74.793 NLP and Speech 2004 Feature Structures Feature Structures and Unification
Dec 19, 2015
74.793 NLP and Speech 2004Feature Structures
Feature Structures and Unification
Feature Structures - General
• Feature structures describe linguistic attributes or features like number, person associated with words or syntactic constituents like noun phrase.
• Feature structures are sets of features and values, e.g. hat [Number sing ]
buys [Person 3 ][Number sing ]
Feature Structures - Agreement
Feature structures can be collected in one ‘variable’ called agreement.
buys agreement [Person 3]
[Number sing]
Feature Structures, Grammar, Parsing Feature Structures• describe additional syntactic-semantic information,
like category, person, number, e.g.goes <verb, 3rd, singular>
• specify feature structure constraints (agreements) as part of the grammar rules
• during parsing, check agreements of feature structures (unification)
example
S → NP VP <NP number> = <VP number>
S → NP VP <NP agreement> = <VP agreement>
Feature Structures as ConstraintsUngrammatical sentences like
“He go”
or “We goes”
can be excluded using feature constraints.
example
S → NP VP <NP agreement> = <VP agreement>
S → NP VP <NP number> = <VP number>
<NP person> = <VP person>
Add to feature structure category cat:
buys cat verb
agreement [Person 3 ]
[Number sing]
Feature Structures and Categories
Compare and combine feature structures:
he buys
buys cat verb
agreement [Person 3 ]
[Number sing]
he cat noun
agreement [Person 3 ]
[Number sing]
Feature Structures and Unification 1
S → NP VP <NP number> = <VP number><NP person> = <VP person>
buys cat verb
agreement [Person 3 ]
[Number sing]
he cat noun
agreement [Person 3 ]
[Number sing]
Using Feature Structures
Unification of Feature StructuresAgreement is checked by the unification operation
according to the following rules:
[featurei valuei] |_| [featurei valuei] = [featurei valuei]
[featurei valuei] |_| [featurei valuej] = fail
if valueivaluej
[featurei valuei] |_| [featurei undef.] = [featurei valuei]
[featurei valuei] |_| [featurej valuej] = featurei valuei
featurej valuej
if featurei
featurej
Features and Subcategorization 1
NP modifiers or Verb complements
central noun + modifiers + agreement
central verb + complements + agreements
“... the man who chased the cat out of the house ...”
“... the man chased the barking dog who bit him ...”
Agreements are passed on / inherited within phrases, e.g. agreement of VP derived from Head-Verb of VP:
<VP agreement> determined by <Verb agreement>
<NP agreement> determined by <Nom agreement>
Features and Subcategorization 2
NP modifiers or Verb complements:
central noun + modifiers + agreement central verb + complements + agreements
“... the man who chased the cat out of the house ...”“... the man chased the barking dog who bit him ...”
Agreements are passed on / inherited within phrases, e.g. agreement of VP derived from Head-Verb of VP:
<VP agreement> determined by <Verb agreement><NP agreement> determined by <Nom agreement>
Semantics
Semantics
Distinguish between
• surface structure (syntactic structure) and
• deep structure (semantic structure) of sentences.
Different forms of Semantic Representation
• logic formalisms
• ontology / semantic representation languages – Case Frame Structures (Filmore)– Conceptual Dependy Theory (Schank)– DL and similar KR languages – Ontologies
Semantic Representations
Semantic Representations based on some form of (formal) Representation Language.
– Semantics Networks– Conceptual Dependency Graphs– Case Frames– Ontologies– DL and similar KR languages
Constructing a Semantic Representation
General: Start with surface structure Derived from parser. Map surface structure to semantic structure
Use phrases as sub-structures. Find concepts and representations for
central phrases (e.g. VP, NP, then PP) Assign phrases to appropriate roles
around central concepts (e.g. bind PP into VP representation).
