introduction to syntax
DESCRIPTION
Introduction to Syntax. Owen Rambow [email protected] October 6 2004. What is Syntax?. Study of structure of language Roughly, goal is to relate surface form (what we perceive when someone says something) to semantics (what that utterance means). What is Syntax Not?. - PowerPoint PPT PresentationTRANSCRIPT
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What is Syntax?• Study of structure of language• Roughly, goal is to relate surface
form (what we perceive when someone says something) to semantics (what that utterance means)
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What is Syntax Not?• Phonology: study of sound systems
and how sounds combine• Morphology: study of how words
are formed from smaller parts (morphemes)
• Semantics: study of meaning of language
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What is Syntax? (2)• Study of structure of language• Specifically, goal is to relate an
interface to phonological component to an interface to a semantic component
• Note: interface to phonological component may look like written text
• Representational device is tree structure
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Simplified Big Picture
/waddyasai/Phonology
Morphology /waddyasai/ what did you say
Syntax what did you say say
you whatobjsubj
Semanticssay
you whatobjsubj P[ x. say(you, x) ]
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What About Chomsky?• At birth of formal language theory (comp sci) and
formal linguistics • Major contribution: syntax is cognitive reality• Humans able to learn languages quickly, but not all
languages universal grammar is biological• Goal of syntactic study: find universal principles
and language-specific parameters• Specific Chomskyan theories change regularly• These ideas adopted by almost all contemporary
syntactic theories (“principles-and-parameters-type theories”)
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Types of Linguistic Theories
• Descriptive: provide account of syntax of a language; often appropriate for NLP engineering work
• Explanatory: provide principles-and-parameters style account of syntax of (preferably) several languages
• Prescriptive: “prescriptive linguistics” is an oxymoron
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Structure in Strings
• Some words: the a small nice big very boy girl sees likes
• Some good sentences:o the boy likes a girl o the small girl likes the big girlo a very small nice boy sees a very nice boy
• Some bad sentences:o *the boy the girlo *small boy likes nice girl
• Can we find subsequences of words (constituents) which in some way behave alike?
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Structure in StringsProposal 1
• Some words: the a small nice big very boy girl sees likes
• Some good sentences:o (the) boy (likes a girl) o (the small) girl (likes the big girl)o (a very small nice) boy (sees a very nice boy)
• Some bad sentences:o *(the) boy (the girl)o *(small) boy (likes the nice girl)
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Structure in StringsProposal 2
• Some words: the a small nice big very boy girl sees likes
• Some good sentences:o (the boy) likes (a girl) o (the small girl) likes (the big girl)o (a very small nice boy) sees (a very nice boy)
• Some bad sentences:o *(the boy) (the girl)o *(small boy) likes (the nice girl)• This is better proposal: fewer types of constituents(blue and red are of same type)
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More Structure in StringsProposal 2 -- ctd
• Some words: the a small nice big very boy girl sees likes
• Some good sentences:o ((the) boy) likes ((a) girl) o ((the) (small) girl) likes ((the) (big) girl)o ((a) ((very) small) (nice) boy) sees ((a) ((very) nice)
girl)• Some bad sentences:
o *((the) boy) ((the) girl)o *((small) boy) likes ((the) (nice) girl)
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From Substrings to Trees• (((the) boy) likes ((a) girl))
boythe
likesgirl
a
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Node Labels?• ( ((the) boy) likes ((a) girl) )• Choose constituents so each one has one
non-bracketed word: the head• Group words by distribution of constituents
they head (part-of-speech, POS):o Noun (N), verb (V), adjective (Adj), adverb (Adv),
determiner (Det)• Category of constituent: XP, where X is POS
o NP, S, AdjP, AdvP, DetP
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Node Labels• (((the/Det) boy/N) likes/V ((a/Det)
girl/N))
boy
the
likes
girl
a
DetP
NP NP
DetP
S
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Types of Nodes• (((the/Det) boy/N) likes/V ((a/Det)
girl/N))
boy
the
likes
girl
a
DetP
NP NP
DetP
S
Phrase-structuretree
nonterminalsymbols= constituents
terminal symbols = words
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Determining Part-of-Speech
o noun or adjective? a blue seat a child seat a very blue seat *a very child seat this seat is blue *this seat is child
blue and child are not the same POS
blue is Adj, child is Noun
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Determining Part-of-Speech (2)
o preposition or particle?
