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Formal Semantics. Slides by Julia Hockenmaier , Laura McGarrity , Bill McCartney, Chris Manning, and Dan Klein. Formal Semantics. It comes in two flavors: Lexical Semantics : The meaning of words

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formal semantics

Formal Semantics

Slides by Julia Hockenmaier, Laura McGarrity, Bill McCartney, Chris Manning, and Dan Klein

formal semantics1
Formal Semantics

It comes in two flavors:

  • Lexical Semantics: The meaning of words
  • Compositional semantics: How the meaning of individual units combine to form the meaning of larger units
what is meaning
What is meaning
  • Meaning ≠ Dictionary entries

Dictionaries define words using words.

Circularity!

reference
Reference
  • Referent: the thing/idea in the world that a word refers to
  • Reference: the relationship between a word and its referent
reference1
Reference

Barack president

Obama

The president is the commander-in-chief.

= Barack Obama is the commander-in-chief.

reference2
Reference

Barack president

Obama

I want to be the president.

≠ I want to be Barack Obama.

reference3
Reference
  • Tooth fairy?
  • Phoenix?
  • Winner of the 2016 presidential election?
what is meaning1
What is meaning?
  • Meaning ≠ Dictionary entries
  • Meaning ≠ Reference
sense
Sense
  • Sense: The mental representation of a word or phrase, independent of its referent.
sense mental image
Sense ≠ Mental Image
  • A word may have different mental images for different people.
    • E.g., “mother”
  • A word may conjure a typical mental image (a prototype), but can signify atypical examples as well.
sense v reference
Sense v. Reference
  • A word/phrase may have sense, but no reference:
    • King of the world
    • The camel in CIS 8538
    • The greatest integer
    • The
  • A word may have reference, but no sense:
    • Proper names: Dan McCloy, Kristi Krein

(who are they?!)

sense v reference1
Sense v. Reference
  • A word may have the same referent, but more than one sense:
    • The morning star / the evening star (Venus)
  • A word may have one sense, but multiple referents:
    • Dog, bird
some semantic relations between words
Some semantic relations between words
  • Hyponymy: subclass
    • Poodle < dog
    • Crimson < red
    • Red < color
    • Dance < move
  • Hypernymy: superclass
  • Synonymy:
    • Couch/sofa
    • Manatee / sea cow
  • Antonymy:
    • Dead/alive
    • Married/single
lexical decomposition
Lexical Decomposition
  • Word sense can be represented with

semantic features:

compositional semantics1
Compositional Semantics
  • The study of how meanings of small units combine to form the meaning of larger units

The dog chased the cat ≠ The cat chased the dog.

ie, the whole does not equal the sum of the parts.

The dog chased the cat = The cat was chased by the dog

ie, syntax matters to determining meaning.

principle of compositionality
Principle of Compositionality

The meaning of a sentence is determined by the meaning of its words in conjunction with the way they are syntactically combined.

exceptions to compositionality
Exceptions to Compositionality
  • Anomaly: when phrases are well-formed syntactically, but not semantically
    • Colorless green ideas sleep furiously. (Chomsky)
    • That bachelor is pregnant.
exceptions to compositionality1
Exceptions to Compositionality
  • Metaphor: the use of an expression to refer to something that it does not literally denote in order to suggest a similarity
    • Time is money.
    • The walls have ears.
exceptions to compositionality2
Exceptions to Compositionality
  • Idioms: Phrases with fixed meanings not composed of literal meanings of the words
    • Kick the bucket = die

(*The bucket was kicked by John.)

    • When pigs fly = ‘it will never happen’

(*She suspected pigs might fly tomorrow.)

    • Bite off more than you can chew

= ‘to take on too much’

(*He chewed just as much as he bit off.)

logical foundations for compositional semantics
Logical Foundations for Compositional Semantics
  • We need a language for expressing the meaning of words, phrases, and sentences
  • Many possible choices; we will focus on
    • First-order predicate logic (FOPL) with types
    • Lambda calculus
truth conditional semantics
Truth-conditional Semantics
  • Linguistic expressions
    • “Bob sings.”
  • Logical translations
    • sings(Bob)
    • but could be p_5789023(a_257890)
  • Denotation:
    • [[bob]] = some specific person (in some context)
    • [[sings(bob)]] = true, in situations where Bob is singing; false, otherwise
  • Types on translations:
    • bob: e(ntity)
    • sings(bob): t(rue or false, a boolean type)
truth conditional semantics1
Truth-conditional Semantics

