Agents That Communicate Chris Bourne Chris Christen Bryan Hryciw - - PowerPoint PPT Presentation

Agents That Communicate

Chris Bourne Chris Christen Bryan Hryciw

Overview

Communication as Action Types of Communicating Agents A Formal Grammar for a Subset of English Syntactic Analysis (Parsing) Definite Clause Grammar (DCG) Augmenting a Grammar Semantic Interpretation Ambiguity and Disambiguity A Communicating Agent Summary

Introduction

Communication is the intentional exchange of information

brought about by the production and perception of signs drawn from a shared system of conventional signs. Most animals employ a fixed set of signs to represent messages that are important to their survival: food here, predator nearby, approach, withdraw, let’s mate. Humans, just as many other animals, use a limited number of signs to communicate (smiling, shaking hands)

Introduction

Humans are the only animal that has developed a complex, structured system of signs, known as language, that enables us to communicate most of what they know about the world. Although other animals such as chimpanzees and dolphins have shown vocabularies of hundreds of symbols, humans are the

• nly species that can communicate an unbounded number of

qualitatively different messages. Although there are other uniquely human attributes, such as wearing clothes and watching TV, Turing created his test based

• n language because language is closely tied to thinking, in a

way these other attributes are not.

Origins & Evolution of Language

Did humans develop the use of language because we are smart, or are we smart because we use language well?

Jerrison, 1991: Human language stems from a need for better cognitive maps of territory. Canines rely heavily on scent marking and their sense of smell to determine where they are and what other animals have been there. Since primates do not have such a highly developed sense of smell, they substituted vocal sounds for scent marking.

Overview

Communication as Action

Types of Communicating Agents A Formal Grammar for a Subset of English Syntactic Analysis (Parsing) Definite Clause Grammar (DCG) Augmenting a Grammar Semantic Interpretation Ambiguity and Disambiguity A Communicating Agent

Communication as Action

Speech Act:

The action available to an agent to produce language includes talking, typing, sign language, etc.

Speaker - An agent that produces a speech act Hearer - An agent that receives a speech act Why would agents choose to perform a speech act?

To be able to:

• Inform, Query, Answer, Request or Command, Promise,

Acknowledge and Share

Communication as Action

Transferring I nformation to Hearer: I nform:

each other about the part of the world each has explored, so other agent has less exploring to do. Ex. There’s a breeze in 3 4.

Answer:

• questions. This is a kind of informing. Ex. Yes, I smelled the

wumpus in 2 3.

Acknowledge:

requests and offers. Ex. Okay.

Share:

feelings and experiences with each other. Ex. You know, that wumpus sure needs deodorant.

Communication as Action

Make the Hearer take some action: Promise:

to do things or offer deals. Ex. I’ll shoot the wumpus if you let me share the gold.

Query:

• ther agents about particular aspects of the world. Ex. Have you

smelled the wumpus anywhere?

Request or Command:

• ther agents to perform actions. It can be seen as impolite to

make direct requests, so often an indirect speech act (a request in the form of a statement or question) is used instead. Ex. I could use some help carrying this or Could you please help me carry this?

Difficulties with Communication

Speaking:

When is a speech act called for? Which speech act, out of all the possibilities is the right one?

Nondeterminism Understanding:

Given ambiguous inputs, what state of the world could have created these inputs?

Fundamentals of Language

Formal Languages: Languages that are invented

and are rigidly defined. A set of strings where each string is a sequence of symbols taken from a finite set called the terminal symbols.

Lisp, C+ + , first order logic, etc.

Natural Languages: Languages that humans use

to talk to one another.

Chinese, Danish, English, etc.

Component Steps of Communication

Three steps take place in the speaker:

I ntention: S want H to believe P Generation: S chooses words W Synthesis: S utters the words W

Four steps take place in the hearer:

Perception: H perceives W1 (ideally, W = W1) Analysis: H infers that W1 has possible meanings P1, … , Pn Disambiguation: H infers that S intended to convey Pi (ideally,

Pi = P)

I ncorporation: H decides to believe Pi (or rejects it if it is out

• f line with what H already believes)

Models of Communication

Encoded Message Model:

Speaker encodes a proposition into words or signs. The hearer then tries to decode this message to retrieve the original

• proposition. The meaning in the speaker’s head, the message that

gets transmitted, and the interpretation that hearer arrives at are all the same, unless there is noise during communication, or an error in encoding or decoding occurs.