A he threw out the garbage B he threw the garbage out the door
A he threw the garbage out B *he threw the garbage the door out
The two out are not same POS; A is particle, B is Preposition
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Word Classes (=POS)• Heads of constituents fall into
distributionally defined classes • Additional support for class
definition of word class comes from morphology
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Some Points on POS Tag Sets
• Possible basic set: N, V, Adj, Adv, P, Det, Aux, Comp, Conj
• 2 supertypes: open- and closed-classo Open: N, V, Adj, Advo Closed: P, Det, Aux, Comp, Conj
• Many subtypes:o eats/V eat/VB, eat/VBP, eats/VBZ, ate/VBD,
eaten/VBN, eating/VBG, o Reflect morphological form & syntactic function
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Phrase Structure and Dependency Structure
likes/V
boy/N girl/N
the/Det a/Detboy
the
likes
girl
a
DetP
NP NP
DetP
S
All nodes are labeled with words!Only leaf nodes labeled with words!
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Phrase Structure and Dependency Structure
(ctd)
likes/V
boy/N girl/N
the/Det a/Detboy
the
likes
girl
a
DetP
NP NP
DetP
S
Representationally equivalent if each nonterminal node has one lexical daughter (its head)
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Types of Dependency
likes/V
boy/N girl/N
a/Detsmall/Adjthe/Det
very/Adv
sometimes/AdvObjSubj
Adj(unct)
FwFw Adj
Adj
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Grammatical Relations• Types of relations between words
o Arguments: subject, object, indirect object, prepositional object
o Adjuncts: temporal, locative, causal, manner, …
o Function Words
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Subcategorization• List of arguments of a word (typically,
a verb), with features about realization (POS, perhaps case, verb form etc)
• In canonical order Subject-Object-IndObj
• Example:o like: N-N, N-V(to-inf)o see: N, N-N, N-N-V(inf)
• Note: J&M talk about subcategorization only within VP
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What About the VP?
boythe
likesgirl
aDetP
NP NP
DetP
S
boy
thelikesDetP
NP
girl
a
NP
DetP
S
VP
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What About the VP?• Existence of VP is a linguistic (i.e., empirical)
claim, not a methodological claim• Semantic evidence???• Syntactic evidence
o VP-fronting (and quickly clean the carpet he did! )o VP-ellipsis (He cleaned the carpets quickly, and so
did she )o Can have adjuncts before and after VP, but not in
VP (He often eats beans, *he eats often beans )• Note: VP cannot be represented in a
dependency representation
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Context-Free Grammars• Defined in formal language theory
(comp sci)• Terminals, nonterminals, start
symbol, rules• String-rewriting system• Start with start symbol, rewrite using
rules, done when only terminals left• NOT A LINGUISTIC THEORY, just a
formal device
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CFG: Example• Many possible CFGs for English, here is an
example (fragment):o S NP VPo VP V NPo NP DetP N | AdjP NPo AdjP Adj | Adv AdjPo N boy | girlo V sees | likeso Adj big | smallo Adv very o DetP a | the
the very small boy likes a girl
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Derivations in a CFG
S NP VPVP V NPNP DetP N | AdjP NPAdjP Adj | Adv AdjPN boy | girlV sees | likesAdj big | smallAdv very DetP a | the
S
S
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Derivations in a CFG
S NP VPVP V NPNP DetP N | AdjP NPAdjP Adj | Adv AdjPN boy | girlV sees | likesAdj big | smallAdv very DetP a | the
NP VP
NP
S
VP
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Derivations in a CFG
S NP VPVP V NPNP DetP N | AdjP NPAdjP Adj | Adv AdjPN boy | girlV sees | likesAdj big | smallAdv very DetP a | the
DetP N VP
DetP
NP
S
VP
N
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Derivations in a CFG
S NP VPVP V NPNP DetP N | AdjP NPAdjP Adj | Adv AdjPN boy | girlV sees | likesAdj big | smallAdv very DetP a | the
the boy VP
boytheDetP
NP
S
VP
N
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Derivations in a CFG
S NP VPVP V NPNP DetP N | AdjP NPAdjP Adj | Adv AdjPN boy | girlV sees | likesAdj big | smallAdv very DetP a | the
the boy likes NP
boythe likes
DetP
NP
NP
S
VP
N V
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Derivations in a CFG
S NP VPVP V NPNP DetP N | AdjP NPAdjP Adj | Adv AdjPN boy | girlV sees | likesAdj big | smallAdv very DetP a | the
the boy likes a girl
boythe likes
DetP
NP
girla
NP
DetP
S
VP
N
N
V
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Derivations in a CFG;Order of Derivation
Irrelevant
S NP VPVP V NPNP DetP N | AdjP NPAdjP Adj | Adv AdjPN boy | girlV sees | likesAdj big | smallAdv very DetP a | the
NP likes DetP girl
likes
NP
girl
NP
DetP
S
VP
N
V
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Derivations of CFGs• String rewriting system: we derive
a string (=derived structure)• But derivation history represented
by phrase-structure tree (=derivation structure)!