Ontology (Interlingua) approach
• Ontology: a language-independent classification of objects, events, relations
• A Semantic Lexicon, which connects lexical items to nodes (concepts) in the ontology
• An analyzer that constructs Interlingua representations and selects (an?) appropriate one
(based on Steve Helmreich's 419 Class, Nov 2003)
Semantic Lexicon
• Provides a syntactic context for the appearance of the lexical item
• Provides a mapping for the lexical item to a node in the ontology (or more complex associations)
• Provides connections from the syntactic context to semantic roles and constraints on these roles
Deriving Basic Semantic Dependency (a toy example)
Input: John makes tools
Syntactic Analysis:cat verbtense presentsubject
root johncat noun-proper
object root toolcat nounnumber plural
Deriving Basic Semantic Dependency
John-n1syn-struc
root johncat noun-proper
sem-struchuman
name john
gender maletool-n1
syn-strucroot toolcat n
sem-structool
Lexicon Entries for John and tool
Relevant Extract from the Specification of the Ontological Concept Used to Describe the Appropriate Meaning of make:
manufacturing-activity...
agent humantheme artifact
…
Meaning Representation - Example make
John-n1syn-struc
root johncat noun-proper
sem-struchuman
name johngender male
tool-n1syn-struc
root toolcat n
sem-structool
Relevant parts of the (appropriate senses of the)lexicon entries for John and tool
The basic semantic dependency component of the TMR for
John makes tools
manufacturing-activity-7
agent uman-3theme set-1
element toolcardinality > 1
…
Semantic Dependency Component
try-v3syn-struc
root trycat vsubj root $var1
cat nxcomp root $var2
cat vform OR infinitive gerund
sem-strucset-1 element-type refsem-1
cardinality >=1refsem-1 sem event
agent ^$var1effect refsem-2
modalitymodality-type epiteucticmodality-scope refsem-2modality-value < 1
refsem-2 value ^$var2sem event
Constructing an IL representation
For each syntactic analysis: Access all semantic mappings and contexts
for each lexical item. Create all possible semantic representations. Test them for coherency of structure and
content.
REQUEST-INFO-130 THEME DEVELOP-2601.PURPOSE DEVELOP-2601.REASON TEXT-POINTER why INSTANCE-OF REQUEST-INFO
DEVELOP-2601THEME SET-2555AGENT NATION-97PHASE CONTINUOUS
TIME FIND-ANCHOR-TIME INSTANCE-OF DEVELOP
TEXT-POINTER developing
NATION-97HAS-NAME Iraq
INSTANCE-OF NATIONTEXT-POINTER Iraq
SET-2555 ELEMENT-TYPE WEAPONCARDINALITY > 1
INSTRUMENT-OF KILL-1864 THEME-OF DEVELOP-2601 INSTANCE-OF WEAPON
TEXT-POINTER weapons
KILL-1864 THEME SET-2556 INSTRUMENT SET-2555 INSTANCE-OF KILL
TEXT-POINTER destruction
SET-2556 THEME-OF KILL-1225 ELEMENT-TYPE HUMAN
CARDINALITY > 100 INSTANCE-OF HUMAN
TEXT-POINTER mass
“Why is Iraq developing weapons of mass destruction?”
Word sense disambiguation Constraint checking – making sure the
constraints imposed on context are met Graph traversal – is-a links are
inexpensive Other links are more expensive The “cheapest” structure is the most
coherent Hunter-gatherer processing
Logic Formalisms
Lambda Calculus
Semantics - Lambda Calculus 1
Logic representations often involve Lambda-Calculus:• represent central phrases (e.g. verb) as -
expressions -expression is like a function which can be applied
to terms• insert semantic representation of complement or
modifier phrases etc. in place of variables
x, y: loves (x, y) FOPL sentence
xy loves (x, y) -expression, function
xy loves (x, y) (John) y loves (John, y)
Semantics - Lambda Calculus 2
Transform sentence into lambda-expression:
“AI Caramba is close to ICSI.”
specific: close-to (AI Caramba, ICSI)
general: x,y: close-to (x, y) x=AI Caramba y=ICSI
Lambda Conversion:
-expr: xy: close-to (x, y) (AI Caramba)
Lambda Reduction:
y: close-to (AI Caramba, y)
close-to (AI Caramba, ICSI)
Semantics - Lambda Calculus 3
Lambda Expressions can be constructed from central expression, inserting semantic representations for complement phrases
Verb serves
{xy e IS-A(e, Serving) Server(e,y) Served(e,x)}
represents general semantics for the verb 'serve
Fill in appropriate expressions for x, y, for example 'meat' for y derived from Noun in NP as complement to Verb.
References
Jurafsky, D. & J. H. Martin, Speech and Language Processing, Prentice-Hall, 2000. (Chapters 9 and 10)
Helmreich, S., From Syntax to Semantics, Presentation in the 74.419 Course, November 2003.