Situated Language Model:

Created because of limitations on the encoded message model. The meaning of the message depends on both the words, and the

situation.

• Ex. “Diamond” refers to one thing when the subject is jewelry,

and a completely different meaning when the subject is baseball.

Overview

Communication as Action

Types of Communicating Agents

A Formal Grammar for a Subset of English Syntactic Analysis (Parsing) Definite Clause Grammar (DCG) Augmenting a Grammar Semantic Interpretation Ambiguity and Disambiguity A Communicating Agent

Types of Communicating Agents

Communicating using Tell and ask:

Agents share a common internal representation language Agents are capable of communicating without any external language at all

Communicating using Formal Language:

Agents make no assumptions about each other’s internal language Agents share a communication language that is a subset of English

Tell and Ask

Communication with Tell and Ask Percepts Actions Percepts Actions

Reasoning Reasoning KB KB Agent A Agent B

TELL(KBB, “P”) TELL(KBA, “P”) ASK(KBB, “Q”) TELL(KBB, “Pit(PA1) ∧ At(PA1,[2,3], SA9)”)

Formal Language

Percepts Actions Percepts Actions

Reasoning Reasoning KB KB Agent A Agent B

Language Language

Overview

Communication as Action Types of Communicating Agents

A Formal Grammar for a Subset of English

Syntactic Analysis (Parsing) Definite Clause Grammar (DCG) Augmenting a Grammar Semantic Interpretation Ambiguity and Disambiguity A Communicating Agent

Formal Grammar for a Subset of English

Lexicon: List of allowable vocabulary words.

Noun -> stench | breeze | glitter | nothing | wumpus | pit | pits | gold | east | … Verb -> is | see | smell | shoot | feel | stinks | go | grab | carry | kill | turn | … Adjective -> right | left | east | south | back | smelly | … Adverb -> here | there | nearby | ahead | right | left | east | south | back | … Pronoun -> me | you | I | it | … Name -> John | Mary | Boston | Aristotle | … Article -> the | a | an | … Preposition -> to | in | on | near | … Conjunction -> and | or | but | … Digit -> 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9

Formal Grammar for a Subset of English

NP -> Pronoun

• > Noun
• > Article Noun
• > Digit Digit
• > NP PP
• > NP RelClause

RelClause -> that VP VP -> Verb

• > VP NP
• > VP Adjective
• > VP PP
• > VP Adverb

PP -> Preposition NP

Grammar:

S -> NP VP S -> S Conjunction S

Overview

Communication as Action Types of Communicating Agents A Formal Grammar for a Subset of English

Syntactic Analysis (Parsing)

Definite Clause Grammar (DCG) Augmenting a Grammar Semantic Interpretation Ambiguity and Disambiguity A Communicating Agent

Parsing Algorithms

There are many algorithms for parsing

Top-down parsing

• Starting with an S and expanding accordingly

Bottom-up parsing Combination of top-down and bottom-up Dynamic programming techniques

• Avoids inefficiencies of backtracking

Bottom-up Parse (example)

function BOTTOM-UP-PARSE(words, grammar) returns a parse tree forest ← words loop do if LENGTH(forest) = 1 and CATEGORY(forest[1]) = START(grammar) then return forest[1] else i ← choose from {1…LENGTH(forest)} rule ← choose from RULES(grammar) n ← LENGTH(RULE-RHS(rule)) subsequence ← SUBSEQUENCE(forest, i, i+n-1) if MATCH(subsequence,RULE-RHS(rule)) then forest[i…i+n-1] ← [MAKE-NODE(RULE-LHS(rule) , subsequence)] else fail end

Article → the Noun → wumpus NP → Article Noun Verb → is Adjective → dead VP → Verb VP → Verb Adjective S → NP VP The wumpus Article Noun is dead Verb VP Adjective NP VP The wumpus is dead Article wumpus is dead Article Noun is dead NP is dead NP Verb dead NP Verb Adjective NP VP Adjective NP VP S rule subsequence forest

Overview

Communication as Action Types of Communicating Agents A Formal Grammar for a Subset of English Syntactic Analysis (Parsing)

Definite Clause Grammar (DCG)

Augmenting a Grammar Semantic Interpretation Ambiguity and Disambiguity A Communicating Agent

Definite Clause Grammer (DCG)

Problems with Backus-Naur Form (BNF)

Need meaning Context sensitive

Introduction of First Order Logic

BNF First Order Logic S → NP VP NP(s1) /\ VP(s2) ⇒ S(Append(s1 ,s2)) Noun → stench | … (s=“stench” \/ …) ⇒ Noun(s)

DCG Notation

Positive:

Easy to describe grammars

Negative:

More verbose than BNF

3 Rules:

• The notation X → Y Z … translate as Y(s1) /\ Z(s2)…⇒ X(Append(s1, s2,…).
• The notation X → word translates as X([“word”]).
• The notation X → Y | Z | … translates as Y’(s) \/ Z’(s) \/…⇒ X(s), where Y’

is the translation into logic of the DCG expression Y.

