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<?xml version="1.0" standalone="yes"?> <Paper uid="P93-1016"> <Title>PRINCIPLE-BASED PARSING WITHOUT OVERGENERATION 1</Title> <Section position="4" start_page="114" end_page="115" type="metho"> <SectionTitle> 3. Modeling Linguistics Devices </SectionTitle> <Paragraph position="0"> GB principles are stated in terms of linguistic concepts such as barrier, government and movement, which are relationships between nodes in syntactic structures. Since we interpret the principles with descriptions of the structures, instead of the structures themselves, we must be able to model these notions with the descriptions.</Paragraph> <Paragraph position="1"> Dominance and m-command: Dominance and m-command are relationships between nodes in syntactic structures. Since an item represent a node in a syntactic structure, relationships between the nodes can be represented by relationships between items: dominance: An item dominates its direct and indirect sources. For example, in Figure 2, i4 dominates il, i2, and iz.</Paragraph> <Paragraph position="2"> m-command: The head daughter of an item representing a maximal category m-commands non-head daughters of the item and their sources.</Paragraph> <Paragraph position="3"> Barrier Chomsky (1986) proposed the notion of barrier to unify the treatment of government and subjacency. In Chomsky's proposal, barrierhood is a property of maximal nodes (nodes representing maximal categories). However, not every maximal node is a barrier. The barrierhood of a node also depends on its context, in terms of L-marking and inheritance.</Paragraph> <Paragraph position="4"> Instead of making barrierhood a property of the nodes in syntactic structures, we define it to be a property of links in the grammar network. That is, certain links in the grammar network are classified as barriers. In Figure 1, barrier links have a black ink-spot on them. Barrierhood is a property of these links, independent of the context. This definition of barrier is simpler than Chomsky's since it is context-free. In our experiments so far, this simpler definition has been found to be adequate.</Paragraph> <Section position="1" start_page="114" end_page="115" type="sub_section"> <SectionTitle> Government </SectionTitle> <Paragraph position="0"> Once the notion of barrier has been defined, the government relationship between two nodes in a structure can be defined as follows: government: A governs B if A is the minimal governor that m-commands B via a sequence of non-barrier links, where governors are N, V, P, A, and tensed I.</Paragraph> <Paragraph position="1"> Items representing governors are assigned +govern attribute. This attribute percolates across head dominance links. If an item has +govern attribute, then non-head sources of the item and their sources are governed by the head of the item if there are paths between them and the item satisfying the conditions: 1. there is no barrier on the path.</Paragraph> <Paragraph position="2"> 2. there is no other item with +govern attribute on the path (minimality condition (Chomsky, 1986, p.10)).</Paragraph> <Paragraph position="3"> Movement :3 Movement is a major source of complexity in principle-based parsing. Directly modeling Move-c~ would obviously generate a large number of invalid movements. Fortunately, movements must also satisfy: null c-command condition: A moved element must c-command its trace (Radford, 1988, p.564), where A c-command B if A does not dominate B but the parent of A dominates B. The c-command condition implies that a movement consists of a sequence of moves in the reverse direction of dominance links, except the last one. Therefore, we can model a movement with a set of attribute values. If an item contains these attribute values, it means that there is a movement out of the structure represented by the item. For example, in Figure 2.b, item i10 contains movement attributes: nppg, npbarr+-er and np-atts. This indicates that there is an np-movement out of the VP whose root node is il0.</Paragraph> <Paragraph position="4"> The movement attributes are generated at the parent node of the initial trace. For example, V:NP is a node representing normal transitive verbs which take an NP as complement. When V:NP receives an item representing the passive sense of the word eaten, V:NP creates another item < \[i,i\] , ((cat v) -npbarrier +nppg (np-atts (cat n))), {}> This item will not be combined with any item from NP node because the NP complement is assumed to be an np-trace. The item is then sent to nodes dominating V:NP. As the item propagates further, the attributes is carried with it, simulating the effect of movement. The np-movement land at IP node when the IP node combines an item from subject NP and another item from Ibar with np-movement attributes. A precondition on the landing is that the attributes of the former can be unified with the value of np-atts of the latter. Wh-movements are dealt with by attributes whpg, whbarrier, wh-atts. This treatment of movement requires that the parent node of a initial trace be able to determine the type of movement. When a movement is generated, the type of the movement depends on the ca (case assigner) attribute of the item:</Paragraph> <Paragraph position="6"> movement examples wh active V, P, finite IP np A, passive V, non-finite IP For example, when IP node receives an item from Ibar, IP attempts to combine it with another item from subject NP. If the subject is not found, then the IP node generates a movement. If the item represent a finite clause, then it has attributes +ca (cform fin) and the movement is of type wh. Otherwise, the movement is of type np.