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Understanding Unification in Grammar

Explore the concept of unification in feature structures and its application in grammatical relationships, rules, and predicates. Learn how feature structures extend and unify, enabling easy expression of grammar rules.

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Understanding Unification in Grammar

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Presentation Transcript

  1. Unification Grammars Allen’s Chapter 4 J&M’s Chapter 11

  2. Unification of Feature Structures • Key concept - extension relationship between two FSs: • F2 extends F1 if every feature-value pair in F1 is also in F2 • Two FSs F1 and F2 unify if there exists a FS F that is an extension of both of them • The Most General Unifier is the minimal FS F that extends both • The Unification operation allows easy expression of grammatical relationships among constituent feature structures

  3. Unification of Feature Structures • F2 extends F1: F1 = (CAT v) F2 = (CAT v ROOT cry) F1 = (AGR {3s 3p}) F2 = (AGR 3s) • F3 is MGU of F1 and F2: F1 = (CAT v ROOT cry), F2 = (CAT v VFORM pres), F3 = (CAT v ROOT cry VFORM pres) • F1 and F2 do not unify: F1 = (CAT v AGR 3s) F2 = (CAT v AGR 3p)

  4. Unification-based Grammars • Grammar rules can be completely specified using unification • Example: X0 --> X1 X2 CAT X0 = S CAT X1 = NP CAT X2 = VP AGR0 =AGR1 = AGR2 VFORM0 = VFORM2 • If a feature (such as CAT) is always specified, it can be associated with the non-terminal of a CFG rule • Examples: S  NP VP AGR0= AGR1 = AGR2 VFORM0 = VFORM2 NP  ART N AGR0 = AGR1 = AGR2

  5. Sample Grammar (Abbreviated Form)

  6. Unification Grammars

  7. Predicative Phrases • VP  (V ROOT be) (NP PRED +) He is a student • VP  (V ROOT be) (PP PRED +) He is in the house • VP  (V ROOT be) (ADJP PRED +) He is happy • X0  X1 X2 CAT0 =VP CAT1 = V CAT2 = {NP PP ADJP} ROOT1 = be PRED2 = +

  8. More Information in Lexicon put : (CAT V SUBCAT (FIRST (CAT NP) SECOND (CAT PP LOC +))) VP  V X2 X3 2 = FIRSTSUBCAT1 3=SECONDSUBCAT1 want : (CAT V SUBCAT (FIRST (CAT NP) SECOND (CAT VP VFORM inf)))

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