Extending the Notation

Non-terminal symbols can be augmented A variable can appear on RHS An arbitrary logical test can appear on RHS

(s=[sem]) ⇒ Digit(sem , s) Digit(sem , s) ⇒ Number(sem , s) Number(sem , s1) /\ Digit(sem , s2) /\ sem = 10 × sem1 + sem2 ⇒ Number(sem , Append(s1 , s2) Digit(sem) → sem { 0 ≤ sem ≤ 9 } Number(sem) → Digit(sem) Number(sem) → Number(sem1) Digit(sem2) {sem = 10 × sem1 + sem2} First Order Logic DCG

Overview

Communication as Action Types of Communicating Agents A Formal Grammar for a Subset of English Syntactic Analysis (Parsing) Definite Clause Grammar (DCG)

Augmenting a Grammar

Semantic Interpretation Ambiguity and Disambiguity A Communicating Agent

Overgeneration

Simple grammar can overgenerate

Ex: “Me smells a stench.”

To fix we must understand Cases of English

Nominative - subjective - “I” Accusative - objective - “me”

New Rules

Use of Augmentation → → → → → → NP(subjective) VP | … Pronoun(case) | Noun | Article Noun VP NP(Objective) | … Preposition NP(Objective) I | you | he | she | … me | you | him | her | … S NP(case) VP PP Pronoun(Subjective) Pronoun(Objective) NPs VP | … Pronouns | Noun | Article Noun Pronouno | Noun | Article Noun VP NPo | … Preposition NPo I | you | he | she | … me | you | him | her | … → → → → → → → S NPs Npo VP PP Pronouns Pronouno Changes needed to handle subjective and objective cases

Verb Subcategorization

Now have slight improvement Must create a sub-categorization list

Believe the smelly wumpus in 2 2 is dead [S] believe Is smelly is in 2 2 is a pit [Adjective] [PP] [NP] is smell a wumpus smell awful smell like a wumpus [NP] [Adjective] [PP] smell give the gold in 3,3 to me give me the gold [NP , PP] [NP , NP] give Example Subcats Verb

Parse Tree

S NP VP([]) VP([NP]) VP([NP,NP]) NP NP Pronoun Pronoun Article Noun Verb([NP,NP]) You give me the gold

Overview

Communication as Action Types of Communicating Agents A Formal Grammar for a Subset of English Syntactic Analysis (Parsing) Definite Clause Grammar (DCG) Augmenting a Grammar

Semantic I nterpretation

Ambiguity and Disambiguity A Communicating Agent

Semantic Interpretation

Semantic I nterpretation: Responsible for combining

meanings compositionally to get a set of possible interpretations

Formal Languages

Compositional Semantics: The semantics of any phrase is a

function of its subphrases

• X + Y

We can handle an infinite grammar with a finite set of rules

Natural Languages

Appears to have a noncompositional semantics

• “The batter hit the ball”

Semantic interpretation alone cannot be certain of the right interpretation of a phrase or sentence

Semantic Interpretation

Semantics as DCG Augmentation

The same idea used to specify the semantics of numbers and digits can applied to the complete language of mathematics

Exp(sem) –> Exp(sem1) Operator(op) Exp(sem2) {sem = Apply(op, sem1, sem2)} Exp(sem) –> ( Exp(sem) ) Exp(sem) –> Number(sem) Digit(sem) –> sem { 0 ≤ sem ≤ 9 } Number(sem) –> Digit(sem) Number(sem) –> Number(sem1) Digit(sem2) { sem = 10 × sem1 + sem2 } Operator(sem) –> sem { sem ∈ {+,–,×,÷}}

The Semantics of E1

Semantic structure is very different from syntactic structure. We use an intermediate form called a quasi-logical form which uses a new construction which we will call a quantified

term.