</Paragraph> </Section> </Section> <Section position="5" start_page="115" end_page="118" type="metho"> <SectionTitle> 4. Interpretation of Principles </SectionTitle> <Paragraph position="0"> We now describe how the principles of GB theory are implemented. ~ -bar Theory: ~N~ * Every syntactic category is a projection of a \] lexical head. / * There two levels of projection of lexical I heads. Only the bar-2 projections can be) complements and adjuncts, j/ The first condition requires that every non-lexical category have a head. This is guaranteed by a constraint in item combination: one of the sources of the two items being combined must be from the head daughter.</Paragraph> <Paragraph position="1"> The second condition is implemented by the structure of the grammar network* The combinations of items represent constructions of larger parse trees from smaller ones. Since the structure of the grammar network satisfies the constraint, the parse trees constructed by item combination also satisfy the X-bar theory.</Paragraph> <Paragraph position="2"> Case Filter: Every lexical NP must be case-~ arked, where A case-marks B iff A is a case as- I ~igner and A governs B (Haegeman, 1991, p.156)fl The case filter is implemented as follows: 1. Case assigners (P, active V, tensed I) have +ca attribute. Governors that are not case assigners (N, A, passive V) have -ca attribute* 2. Every item at NP node is assigned an attribute value -cm, which means that the item needs to be case-marked. The -cm attribute then propagates with the item. This item is said to be the origin of the -era attribute.</Paragraph> <Paragraph position="3"> 3. Barrier links do not allow any item with -cm to pass through, because, once the item goes beyond the barrier, the origin of-cm will not be governed, let alone case-marked.</Paragraph> <Paragraph position="4"> 4. Since each node has at most one governor, if' the governor is not a case assigner, the node will not be case-marked. Therefore, a case-filter violation is detected if +govern -era -ca co-occur in an item.</Paragraph> <Paragraph position="5"> 5. If +govern +ca -cm co-occur in an item, then the head daughter of the item governs and case-marks the origin of -cm. The case-filter condition on the origin of -era is met. The -era attribute is cleared.</Paragraph> <Paragraph position="6"> For example, consider the following sentences: (2) a. I believe John to have left.</Paragraph> <Paragraph position="7"> b. *It was believed John to have left.</Paragraph> <Paragraph position="8"> c. I would hope for John to leave* d. *I would hope John to leave.</Paragraph> <Paragraph position="9"> The word &quot;believe&quot; belongs to a subcategory of verb (V:IP) that takes an IP as the complement. Since there is no barrier between V:IP and the subject of IP, words like &quot;believe&quot; can govern into the IP complement and case-mark its subject (known as exceptional case-marking in literature). In (2a), the -cm attribute assigned to the item representing \[NP John\] percolates to V:IP node without being blocked by any barrier. Since +govern +ca -cm co-occur in the item at V:IP node, the case-filter is satisfied (Figure 3.a). On the other hand, in (25) the pas- null violation is detected at V:IP node (Figure 3.b). The word &quot;hope&quot; takes a CP complement. It does not govern the subject of CP because there is a barrier between them. The subject of an infinitive CP can only be governed by complement &quot;for&quot; (Figure 3.c and 3.d).</Paragraph> <Paragraph position="10"> criterion: Every chain must receive and one~ ly one 0-role, where a chain consists of an NP I d the traces (if any) coindexed with it (van I emsdijk and Williams, 1986, p.245). / We first consider chains consisting of one element. The 0-criterion is implemented as the following constraints: null 1. An item at NP node is assigned +theta if its nform attribute is norm. Otherwise, if the value of nform is there or it, then the item is assigned -theta.</Paragraph> <Paragraph position="11"> 2. Lexical nodes assign +theta or -theta to items depending on whether they are 0-assigners (V, A, P) or not (N, C).</Paragraph> <Paragraph position="12"> 3. Verbs and adjectives also have a subj-theta attribute.</Paragraph> <Paragraph position="13"> value O-role* examples +subj-theta yes &quot;take&quot;, &quot;sleep&quot; -subj-theta no &quot;seem&quot;, passive verbs *assigning O-role to subject This attribute percolates with the item from V to IP. The IP node then check the value of theta and subj-theta to make sure that tile verb assigns a 0-role to the subject if it re- null has attribute +theta, When the items representing &quot;love&quot; and &quot;Mary&quot; are combined. Their theta attribute are unifiable, thus satisfying the 0-criterion. The +subj-theta attribute of &quot;love&quot; percolates with the item representing &quot;love Mary&quot;, which is propagated to IP node. When the item from NP and Ibar are combined at IP node, the new item has both -theta and +subj-theta attribute, resulting in a 0-criterion violation.</Paragraph> <Paragraph position="14"> The above constraints guarantee that chains with only one element satisfy 0-criterion. We now consider chains with more than one element. The base-position of a wh-movement is case-marked and assigned a 0-role. The base position of an np-movement is assigned a 0-role, but not case-marked. To ensure that the movement chains satisfy 0-criterion we need only to make sure that the items representing the parents of intermediate traces and landing sites of the movements satisfy these conditions: null None of +ca, +theta and +subj-theta is present in the items representing the parent of intermediate traces of (wh- and np-) movements as well as the landing sites of whmovements, thus these positions are not case-marked and assigned a O-role.