Formal semantics
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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 semantics

Compositional Semantics


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.)


Idioms in other languages

Idioms in other languages


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)<now)

    • 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.


Mapping sentences to logical forms

Mapping Sentences to Logical Forms


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)


Some training examples

Some Training Examples


Ccg lexicon

CCG Lexicon


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


Ccg parsing example

CCG Parsing Example


Parsing a question

Parsing a Question


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


Genlex output lexicon

GENLEX 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


Example learned lexical entries

Example Learned Lexical Entries


Challenge revisited

Challenge Revisited


Disharmonic application

Disharmonic Application


Missing content words

Missing Content Words


Missing content free words

Missing content-free words


A complete parse

A complete parse


Geo880 test set

Geo880 Test Set


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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