6 863J Natural Language Processing Lecture 9 Writing grammars feature based grammars Robert C Berwick berwick ai mit edu The Menu Bar Administrivia Schedule alert Lab 3 out due next Weds Lab time today tomorrow Please read notes3 pdf englishgrammar pdf on web Agenda Building grammars basics to complex Limits of context free grammars the trouble with tribbles Foundation for the laboratory 6 863J 9 611J Lecture 9 Sp03 Grammars for natural languages Where do the rules come from Roughly read them off of parse trees A rule based construction based point of view Take surface phrase patterns mostly But we still want to map to an underlying logical form How do we start out 6 863J 9 611J Lecture 9 Sp03 Reading rules from parse trees S NP VP VP V NP Can t we get a computer to do this NP Det N NP N 6 863J 9 611J Lecture 9 Sp03 Key elements part 1 Establish basic phrase types S VP NP PP Where do these come from 6 863J 9 611J Lecture 9 Sp03 What kinds of phrases are there Noun phrases verb phrases adjectival phrases green with envy adverbial phrases quickly up the hill prepositional phrases off the wall etc In general grounded on lexical items Shows us the constraints on context free rules for natural grammars Example 6 863J 9 611J Lecture 9 Sp03 Phrase types are constrained by lexical projection Verb Phrase Verb Noun Phrase is a kick the ball Prepositional Phrase Preposition Noun Phrase on the table Adjective Phrase Adjective Prep Phrase green with envy Etc what is the pattern 6 863J 9 611J Lecture 9 Sp03 Function argument relation XP X arguments where X Noun Verb Preposition Adjective all lexical categories in the language Like function argument structure so called Xbar theory Constrains what grammar rules cannot be Verb Phrase Noun Noun Phrase or even Verb Phrase Noun Phrase Verb Noun Phrase 6 863J 9 611J Lecture 9 Sp03 English is function argument form function args sold the stock at a bargain price green with envy the over priced stock 6 863J 9 611J Lecture 9 Sp03 Other languages are the mirrorinverse arg function This is like Japanese the over priced stock green with envy sold at a bargain price the stock 6 863J 9 611J Lecture 9 Sp03 Key elements part 2 Establish verb subcategories What are these Different verbs take different arguments 0 1 2 arguments complements Poirot thought Poirot thought the gun Poirot thought the gun was the cause Some verbs take certain sentence complements I know who John saw I think who John saw propositional types Embedded questions I wonder whether Embedded proposition I think that John saw Mary 6 863J 9 611J Lecture 9 Sp03 Key elements Subtlety to this Believe know think wonder I believe why John likes ice cream I know why John likes ice cream I believe that John likes ice cream I believe that John likes ice cream args type Verb subcategories How many subcategories are there What is the structure 6 863J 9 611J Lecture 9 Sp03 Idea for phrases They are based on projections of words lexical items imagine features percolating up XP V proposition know V proposition 6 863J 9 611J Lecture 9 Sp03 Heads of phrases V proposition know V proposition 6 863J 9 611J Lecture 9 Sp03 The parse structure for embedded sentences I believe that John likes ice cream S NP VP I V that J likes ice cream believe 6 863J 9 611J Lecture 9 Sp03 New phrase type S bar S NP VP I V Sbar believe that J likes ice cream 6 863J 9 611J Lecture 9 Sp03 Sbar VP V Sbar believe Comp S that J likes ice cream 6 863J 9 611J Lecture 9 Sp03 Sbar VP V Sbar believe Comp S J likes ice cream 6 863J 9 611J Lecture 9 Sp03 In fact true for all sentences Sbar Comp John likes ice cream S Why J likes ice cream 6 863J 9 611J Lecture 9 Sp03 What rules will we need U do it 6 863J 9 611J Lecture 9 Sp03 Verb types continued What about Clinton admires honesty Honesty admires Clinton How do we encode these in a CFG Should we encode them Colorless green ideas sleep furiously Revolutionary new ideas appear infrequently 6 863J 9 611J Lecture 9 Sp03 Features 6 863J 9 611J Lecture 9 Sp03 The trouble with tribbles morphology of a single word Verb head thrill tense present num sing person 3 thrills projection of features up to a bigger phrase VP head tense num V head tense num NP provided is in the set TRANSITIVE VERBS agreement between sister phrases S head tense NP num VP head tense num provided is in the set TRANSITIVE VERBS 6 863J 9 611J Lecture 9 Sp03 3 Common Ways to Use Features Verb head thrill tense present num sing person 3 thrills VP head tense num V head tense num NP S head tense NP num VP head tense num S comprehension perspective NP VP num sing num sing Verb num sing NP A roller coaster thrills every teenager 6 863J 9 611J Lecture 9 Sp03 CFG Solution Encode constraints into the non terminals Noun verb agreement S SgS S PlS SgS SgNP SgVP SgNP SgDet SgNom Verb subcategories IntransVP IntransV TransVP TransV NP 6 863J 9 611J Lecture 9 Sp03 Problems with this how much info 6 863J 9 611J Lecture 9 Sp03 Agreement gets complex VAR POSSN GENDER POS CASE PERSONNEG Czech AGFS3 1A POSSG DCOMP VOICE SUBPOS TENSE NUMBER 6 863J 9 611J Lecture 9 Sp03 Lots of features tense number person gaps vowels commas wh etc etc He has Sorry that s just how language is You know too much to write it down easily gone Other sentence types Questions Will John eat ice cream Did John eat ice cream How do we encode this 6 863J 9 611J Lecture 9 Sp03 Empty elements or categories Where surface phrase is displaced from its canonical syntactic position Examples The ice cream was eaten vs John ate the ice cream What did John eat What did Bill say that that John thought the cat ate For What x did Bill say the cat ate x Bush is too stubborn to talk to Bush is too stubborn x to talk to Bush Bush is too stubborn to talk to the Pope Bush is too stubborn Bush to talk to the Pope 6 863J 9 611J Lecture 9 Sp03 More interesting clause types Apparently long distance effects displacement of phrases from their base positions 1 So called wh movement What did John eat 2 Topicalization actually the same On this day it snowed two feet 3 Other cases so called passive The eggplant was eaten by John How to handle this 6 863J 9 611J Lecture 9 Sp03 We can think of this as fillers and gaps Filler the displaced item Gap the place where it belongs as argument Fillers can be NPs PPs S s Gaps are invisible so hard to parse we have to guess Can be complex Which book did you file without …
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