 
indeterminacy for the Lexicat model but well before this 
point for the Shicber/Pcreira model. This corroborates the 
far stronger prediction made by Lcxicat, and also makes 
the mechanism of interaction for this model simpler (sec 
Briscoc, in press). 
Finally, we should note that in the garden paths in (7) 
it is intuitively clear that examples b) and c) would be 
spoken with prosodic boundaries at the correct attachment 
point, and probably written with commas otherwise. 
Example a) on the other hand, is more subtle, but the 
experimental results reported above suggest that want 
would be lengthened in this context signalling the early 
attachment point. Thus, the IDH's prediction that garden 
paths are the result of the removal or distortion of non- 
syntactic information which functions to prevent the 
parser's default analysis in the face of indeterminacy is 
corroborated. 
CONCLUSION 
The paper has presented two approaches to the 
deterministic analysis of unbounded dependencies. The 
LR(1) technique is capable of resolving the type of local 
ambiguities which appear to occur in these constructions, 
suggesting that Church and Johnson-Laird were wrong to 
reject deterministic parsing on the basis of this data. 
However, we have argued that the Lexicat parser 
provides a better psychological model of the parsing of 
unbounded dependencies because a) it predicts the garden 
path data and b), in conjunction with the IDH, it predicts 
the apparent distribution Of prosodic boundaries in these 
constructions more successfully, and c) it provides a 
unified account of the resolution of local and global 
216 
ambiguities, and d) it is a simpler model of deterministc 
parsing which does not require preprocessing the 
grammar or maintaining state information concerning the 
left context• 
REFERENCES 
Ades, A. & Steedman, M. (1982). On the order of words. 
Linguistics & Philosophy, 4, 517-558. 
Aho, A. & Ullman, J. (1972). The Theory of Parsing, 
Translating and Compiling. Vol. I, Englewood Cliffs, NJ: 
Prentice-Hall. 
Berwick, R. (1985). The Acquisition of Syntactic 
Knowledge. Cambridge, Mass.: MIT Press. 
Berwick, R. & Weinberg, A. (1984). The Grammatical 
Basis of Linguistic Performance. Cambridge, Mass.: MIT 
Press. 
Briscoe, E. (In press). Modelling Human Speech 
Comprehension; A Computational Approach. Chichester, 
UK: Ellis Horwood, 
Briscoe, E. & Boguraev, B. (1984). Control structures 
and theories of interaction in speech understanding 
systems. In Proc. of Coling84, Stanford, Ca, pp. 259-266. 
Chomsky, N. (1981). Lectures on Government and 
Binding• Dordrecht, Holland: Foris. 
Church, K. (1980). On Memory Limitations in Natural 
Language Processing• Bloomington, Ind.: Indiana 
University Linguistics Club. 
Cooper, W. & Paccia-Cooper, J. (1980). Syntax & 
Speech. Cambridge, Mass.: Harvard University Press. 
Fodor, J.D. (1979). Superstrategy. In Walker, E. & 
Cooper, W. (eds.) Sentence Processing., Hillsdale, NJ: 
Lawrence Erlbaum. 
Fodor, J.D. (1985)• Deterministic parsing and subjacency. 
Language and Cognitive Processes, 1.1, 3-42. 
Frazier, L. (1979). On Comprehending Sentences: 
Syntactic Parsing Strategies. Bloomington, Ind.: Indiana 
University Linguistics Club. 
Gazdar, G., Klein, E., Pullum, G., & Sag, I. (1985). 
Generalized Phrase Structure Grammar. Oxford, UK: 
Blackwell. 
Johnson-Laird, P. (1983). Mental Models. Cambridge, 
UK: CUP. 
Marcus, M. (1980)• A Theory of Syntactic Recognition for 
Natural Language. Cambridge, Mass.: MIT Press. 
Nespor, M. & Vogel, I. (1986). Prosodic Phonology. 
Dordrecht, Holland: Foris. 
Pereira, F. (1985). A new characterisation of attachment 
preferences• In Dowty, D., Karttunen, L., & Zwicky, A. 
(eds.) Natural Language Parsing. Cambridge, UK: CUP. 
Shieber, S. (1983)• Sentence disambiguation by a shift- 
reduce parsing technique. In Proc. of 21st ACL, 
Cambridge, Mass., pp. 113-118. 
Steedman, M. (1985). Dependency and coordination in 
the grammar of Dutch and English. Language 55, 523-68. 
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