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Agents That Communicate

Chris BourneChris ChristenBryan Hryciw

OverviewCommunication as ActionTypes of Communicating AgentsA Formal Grammar for a Subset of EnglishSyntactic Analysis (Parsing)Definite Clause Grammar (DCG)Augmenting a GrammarSemantic InterpretationAmbiguity and DisambiguityA Communicating AgentSummary

IntroductionCommunication 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)

IntroductionHumans 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 only 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 on 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.

OverviewCommunication as ActionTypes of Communicating AgentsA Formal Grammar for a Subset of EnglishSyntactic Analysis (Parsing)Definite Clause Grammar (DCG)Augmenting a GrammarSemantic InterpretationAmbiguity and DisambiguityA Communicating Agent

Communication as ActionSpeech Act:

The action available to an agent to produce languageincludes talking, typing, sign language, etc.Speaker - An agent that produces a speech actHearer - 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 ActionTransferring Information to Hearer:

Inform: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 ActionMake 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:other agents about particular aspects of the world. Ex. Have you smelled the wumpus anywhere?

Request or Command:other 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 CommunicationSpeaking:

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 LanguageFormal 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 CommunicationThree steps take place in the speaker:

Intention: S want H to believe PGeneration: S chooses words WSynthesis: 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)Incorporation: H decides to believe Pi (or rejects it if it is out of line with what H already believes)

Models of CommunicationEncoded 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.

Types of Communicating AgentsCommunicating using Tell and ask:

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

Communicating using Formal Language:Agents make no assumptions about each other’s internal languageAgents share a communication language that is a subset of English

Tell and AskCommunication with Tell and Ask

Percepts Actions Percepts Actions

Reasoning Reasoning

KB KBAgent 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 ActionsReasoning Reasoning

KB KBAgent A Agent BLanguage

Language

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 VPS -> S Conjunction S

Parsing AlgorithmsThere are many algorithms for parsing

Top-down parsing• Starting with an S and expanding accordingly

Bottom-up parsingCombination of top-down and bottom-upDynamic programming techniques• Avoids inefficiencies of backtracking

Bottom-up Parse (example)

function BOTTOM-UP-PARSE(words, grammar) returns a parse tree

forest ← wordsloop do

if LENGTH(forest) = 1 and CATEGORY(forest[1]) = START(grammar) thenreturn forest[1]

elsei ← 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 → theNoun → wumpusNP → Article NounVerb → isAdjective → deadVP → VerbVP → Verb AdjectiveS → NP VP

ThewumpusArticle NounisdeadVerbVP AdjectiveNP VP

The wumpus is deadArticle wumpus is deadArticle Noun is deadNP is deadNP Verb deadNP Verb AdjectiveNP VP AdjectiveNP VPS

rulesubsequenceforest

Definite Clause Grammer (DCG)Problems with Backus-Naur Form (BNF)

Need meaningContext sensitive

Introduction of First Order Logic

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

DCG NotationPositive:

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 NotationNon-terminal symbols can be augmentedA variable can appear on RHSAn 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 LogicDCG

OvergenerationSimple grammar can overgenerate

Ex: “Me smells a stench.”

To fix we must understandCases of English

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

New Rules

Use of Augmentation

→→→→→→

NP(subjective) VP | …Pronoun(case) | Noun | Article NounVP NP(Objective) | …Preposition NP(Objective)I | you | he | she | …me | you | him | her | …

SNP(case)

VPPP

Pronoun(Subjective)Pronoun(Objective)

NPs VP | …Pronouns | Noun | Article NounPronouno | Noun | Article NounVP NPo | …Preposition NPoI | you | he | she | …me | you | him | her | …

→→→→→→→

SNPsNpoVPPP

PronounsPronouno

Changes needed to handle subjective and objective cases

Verb SubcategorizationNow have slight improvementMust create a sub-categorization list

Believe the smelly wumpus in 2 2 is dead[S]believe

Is smellyis in 2 2is a pit

[Adjective][PP][NP]

is

smell a wumpussmell awfulsmell like a wumpus

[NP][Adjective][PP]

smell

give the gold in 3,3 to megive me the gold

[NP , PP][NP , NP]

giveExampleSubcatsVerb

Parse TreeS

NP

VP([])

VP([NP])

VP([NP,NP]) NP NP

Pronoun Pronoun Article NounVerb([NP,NP])

You give me the gold

Semantic InterpretationSemantic Interpretation: Responsible for combining meanings compositionally to get a set of possible interpretationsFormal Languages

Compositional Semantics: The semantics of any phrase is a function of its subphrases

• X + YWe can handle an infinite grammar with a finite set of rules

Natural LanguagesAppears 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 InterpretationSemantics 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 E1Semantic structure is very different from syntactic structure.We use an intermediate form called a quasi-logical formwhich 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 InterpretationThrough semantic interpretation, an agent can perceive a string of 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 InterpretationIndexicals: 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.

Ambiguity and DisambiguationThe 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.

AmbiguityLexical Ambiguity

a word has more than one meaning

Syntactic Ambiguity (Structural Ambiguity)more than one possible parse for the phrase

Semantic Ambiguityfollows from lexical or syntactic ambiguity

Referential Ambiguitysemantic ambiguity caused by anaphoric expressions

AmbiguityPragmatic Ambiguity

Speaker and hearer disagree on what the current situation is.

Local AmbiguityA substring can be parsed several ways.

VaguenessNatural languages are also vague

• “It’s hot outside.”

DisambiguationDisambiguation is a question of diagnosis.

Models of the world are used to provide possible interpretations of a speech act.

Models of the speakerModels of the hearer

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

DisambiguationIn general, disambiguation requires the combination of four models:

the world modelthe mental modelthe language modelthe acoustic model

Natural language often uses deliberate ambiguity.Most language understanding programs ignore this possibility

DisambiguationContext 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)

A Communicating AgentHow 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 AgentRules for commands and statementsS(Command(rel(Hearer)) –> VP(rel)S(Statement(rel(obj)) –> NP(obj) VP(rel)

Rules for acknowledgementsS(Acknowledge(sem)) –> Ack(sem)Ack(True) –> yesAck(True) –> OKAck(False) –> no

SummaryAgents 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 moreFormal language theory and phrase structure grammars are useful tools for dealing with some aspects of natural languageCommunication involves

three steps by the speaker • intention, generation and synthesis

four steps by the hearer • perception, analysis, disambiguation and incorporation

SummaryThe 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 propositionThe 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 allowsfor augmentations

There are many algorithms for parsing strings.I.e. bottom up, top down, combination, and dynamic

SummaryPragmatic 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 toconvey.

The End

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

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