Logic, Language and Learning

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Logic, Language and Learning
Chapter 8 Morphology Luc De Raedt (Following
Covington)
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Outline

• Todays lecture- morphology

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Morphology
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Introduction (1)

• Morphology how words are formed from components
  (morphemes)
• Consists of two processes inflection and
  derivation
• Inflection creating the various forms of each
  word
• Create -gt created or creating

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Introduction (1)

• Derivation creating new words from pre-existing
  words often of different syntactic categories
• Create -gt creation
• Contrast to syntax in most languages morphology
  is finite!
• Thus, one could simply list all possible forms in
  computer implementations.

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Introduction (2)

• Irregularitychild - children,corpus - corpora,
  etc.
• Practical questionHow many rules? How many
  facts?
• Trade-off

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Introduction (3)

• Morphologically complex words can be broken up
  into several meaningful units called morphs.
• dogs dogs dishes dishes undoing
  undoing unrealities unrealitys
• If two morphs are equivalent, then they are
  allomorphs of the same morpheme (e.g. s of dogs
  and es of dishes are allomorphs of the English
  plural morpheme).

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Introduction (4)

• English words zero or more prefixes, followed by
  root, followed by zero or more suffixes
• untouchables untouchables
• Focus here on English- a morphological parser
  in Prolog- two-level morphology

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A Morphological Parser in Prolog
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A morphological parser for English inflection (1)
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A morphological parser for English inflection (2)
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Morphographemic rules

• Final e deletion if suffix begins with a
  vowelrakeing rakingrakeed raked
• y-to-i rule final y changes to i before any
  suffix that does not begin with icarryed
  carriedbut carrying carrying and
  delayeddelayed
• s-to-es rule suffix s, on both nouns and verbs,
  appears as es after s, z, x, sh, ch, and after y
  which has changed to igrasss grasses, dishs
  dishes
• Final consonant doubling single final consonant
  doubles before any suffix that begins with a
  vowelgrabedgrabbed, bigerbigger

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Lexical look-up by letter trees (tries) (1)
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Lexical look-up by letter trees (tries) (2)

• ltree(?Tree) Stores the lexicon as a
  letter tree.ltree( b, a, r, k, bark,
  c, a, r, r, y, carry,
  t, cat, e, g, o, r,
  y, category, d, e, l, a, y,
  delay, h, e, l, p, help,
  o, p, hop, e,
  hope, q, u, a, r, r, y,
  quarry, i, z, quiz,
  o, t, e, quote ).

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Use of ltree (1)

• mparse(Word,-Result) Retrieves morph.
  description of Word (a charlist).mparse(Word,Res
  ult) - ltree(T), find_word(Word,T,Result).
  find_word(Word,Tree,-LexEntry) Finds Word
  in Tree retrieving LexEntry.find_word(HT,Tree
  ,LexEntry) - find_branch(H,Tree,SubTree),
  find_word(T,SubTree,LexEntry).find_word(,LexE
  ntry_,LexEntry) - \ (LexEntry __).

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Use of ltree (2)

• find_branch(Letter,Tree,-LexEntry) Given
  a letter and a tree, returns the appropriate
  (unique) subtree. Deterministic.find_branch(Let
  ter,LetterLexEntry_,LexEntry) - !.
  Found it there is only one.find_branch(Letter,
  L__,_) - Letter _at_lt L, !, fail. Went
  past where it should be don't search any
  further.find_branch(Letter,_Rest,LexEntry)
  - find_branch(Letter,Rest,LexEntry).
  Haven't found it yet, so advance to next branch.

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Some English inflectional endings
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Revised letter tree with more information
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Morphological parser (1)
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Morphological parser (2)
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Morphological parser (3)
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Morphological parser (4)
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Two-Level Morphology
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Introduction (1)

• Morphographemics relationship between underlying
  form and surface form
• Underlying form listed in lexicon
• Surface form actually occurring

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Introduction (1)

• Morphographemic rules map from underlying form to
  surface form
• From quizs to quiz-es to quizzes
• Often, the morphological parser has to "deal with
  elements that aren't there" (e.g., e-deletion)

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Introduction (2)

• Needed morphological parser that is guided by
  the lexicon
• Problem of string comparison step through
  surface form, letter by letter, and
  simultaneously step through the underlying form
  given by one of the rules
• u n r a k e e d
  u n 0 r a k 0 0 e d

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Two-level morphology

• (Koskenniemi, 1983), implemented in KIMMO system
  by (Karttunen, 1983)
• Key insights- multi-level rules can be
  combined- most rules are independent and can
  operate in parallel- each rule can be
  implemented efficiently as a
  deterministic finite-state transducer

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Finite-state transducer
for final e-deletion
Rule e0 ltgt CC _ 0 VV Map into XY
underlying X corresponds to surface Y
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Finite state transducer in Prolog (1)

• To test ?- transduce(1,State,r,a,k,e,,e,d,S
  urface). state(Old,-New,?Undl,?Surf)
  Allows moving from state Old to state New by
  accepting Undl and Surf characters.state(1,2,C,C
  ) - \ vowel(C), !.state(1,1,X,X).state(2,3,e,0
  ) - !.state(2,2,C,C) - \ vowel(C),
  !.state(2,1,X,X).state(3,4,,0).state(4,5,V,V)
  - vowel(V).state(5,1,X,X).

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Finite state transducer in Prolog (2)

• final_state(?N) - N is a state in which
  the transducer can stop.final_state(1).final_st
  ate(2).final_state(5). vowel(V) - V is a
  vowel.vowel(a). vowel(e). vowel(i).
  vowel(o). vowel(u). vowel(y).
  transduce(Start,Finish,UndlString,SurfString)tra
  nsduce(Start,Finish,UUndlString,SSurfString)
  - state(Start,Next,U,S), transduce(Next,Fini
  sh,UndlString,SurfString).transduce(Start,Start,
  ,) - final_state(Start).