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CSA2050 Introduction to Computational Linguistics

Lecture DCG3

Handling Subcategorisation

Handling Relative Clauses

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Subcategories of Verbs

• English includes the following sentences:– John disappeared– John broke the window– John gave Mary the book

• In each case the verb phrase has a different structure corresponding to the number of noun phrases that follow.

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Subcategories of VP

VP

V

disappeared

VP

V NP

broke the window

VP

V NP NP

gave Mary the book

• To handle this we need separate rules for VP

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Some Rules forSubcategories of Verb

vp -> v.

vp -> v, np.

vp -> v, np, np.

• Checkpoint: how good are these rules for handling English?

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Counterexamples

• The following sentences obey the above rules:– John broke– Mary disappeared the window– Mike told Mary the book

• All of them violate the subcategorisation constraints of the individual verbs.

• How can we make sure that the right rule goes with the right verb?

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Handling Subcategorisation - I

vp --> v1.

vp --> v2, np.

vp --> v3 np, np.

• This solution has the problem that the vi

are distinct symbols with no relationship between them. We thus miss the fact that they are all verbs

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Handling Subcategorisation - II

• A slightly better method is to adopt rules of the following shape:

vp --> v(intrans)

vp --> v(trans), np.

vp --> v(ditrans), np, np.• This is essentially the same method that

we used to avoid multiplying categories when handling agreement.

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Handling Subcategorisation:The Lexicon

• Assume that the lexicon looks something like this:

lex(disappeared, v, intrans).

lex(broke,v, trans).

lex(gave, v, ditrans).

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Two Approaches – One Lexicon

• Approach 1

v1 --> [X] {lex(X,v,intrans)}.

v2 --> [X] {lex(X,v,trans)}.

v2 --> [X] {lex(X,v,ditrans)}.

• Approach 2

v(Type) --> [X], {lex(X,v,Type)}.

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Other Verb Complements

• So far we have only discussed three possible VP rules. But in practice there are many more:– np, pp: John told Bill about the accident– np, vp: John persuaded Bill to come– np, pp,pp: John rented an apartment to Bill for

Lm100– np, s: John informed Bill that Jack was coming

• Taking prepositions into account, about 40 different patterns are required to handle verbs in English.

• This will lead to an undesirable proliferation of grammar rules.

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Subcategorisation:Approach III

• Shift responsibility towards the lexicon

• Key idea: lexical entry for each verb contains a complement list.lex(told,v, [np,pp]).lex(persuaded,v,[np,vp]).lex(rented,v,[np,pp,pp].

• The list is then managed by the grammar.

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Grammar Rules for VP

• Basic idea is to avoid multiple grammar rules of the formvp --> v([]).vp --> v([np]), np(...).vp --> v([np,np]), np(...), np(...).vp --> v([np,pp]), np(...), pp(...). vp --> v([np,s]), np(...), s(...).

• Instead have a single rule of this formvp --> v(SubCatList), comps(SubCatList).which allows a verb phrase to be formed from a verb followed by a sequence of zero or more complements.

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The Grammar

s --> np, vp.

np --> n.

np --> d, n.

vp --> v(SC), comps(SC).

comps([]) --> [].

comps([X|R]) --> x(X), comps(R).

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The Comps Rule

comps([]) --> [].

comps([X|R]) --> x(X), comps(R)

• Challenge: how to write the definition of the "x" predicate.

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Relative Clauses

• Sentences like

Terry runs a program that Bertrand wroteThe man who came to dinner vomited

include relative clauses (shown in boldface)

• Relative clauses are interesting because they involve "filler-gap dependency"

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Structure of Relative Clauses

• Relative clauses such asthat Bertrand wroteare not well described by

relclause --> [that], vp.

• This is because the underlying structure is not the concatenation of "that" and a VP, i.e.

• If this is not the structure, what is?

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The Structure of the Relative Clause

• To understand the structure we must first look at the wider context in which it occurs, in this case the noun phrase:a program that Bertrand wrote.

• This noun phrase is derived from the underlying sentenceBertrand wrote a program.

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Derivation of Relative Clause

• Basic sentence: Bertrand wrote a program• Step 1: identify object NP

Bertrand wrote [a program]• Step 2: move to the front, leaving gap where object NP

was.[A program] Bertrand wrote npgap

• Note that underlined structure is a sentence with a gap instead of an object.

• Step 3: insert relative pronoun just before sentence containing gap.

• [A program] that Bertrand wrote npgap• N.B. Gap is invisible

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Structure of the Noun Phrase

NP

d N

NP

rel Sgap

VPgap

v NPgap

a program that

n

bertrand wrote

RelCl

filler

gap

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Long Distance Dependencies

• A filler-gap dependency occurs in an NL sentence when a subpart of some phrase is missing from its normal location and another phrase, outside the incomplete one, stands in its place.

• Filler-gap dependencies are examples of long-distance dependencies.

• The amount of material between dependents (in this case filler and gap) is unbounded, at least in principle, e.g.Terry read a book that Bertrand told a student to ask a professor to write.

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Handling Gaps within a DCG

• Use a nonterminal argument position to indicate the presence or absence a gap, e.g.np(nogap) = an NP without a gapnp(gap) = an NP with a gap

• Introduce the term gap(T) to indicate the presence or absence of a gap of category T. So gap(np) will indicate where an NP used to be.

• For each category c that admits gaps, introduce the rule c(gap(c)) rewrites nothing, e.g.

np(gap(np)) --> [ ].