1. Introduction to English Morphology Finite State Transducers
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Introduction to English Morphology
Finite State Transducers
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2. Acknowledgement For further details see Jurafsky & Martin Ch.3
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3. Morphology
Morphology is the study of how word-parts combine to form word wholes.
Several different dimensions:
Orthographic - rules for combining strings of characters together.
Syntax - effect on syntactic category.
Semantic - effect on meaning.
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4. Examples of Morphological Processes
Affixation
prefix
suffix
circumfix: German ge + stem + t e.g. sagen, gesagt
infix: unbloodylikely
Vowel change: swim/swam
Consonant change: send/sent
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5. Inflectional/Derivational Morphology
Inflectional +s plural +ed past
category preserving
productive: always applies (esp. new words, e.g. fax)
systematic: same semantic effect
Derivational +ment
category changing escape+ment
not completely productive: detractment*
not completely systematic: catchment
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6. English Inflectional Morphology
Applies to nouns, verbs and adjectives only
Number of inflections relatively small
Nouns
Plural, Possessive
Verbs
Verb forms
Adjectives
Comparison
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7. Noun Inflections Regular Irregular Singular cat church mouse ox Plural
cats
churches
mice
oxen
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8. Regular Verb Inflections
stem
walk
merge
try
map
-s form
walks
merges
tries
maps
-ing participle
walking
merging
trying
mapping
-ed participle
or past
walked
merged
tried
mapped
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9. Irregular Verb Inflections
stem
eat
catch
cut go
-s form
eats
catches
cuts
goes
-ing participle
eating
catching
cutting
going
Past
ate
caught
went
-ed participle
eaten
gone
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10. Morphological Parsing
Output
Analysis
cat + PL
Input Word
cats
Morphological
Parser
Output is a string of morphemes
Reversibility?
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11. Morphological Parsing: Examples
Input word
Output morphemes
cats
cat +N +PL
cat
cat + N + SG
cities
city + N + PL
walks
walk + V + 3SG
cook
cook +N +SG or
cook +V
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12. Morphemes Morpheme is a theoretical contruct ...
but has a practical use
Choice of morpheme vocabulary: theoretical and practical motivation
Distinction between underlying morpheme and its realisation.
String of morphemes could be turned into another representation later
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13. Morphological Parsing Requires
Lexicon: list of stems and affixes + related information (e.g syntactic category)
Morphotactics: a model of ordering constraints over morphemes (e.g. the fact that +s comes after the stem not before).
Correspondences between input and output strings
Spelling Rules: city + s  cities
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14. Lexicon Lexicon is generally divided into sublexicons
Stem Lexicon
Noun Stems
Verb Stems
etc
Suffix Lexicon
Prefix Lexicon
Can all be represented as FSAs
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15. FSA for Sublexicon Fragmenth e s a e i t s
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16. FSA for Morphotactics for Noun Inflection
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17. Morphotactics for Verb Inflection
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18. Input/Output Correspondences
Problem: how to specify correspondence between input word, and output analysis.
Given: both input and output are strings.
Two level morphology (Koskenniemi 1983) proposes
Surface Tape (words)
Lexical Tape (concatenation of morphemes)
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19. 2 Level Model The automaton used to perform the mapping
Between these levels is the finite state transducer
(FST).
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20. Basic FS Transducer
Each transition of a transducer is labelled with a pair of symbols
Input symbols are matched against the lower-side symbols on transitions.
If analysis succeeds, return the string of upper-side symbols
output symb
input symb
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21. Morphological Analysis
+PL T A C S e +N
{ ("CATS", "CAT+N+PL"),
("CAT", "CAT+N+SG") }
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22. FST Formal Definition States, initial state, final states: same as FSA
Alphabets I and O are input and output alphabets, not necessarily disjoint.
FST Alphabet Σ  I x O
Transition function δ(q, i:o), defines the state q' that ensues when the machine is in state q and encounters complex symbol i:o.
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23. FST Alphabet Example I x O O I Σ c a t ε a:c a:a c:c a:t c:ac:t a:ε
':ε '
t:c
t:a
t:t
t:ε
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24. Summary
Morphological processing can be handled by finite state machinery
Finite State Transducers are formally very similar to Finite State Automata.
They are formally equivalent to regular relations, i.e. sets of pairings of sentences of regular languages.
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