</Paragraph> <Paragraph position="15"> Both +ca and +subj-theta are present in the items representing parents of landing sites of np-movements.</Paragraph> <Paragraph position="16"> Subjacency: Movement cannot cross more thanJ ne barrier (Haegeman, 1991, p.494).</Paragraph> <Paragraph position="17"> A wh-movement carries a whbarrier attribute. The value -whbarrier means that the movement has not crossed any barrier and +whbarrier means that the movement has already crossed one barrier. Barrier links allow items with -whbarrier to pass through, but change the value to +whbarrier. Items with +whbarrier are blocked by barrier links. When a wh-movement leaves an intermediate trace at a position, the corresponding whbarrier becomes -. The subjacency of np-movements is similarly bandied with a npbarrier attribute.</Paragraph> <Paragraph position="18"> Ermpty Category Principle (ECP): A traceJ its parent must be properly governed.</Paragraph> <Paragraph position="19"> In literature, proper government is not, as the term suggests, subsumed by government. For example, in (4) Who do you think \[cP e' \[IP e came\]\] the tensed I in liP e came\] governs but does not properly govern the trace e. On the other hand, # properly governs but does not govern e (Haegeman, 1901, p.4 6).</Paragraph> <Paragraph position="20"> Here, we define proper government to be a sub-class of government: Proper government: A properly governs B iff A governs B and A is a 0-role assigner (A do not have to assign 0-role to B).</Paragraph> <Paragraph position="21"> Therefore, if an item have both +govern and one of +theta or +subj-theta, then the head of the item properly governs the non-head source items and their sources that are reachable via a sequence of non-barrier links. This definition unifies the notions of government and proper government. In (4), e is properly governed by tensed I, e I is properly governed by &quot;think&quot;.</Paragraph> <Paragraph position="22"> This definition won't be able to account for difference between (4) and (5) (That-Trace Effect, (Haegeman, 1991, p.456)): (5) *Who do you think \[CP e' that \[IP e came\]\] However, That-Trace Effect can be explained by a separate principle.</Paragraph> <Paragraph position="23"> The proper government of wh-traces are handled by an attribute whpg (np-movements are similarly dealt with by an nppg attribute):</Paragraph> <Section position="1" start_page="117" end_page="118" type="sub_section"> <SectionTitle> Value Meaning </SectionTitle> <Paragraph position="0"> -whpg the most recent trace has yet to be properly governed.</Paragraph> <Paragraph position="1"> +~hpg the most recent trace has already been properly governed.</Paragraph> <Paragraph position="2"> 1. If an item has the attributes -whpg, -theta, +govern, then the item is an ECP violation, because the governor of the trace is not a 0-role assigner. If an item has attributes -whpg, +theta, +govern, then the trace is properly governed. The value of whpg is changed to +. 2. Whenever a wh-movement leaves an intermediate trace, whpg becomes -.</Paragraph> <Paragraph position="3"> 3. Barrier links block items with -~hpg. For example, the word claim takes a CP complement. In the sentence: (6) *Whol did you make the claim e~ that Reagan met ei there is a wh-movement out of the complement CP of claim. When the movement left an intermediate trace at CSpec, the value of whpg became -. When the item with -whpg is combined with the item representing claim, their unification has attributes (+govern -theta -whpg), which is an ECP violation. The item is recognized as invalid and discarded. PRO Theorem: PRO must be ungoverned 1 Haegeman, 1991, p.263).</Paragraph> <Paragraph position="4"> When the IP node receives an item from Ibar with cform not being fin, the node makes a copy of the item and assign +pro and -ppro to the copy and then send it further without combining it with any item from (subject) NP node. The attribute +pro represents the hypothesis that the subject of the clause is PRO. The meaning of -ppro is that the subject PRO has not yet been protected (from being governed).</Paragraph> <Paragraph position="5"> When an item containing -ppro passes through a barrier link, -ppro becomes +ppro which means that the PRO subject has now been protected. A PROtheorem violation is detected if +govern and -ppro co-occur in an item.</Paragraph> </Section> </Section> <Section position="6" start_page="118" end_page="118" type="metho"> <SectionTitle> 5. Objected-oriented Implementation </SectionTitle> <Paragraph position="0"> The parser has been implemented in C++, an object-oriented extension of C. The object-oriented paradigm makes the relationships between nodes and links in the grammar network and their software counterparts explicit and direct. Communication via message passing is reflected in the message passing metaphor used in object-oriented languages.</Paragraph> <Paragraph position="2"> constraints that implement the principles are distributed over the nodes and links in the network.</Paragraph> <Paragraph position="3"> The implementation of the constraints is modular because they are defined in class definitions and all the instances of the class and its subclasses inherit these constraints. The object-oriented paradigm allows the subclasses to modify the constraints.</Paragraph> <Paragraph position="4"> The implementation of the parser has been tested with example sentences from Chapters 410, 15-18 of (van Riemsdijk and Williams, 1986).</Paragraph> <Paragraph position="5"> The chapters left out are mostly about logical form and Binding Theory, which have not yet been implemented in the parser. The average parsing time for sentences with 5 to 20 words is below half of a second on a SPARCstation ELC.</Paragraph> </Section> class="xml-element"></Paper>