“every agent” -> [∀ a Agent(a)]

“Every agent smells a wumpus”

∃ e (e ∈ Perceive([∀ a Agent(a)], [∃ w Wumpus(w)],Nose) ∧ During(Now, e))

Pragmatic Interpretation

Through semantic interpretation, an agent can perceive a string

• f words and use a grammar to derive a set of possible

semantic interpretations. Now we address the problem of completing the interpretation by adding information about the current situation

Information which is noncompositional and context-

dependant

Pragmatic Interpretation

I ndexicals: Phrases that refer directly to the current situation

“I am in Boston today.”

Anaphora: Phrases referring to objects that have been

mentioned previously

“John was hungry. He entered a restaurant.” “After John proposed to Marsha, they found a preacher and got

• married. For the honeymoon, they went to Hawaii.”

Deciding which reference is the right one is a part of

disambiguation.

Overview

Communication as Action Types of Communicating Agents A Formal Grammar for a Subset of English Syntactic Analysis (Parsing) Definite Clause Grammar (DCG) Augmenting a Grammar Semantic Interpretation

Ambiguity and Disambiguity

A Communicating Agent

Ambiguity and Disambiguation

The biggest problem in communicative exchange is that most utterances are ambiguous.

Squad helps dog bite victim. Red-hot star to wed astronomer. Helicopter powered by human flies. Once-sagging cloth diaper industry saved by full dumps.

Ambiguity

Lexical Ambiguity

a word has more than one meaning

Syntactic Ambiguity (Structural Ambiguity)

more than one possible parse for the phrase

Semantic Ambiguity

follows from lexical or syntactic ambiguity

Referential Ambiguity

semantic ambiguity caused by anaphoric expressions

Ambiguity

Pragmatic Ambiguity

Speaker and hearer disagree on what the current situation is.

Local Ambiguity

A substring can be parsed several ways.

Vagueness

Natural languages are also vague

• “It’s hot outside.”

Disambiguation

Disambiguation is a question of diagnosis. Models of the world are used to provide possible

interpretations of a speech act.

Models of the speaker Models of the hearer

It is difficult to pick the right interpretation because there may be several right ones.

Disambiguation

In general, disambiguation requires the combination

• f four models:

the world model the mental model the language model the acoustic model

Natural language often uses deliberate ambiguity.

Most language understanding programs ignore this possibility

Disambiguation

Context free grammars do not provide a very useful language model.

Probabilistic context-free grammars (PCFG’s)

each rewrite rule has a probability associated with it

S –> NP VP (0.9) S –> S Conjunction S (0.1)

Overview

Communication as Action Types of Communicating Agents A Formal Grammar for a Subset of English Syntactic Analysis (Parsing) Definite Clause Grammar (DCG) Augmenting a Grammar Semantic Interpretation Ambiguity and Disambiguity

A Communicating Agent

A Communicating Agent

How does this all fit in to an agent that can communicate?

Start with the wumpus world robot slave.

Extend the grammar to accept commands

“Go east” “Go to 2 2”

Identify the kind (i.e, command or statement) of speech as part of the quasi-logical form.

A Communicating Agent

Rules for commands and statements

S(Command(rel(Hearer)) –> VP(rel) S(Statement(rel(obj)) –> NP(obj) VP(rel)

Rules for acknowledgements

S(Acknowledge(sem)) –> Ack(sem) Ack(True) –> yes Ack(True) –> OK Ack(False) –> no

Summary

Agents send signals to each other using a speech act. All animals use some conventional signs to communicate, but humans use language in a more sophisticated way that enables them to communicate much more Formal language theory and phrase structure grammars are useful tools for dealing with some aspects of natural language Communication involves

three steps by the speaker

• intention, generation and synthesis

four steps by the hearer

• perception, analysis, disambiguation and incorporation

Summary

The encoded message model of communication says that a speaker encodes a representation of a proposition into language, and the hearer decodes the message to uncover the proposition The situated language model states that the meaning of a message is a function of both the message and the situation in which it occurs.

Augmenting a grammar allows us to handle many problems

Definite Clause Grammar (DCG) is an extension of BNF that allows for augmentations

There are many algorithms for parsing strings.

I.e. bottom up, top down, combination, and dynamic

Summary

Pragmatic interpretation takes the current situation into

account to determine the effect of an utterance in context

Disambiguation is the process of deciding which of the

possible interpretations is the one that the speaker intended to convey.

The End

Bibliography

Norvig, Peter and Russell, Stuart, 1995. “Artificial Intelligence A Modern Approach”, Prentice-Hall Inc., Upper Saddle River, New Jersey.