English SyntaxTwo Kinds of IssuesWhat is Syntax?Slide 4Why Do We Care about Syntax?Sometimes We Need it Even if We Don’t Go All the WayFinding Constituents in SentencesFinding Constituents in Sentences, con’tHow Many Kinds of Constituents are There?How Many Kinds of Constituents are There, con’tSingle Word ConstituentsSimple Constituent Types Don’t Capture EverythingSubcategorizationThe Role of the Lexicon in ParsingOne Other Barrier to a Small Number of Kinds of Constituents - AgreementThe Solution – Augmenting the Constituent TypesSpecifying a LanguageA Context-Free Grammar for EnglishContext-Free GrammarsDerivations Using CFGsSlide 21Context-Free Grammars and Parse TreesLong Distance DependenciesLong Distance Dependencies – A Linguistic SolutionLong Distance Dependencies – Computational SolutionsSpoken Language SyntaxSlide 27English SyntaxRead J & M Chapter 9.Two Kinds of Issues•Linguistic – what are the facts about language?The rules of syntax (grammar)•Algorithmic – what are effective computational procedures for dealing with those facts?Building parsersWhat is Syntax?Try 1: the rules for stringing words together to form sentences.The boys hit the ball. vs. Ball boys hit the the.I gave Sue a ride to the store vs. I gave Sue ride to store.I saw the book that Mary had written. vs.I saw the book what Mary had written.But if that’s all it were, we wouldn’t have to do much for understanding assuming legal input.What is Syntax?Try 2: The rules for forming constituents that correspond to meaningful entities.Example: The cat with the furry tail purred.Why Do We Care about Syntax?MorphologyPOS TaggingSyntaxSemanticsDiscourse IntegrationGeneration goes backwards. For this reason, we generally want declarative representations of the facts.Sometimes We Need it Even if We Don’t Go All the WayQuestion answering:Lawyers whose clients committed fraudvsLawyers who committed fraudvsClients whose lawyers committed fraudFinding Constituents in SentencesA constituent is a word or group of words that functions as a unit.How can we discern constituents?•Semantically: The cat with the furry tail purred.•What can be chopped out and replaced by a single word?Agnes purred. * Agnes tail purred.Finding Constituents in Sentences, con’t•Preposed and postposed constructions: Early next year I’d like to go to Paris. I’d like to go to Paris early next year. I’d like early next year to go to Paris.* Early I’d like to go to Paris next year.* I’d like early to go to Paris next year.* The early next year old man would like to go to Paris.How Many Kinds of Constituents are There?Although there may be an infinite number of possible constituent tokens, there’s quite a small number of constituent types, e.g., NP, PP, VP.On what basis can we group tokens into types? Occurrence in similar contexts.How Many Kinds of Constituents are There, con’tThe cat with the furry tail purred.Every dog wore a collar.Most of the children in the room brought a dog with a furry tail and a collar.The furry tail brought a room.Every room purred.A dog with a furry tail and a collar purred.Mary saw most of the children in the room.NPs occur as subjects, objects of verbs, and objects of prepositions.Single Word ConstituentsSingle word constituents are exactly the parts of speech that we have already considered.How many of these single word constituent types are there? Look at sizes of tagsets.Lots of design decisions: Sue bought the big white house.* Sue bought the white big house.Are big and white the same POS?Simple Constituent Types Don’t Capture Everything* The cat with a furry tail purred a collar. Mary imagined a cat with a furry tail. Mary decided to go.* Mary decided a cat with a furry tail. Mary decided a cat with a furry tail would be her next pet. Mary gave Lucy the food.* Mary decided Lucy the food.SubcategorizationFrame Verb ExampleØ eat, sleep, … I want to eatNP prefer, find, leave, ... Find [NP the flight from Pittsburgh to Boston]NP NP show, give, … Show [NP me] [NP airlines with flights from Pittsburgh]PPfrom PPto fly, travel, … I would like to fly [pp from Boston] [pp to Philadelphia]NP PPwith help, load, … Can you help [NP me] [pp with a flight]VPto prefer, want, need, … I would prefer [VPto to go by United airlines]VPbrst can, would, might, … I can [VPbrst go from Boston]S mean Does this mean [S AA has a hub in Boston]?The Role of the Lexicon in Parsing•Serves as the starting point for POS tagging.•Provides additional information such as subcategorization:•For verbs•For adjectives:I’m angry with Mary. I’m angry at Mary.I’m mad at Mary. * I’m mad with Mary.•For nouns:Jane has a passion for old movies.Jane has an interest in old movies.One Other Barrier to a Small Number of Kinds of Constituents - AgreementNumber agreement: The boys want to go to the game(s).* The boy want to to to the game(s).Case agreement: I want to give it to him.* Me want to give it to he.In English it’s just pronouns, but not so in many other languages.The Solution – Augmenting the Constituent TypesTo solve these and other problems, one strategy is to augment constituent types with other sorts of information:V +pl +[NP NP] VP/NP/NP +pl ShowVP/NP +pl Show me VP +pl Show me the book.Specifying a Language•The set of sentences in English is large (maybe even infinite).•We want a concise (i. e., much shorter than a list of sentences) definition of it.•We have a finite (in fact quite small) set of constituent types (NP, VP, etc.) from which to build our description.So we appeal to recursion and write grammar rules such as:S NP VPVP V NPNP NP PP NP NP S (The boy who went to the store won the game.)PP prep NPA Context-Free Grammar for EnglishIf we ignore:•subcategorization•agreement•gappingThen we can build a context-free grammar for English that does a pretty good job of:•generating all and only the acceptable sentences, and of •building reasonable parse trees for those sentences.We’ll look at whether English is formally context free later.Context-Free GrammarsA context-free grammar (CFG) is a 4-tuple: 1. A set of non-terminal symbols N2. A set of terminals (disjoint from N)3. A set of productions P, each of the form A , where A is a non-terminal and is a string of symbols from the infinite set of strings (N)*4. A designated start symbol
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