CS 4705 Lecture 3 Morphology: Parsing Words

What is morphology? The study of how words are composed from smaller, meaning-bearing units (morphemes) –Stems: children, undoubtedly, –Affixes (prefixes, suffixes, circumfixes, infixes) Immaterial Trying Gesagt Absobl**dylutely –Concatenative vs. non-concatenative (e.g. Arabic root- and-pattern) morphological systems

Morphology Helps Define Word Classes AKA morphological classes, parts-of-speech Closed vs. open (function vs. content) class words –Pronoun, preposition, conjunction, determiner,… –Noun, verb, adverb, adjective,…

(English) Inflectional Morphology Word stem + grammatical morpheme –Usually produces word of same classclass –Usually serves a syntactic function (e.g. agreement) like  likes or liked bird  birds Nominal morphology –Plural forms s or es Irregular forms (goose/geese) Mass vs. count nouns (fish/fish, or s?) –Possessives (cat’s, cats’)

Verbal inflection –Main verbs (sleep, like, fear) verbs relatively regular -s, ing, ed And productive: ed, instant-messaged, faxed, homered But some are not regular: eat/ate/eaten, catch/caught/caught –Primary (be, have, do) and modal verbs (can, will, must) often irregular and not productive Be: am/is/are/were/was/been/being –Irregular verbs few (~250) but frequently occurring –So….English inflectional morphology is fairly easy to model….with some special cases...

(English) Derivational Morphology Word stem + grammatical morpheme –Usually produces word of different class –More complicated than inflectional E.g. verbs --> nouns –-ize verbs  -ation nouns –generalize, realize  generalization, realization E.g.: verbs, nouns  adjectives –embrace, pity  embraceable, pitiable –care, wit  careless, witless

E.g.: adjective  adverb –happy  happily But “rules” have many exceptions –Less productive: *evidence-less, *concern-less, *go- able, *sleep-able –Meanings of derived terms harder to predict by rule clueless, careless, nerveless

Parsing Taking a surface input and identifying its components and underlying structure Morphological parsing: parsing a word into stem and affixes, identifying its parts and their relationships –Stem and features: goose  goose +N +SG or goose + V geese  goose +N +PL gooses  goose +V +3SG –Bracketing: indecipherable  [in [[de [cipher]] able]]

Why parse words? For spell-checking –Is muncheble a legal word? To identify a word’s part-of-speech (pos) –For sentence parsing, for machine translation, … To identify a word’s stem –For information retrieval Why not just list all word forms in a lexicon?

How do people represent words? Hypotheses: –Full listing hypothesis: words listed –Minimum redundancy hypothesis: morphemes listed Experimental evidence: –Priming experiments (Does seeing/hearing one word facilitate recognition of another?) suggest neither –Regularly inflected forms prime stem but not derived forms –But spoken derived words can prime stems if they are semantically close (e.g. government/govern but not department/depart)

Speech errors suggest affixes must be represented separately in the mental lexicon –easy enoughly

What do we need to build a morphological parser? Lexicon: list of stems and affixes (w/ corresponding pos) Morphotactics of the language: model of how and which morphemes can be affixed to a stem Orthographic rules: spelling modifications that may occur when affixation occurs –in  il in context of l (in- + legal)

Using FSAs to Represent English Plural Nouns English nominal inflection q0q2q1 plural (-s) reg-n irreg-sg-n irreg-pl-n Inputs: cats, geese, goose

Derivational morphology: adjective fragment q3 q5 q4 q0 q1q2 un- adj-root 1 -er, -ly, -est  adj-root 1 adj-root 2 -er, -est Adj-root 1 : clear, happy, real (clearly) Adj-root 2 : big, red (~bigly)

FSAs can also represent the Lexicon Expand each non-terminal arc in the previous FSA into a sub-lexicon FSA (e.g. adj_root 2 = {big, red}) and then expand each of these stems into its letters (e.g. red  r e d) to get a recognizer for adjectives q0 q1 un- r e q2 q4 q3 -er, -est d b g q5 q6i q7

But….. Covering the whole lexicon this way will require very large FSAs with consequent search and maintenance problems –Adding new items to the lexicon means recomputing the whole FSA –Non-determinism FSAs tell us whether a word is in the language or not – but usually we want to know more: –What is the stem? –What are the affixes and what sort are they? –We used this information to recognize the word: can we get it back?

Parsing with Finite State Transducers cats  cat +N +PL (a plural NP) Koskenniemi’s two-level morphology –Idea: word is a relationship between lexical level (its morphemes) and surface level (its orthography) –Morphological parsing : find the mapping (transduction) between lexical and surface levels cat+N+PL cats

Finite State Transducers can represent this mapping FSTs map between one set of symbols and another using an FSA whose alphabet  is composed of pairs of symbols from input and output alphabets In general, FSTs can be used for –Translators (Hello:Ciao) –Parser/generator s(Hello:How may I help you?) –As well as Kimmo-style morphological parsing

FST is a 5-tuple consisting of –Q: set of states {q0,q1,q2,q3,q4} –  : an alphabet of complex symbols, each an i/o pair s.t. i  I (an input alphabet) and o  O (an output alphabet) and  is in I x O –q0: a start state –F: a set of final states in Q {q4} –  (q,i:o): a transition function mapping Q x  to Q –Emphatic Sheep  Quizzical Cow q0 q4 q1q2q3 b:ma:o !:?

FST for a 2-level Lexicon E.g. Reg-nIrreg-pl-nIrreg-sg-n c a tg o:e o:e s eg o o s e q0q1q2 q3 c:ca:at:t q4q6q7q5 se:o eg

FST for English Nominal Inflection q0q7 +PL:^s# q1q4 q2q5 q3q6 reg-n irreg-n-sg irreg-n-pl +N:  +PL:-s# +SG:-# +N:  stac c+PL+Nta

Useful Operations on Transducers Cascade: running 2+ FSTs in sequence Intersection: represent the common transitions in FST1 and FST2 (ASR: finding pronunciations) Composition: apply FST2 transition function to result of FST1 transition function Inversion: exchanging the input and output alphabets (recognize and generate with same FST) cf AT&T FSM Toolkit and papers by Mohri, Pereira, and RileyAT&T FSM ToolkitMohri Pereira

Orthographic Rules and FSTs Define additional FSTs to implement rules such as consonant doubling (beg  begging), ‘e’ deletion (make  making), ‘e’ insertion (watch  watches), etc. Lexical fox+N+PL Intermediate fox^s# Surface foxes

Porter Stemmer Used for tasks in which you only care about the stem –IR, modeling given/new distinction, topic detection, document similarity Rewrite rules (e.g. misunderstanding --> misunderstand --> understand --> …) Not perfect …. But sometimes it doesn’t matter too much Fast and easy

Summing Up FSTs provide a useful tool for implementing a standard model of morphological analysis, Kimmo’s two-level morphology But for many tasks (e.g. IR) much simpler approaches are still widely used, e.g. the rule- based Porter Stemmer Next time: –Read Ch 4 –Read over HW1 and ask questions nowHW1