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Grammar Equivalence• Can have different grammars that generate
same set of strings (weak equivalence)o Grammar 1: NP DetP N and DetP a | theo Grammar 2: NP a N | NP the N
• Can have different grammars that have same set of derivation trees (strong equivalence)o With CFGs, possible only with useless ruleso Grammar 2: NP a N | NP the No Grammar 3: NP a N | NP the N, DetP many
• Strong equivalence implies weak equivalence
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Normal Forms &c• There are weakly equivalent
normal forms (Chomsky Normal Form, Greibach Normal Form)
• There are ways to eliminate useless productions and so on
• See your formal language textbook
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“Generative Grammar”• Formal languages: formal device to
generate a set of strings (such as a CFG)• Linguistics (Chomskyan linguistics in
particular): approach in which a linguistic theory enumerates all possible strings/structures in a language (=competence)
• Chomskyan theories do not really use formal devices – they use CFG + informally defined transformations
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Nobody Uses CFGs Only (Except Intro NLP Courses)• All major syntactic theories (Chomsky,
LFG, HPSG, TAG-based theories) represent both phrase structure and dependency, in one way or another
• All successful parsers currently use statistics about phrase structure and about dependency
• Derive dependency through “head percolation”: for each rule, say which daughter is head
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Massive Ambiguity of Syntax
• For a standard sentence, and a grammar with wide coverage, there are 1000s of derivations!
• Example:o The large portrait painter told the
delegation that he sent money orders in a letter on Wednesday
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Penn Treebank (PTB), Again
• Syntactically annotated corpus of newspaper texts (phrase structure)
• The newspaper texts are naturally occurring data, but the PTB is not!
• PTB annotation represents a particular linguistic theory (but a fairly “vanilla” one)
• Particularitieso Very indirect representation of grammatical
relations (need for head percolation tables)o Completely flat structure in NP (brown bag
lunch, pink-and-yellow child seat )o Has flat Ss, flat VPs
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Types of syntactic constructions
• Is this the same construction?o An elf decided to clean the kitcheno An elf seemed to clean the kitchen An elf cleaned the kitchen
• Is this the same construction?o An elf decided to be in the kitcheno An elf seemed to be in the kitchenAn elf was in the kitchen
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Types of syntactic constructions (ctd)
• Is this the same construction?There is an elf in the kitcheno *There decided to be an elf in the
kitcheno There seemed to be an elf in the kitchen
• Is this the same construction?It is raining/it rainso ??It decided to rain/be rainingo It seemed to rain/be raining
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Types of syntactic constructions (ctd)
Conclusion: • to seem: whatever is embedded
surface subject can appear in upper clause
• to decide: only full nouns that are referential can appear in upper clause
• Two types of verbs
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Types of syntactic constructions: Analysis
to seem: lower surface subject raises to upper clause; raising verb
seems (there to be an elf in the kitchen)there seems (t to be an elf in the kitchen)it seems (there is an elf in the kitchen)
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Types of syntactic constructions: Analysis
(ctd)• to decide: subject is in upper clause and co-
refers with an empty subject in lower clause; control verb
an elf decided (an elf to clean the kitchen)an elf decided (to clean the kitchen)an elf decided (he cleans/should clean the kitchen)*it decided (an elf cleans/should clean the kitchen)
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Lessons Learned from the Raising/Control Issue
• Use distribution of data to group phenomena into classes
• Use different underlying structure as basis for explanations
• Allow things to “move” around from underlying structure -> transformational grammar
• Check whether explanation you give makes predictions