Some more complicated logical descriptions of language:

  • “All girls like a video game.”
  • x:e . y:e . girl(x)  [video-game(y)  likes(x,y)]
  • “Alice is a former teacher.”
  • (former(teacher))(Alice)
  • “Alice saw the cat before Bob did.”
  • x:e, y:e, z:e, t1:e, t2:e .

cat(x)  see(y)  see(z) 

agent(y, Alice)  patient(y, x) 

agent(z, Bob)  patient(z, x) 

time(y, t1)  time(z, t2)  <(t1, t2)

fopl syntax summary
FOPL Syntax Summary
  • A set of types T = {t1, … }
  • A set of constants C = {c1, …}, each associated with a type from T
  • A set of relations R = {r1, …}, where each ri is a subset of Cn for some n.
  • A set of variables X = {x1, …}
  • , , , , , , ., :
truth conditional semantics2
Truth-conditional semantics
  • Proper names:
    • Refer directly to some entity in the world
    • Bob: bob
  • Sentences:
    • Are either t or f
    • Bob sings: sings(bob)
  • So what about verbs and VPs?
    • sings must combine with bob to produce sings(bob)
    • The λ-calculus is a notation for functions whose arguments are not yet filled.
    • sings: λx.sings(x)
    • This is a predicate, a function that returns a truth value. In this case, it takes a single entity as an argument, so we can write its type as e  t
  • Adjectives?
lambda calculus
Lambda calculus
  • FOPL + λ (new quantifier) will be our lambda calculus
  • Intuitively, λ is just a way of creating a function
    • E.g., girl() is a relation symbol; but

λx. girl(x) is a function that takes one argument.

  • New inference rule: function application

(λx. L1(x)) (L2) → L1(L2)

E.g.,(λx. x2) (3) → 32

E.g., (λx. sings(x)) (Bob) → sings(Bob)

  • Lambda calculus lets us describe the meaning of words individually.
    • Function application (and a few other rules) then lets us combine those meanings to come up with the meaning of larger phrases or sentences.
compositional semantics with the calculus
Compositional Semantics with the λ-calculus
  • So now we have meanings for the words
  • How do we know how to combine the words?
  • Associate a combination rule with each grammar rule:
    • S : β(α) NP : αVP : β (function application)
    • VP : λx. α(x) ∧β(x)  VP : αand : ∅VP : β(intersection)
  • Example:
composition some more examples
Composition: Some more examples
  • Transitive verbs:
    • likes : λx.λy.likes(y,x)
    • Two-places predicates, type e(et)
    • VP “likes Amy” : λy.likes(y,Amy) is just a one-place predicate
  • Quantifiers:
    • What does “everyone” mean?
    • Everyone : λf.x.f(x)
    • Some problems:
      • Have to change our NP/VP rule
      • Won’t work for “Amy likes everyone”
    • What about “Everyone likes someone”?
    • Gets tricky quickly!
composition some more examples1
Composition: Some more examples
  • Indefinites
    • The wrong way:
      • “Bob ate a waffle” : ate(bob,waffle)
      • “Amy ate a waffle” : ate(amy,waffle)
    • Better translation:
      • ∃x.waffle(x) ^ ate(bob, x)
      • What does the translation of “a” have to be?
      • What about “the”?
      • What about “every”?
denotation
Denotation
  • What do we do with the logical form?
    • It has fewer (no?) ambiguities
    • Can check the truth-value against a database
    • More usefully: can add new facts, expressed in language, to an existing relational database
    • Question-answering: can check whether a statement in a corpus entails a question-answer pair:

“Bob sings and dances” 

Q:“Who sings?” has answer A:“Bob”

    • Can chain together facts for story comprehension
grounding
Grounding
  • What does the translation likes : λx. λy. likes(y,x) have to do with actual liking?
  • Nothing! (unless the denotation model says it does)
  • Grounding: relating linguistic symbols to perceptual referents
    • Sometimes a connection to a database entry is enough
    • Other times, you might insist on connecting “blue” to the appropriate portion of the visual EM spectrum
    • Or connect “likes” to an emotional sensation
  • Alternative to grounding: meaning postulates
    • You could insist, e.g., that likes(y,x) => knows(y,x)
more representation issues
More representation issues
  • Tense and events
    • In general, you don’t get far with verbs as predicates
    • Better to have event variables e
      • “Alice danced” : danced(Alice) vs.
      • “Alice danced” : ∃e.dance(e)^agent(e, Alice)^(time(e)
    • Event variables let you talk about non-trivial tense/aspect structures:

“Alice had been dancing when Bob sneezed”

more representation issues1
More representation issues
  • Propositional attitudes (modal logic)
    • “Bob thinks that I am a gummi bear”
      • thinks(bob, gummi(me))?
      • thinks(bob, “He is a gummi bear”)?
    • Usually, the solution involves intensions (^p) which are, roughly, the set of possible worlds in which predicate p is true.
      • thinks(bob, ^gummi(me))
    • Computationally challenging
      • Each agent has to model every other agent’s mental state
      • This comes up all the time in language –
        • E.g., if you want to talk about what your bill claims that you bought, vs. what you think you bought, vs. what you actually bought.
more representation issues2
More representation issues
  • Multiple quantifiers:

“In this country, a woman gives birth every 15 minutes.

Our job is to find her, and stop her.”

-- Groucho Marx

      • Deciding between readings
        • “Bob bought a pumpkin every Halloween.”
        • “Bob put a warning in every window.”
more representation issues3
More representation issues
  • Other tricky stuff
    • Adverbs
    • Non-intersective adjectives
    • Generalized quantifiers
    • Generics
      • “Cats like naps.”
      • “The players scored a goal.”
    • Pronouns and anaphora
      • “If you have a dime, put it in the meter.”
    • … etc., etc.
ccg parsing
CCG Parsing
  • Combinatory Categorial Grammar
    • Lexicalized PCFG
    • Categories encode argument sequences
      • A/B means a category that can combine with a B to the right to form an A
      • A \ B means a category that can combine with a B to the left to form an A
    • A syntactic parallel to the lambda calculus
learning to map sentences to logical form
Learning to map sentences to logical form
  • Zettlemoyer and Collins (IJCAI 05, EMNLP 07)
parsing rules combinators
Parsing Rules (Combinators)

Application

Right: X : f(a)  X/Y : f Y : a

Left: X : f(a)  Y : a X\Y : f

Additional rules:

  • Composition
  • Type-raising
lexical generation
Lexical Generation

Input Training Example

Sentence: Texas borders Kansas.

Logical form: borders(Texas, Kansas)

genlex
GENLEX
  • Input: a training example (Si, Li)
  • Computation:
    • Create all substrings of consecutive words in Si
    • Create categories from Li
    • Create lexical entries that are the cross products of these two sets
  • Output: Lexicon Λ
genlex cross product
GENLEX Cross Product

Input Training Example

Sentence: Texas borders Kansas.

Logical form: borders(Texas, Kansas)

Output Lexicon

weighted ccg
Weighted CCG

Given a log-linear model with a CCG lexicon Λ, a feature vector f, and weights w:

The best parse is: y* = argmax w ∙ f(x,y)

where we consider all possible parses y for the sentence x given the lexicon Λ.

y

parameter estimation for weighted ccg parsing
Parameter Estimation for Weighted CCG Parsing

Inputs: Training set {(Si,Li) | i = 1, …, n}

Initial lexicon Λ, initial weights w, num. iter. T

Computation: For t=1 … T, i = 1 … n:

Step 1: Check correctness

If y* = argmax w ∙ f(Si,y) is Li, skip to next i

Step 2: Lexical generation

Set λ = Λ∪ GENLEX(Si,Li)

Let y’ = argmax w ∙ f(Si,y)

Define λi to be the lexical entries in y’

Set Λ = Λ∪λi

Step 3: Update Parameters

Let y’’ = argmax w ∙ f(Si,y)

If y’’ ≠ Li

Set w = w + f(Si, y’) – f(Si,y’’)

Output: Lexicon Λ and parameters w

y s.t. L(y) = Li

y

summing up
Summing Up
  • Hypothesis: Principle of Compositionality
    • Semantics of NL sentences and phrases can be composed from the semantics of their subparts
  • Rules can be derived which map syntactic analysis to semantic representation (Rule-to-Rule Hypothesis)
    • Lambda notation provides a way to extend FOPC to this end
    • But coming up with rule2rule mappings is hard
  • Idioms, metaphors and other non-compositional aspects of language makes things tricky (e.g. fake gun)
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