11-711: Nyberg's Lecture Notes

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Introduction to Semantic Processing (2)

In this lecture, we examine case-frame parsing techniques in more detail, and finish our survey of semantic processing techniques with a discussion of compositionality.

Case Frames and Case-Frame Parsing

  1. Top-Down
    • Easy algorithm to code
    • Less efficient with lots of top-level frames
    • Examples from Xcalibur

  2. Bottom-Up
    • Harder to code
    • More efficient

  3. Top-Down with Bottom-Up Indexing
    • Hybrid approach
    • Requires pre-indexing
    • Faster run-time
    • Trade space (index size) for speed
    • Examples from BPARSE

Compositionality

Compositionality Principle
SEMANTICS: a principle (attributed to Frege, hence sometimes called Frege's Principle) that constrains the relation between form and meaning by requiring that the meaning of a composite expression is built up from the meanings of its basic expressions. This principle plays an important role in formal semantic theories, like Montague Grammar. Here the Compositionality Principle takes the form of a homomorphism, a mapping that assigns meanings to the basic expressions of the language and semantic operations to syntactic rules.

(from Hans Leidekker's Lexicon of Linguistics)

  • KANT Interlingua

    • Head mapping
    • Feature mapping
    • Grammatical function => semantic role
    • Example: "Attach the cable to the harness and the terminal."

  • Where is semantic rep. non-isomorphic with surface syntax?

    • Case absorption
      "John propelled the ball with his foot"
(*A-KICK (AGENT *O-JOHN) (PATIENT *O-BALL))

    • Case creation
      "Bob sued Acme"
(*A-FILE-LEGAL-ACTION 
  (AGENT *O-BOB) 
  (PATIENT *O-SUIT) 
  (RECIPIENT *O-ACME))

    • Constituent => Feature
      "the man"
((cat n)(root "man")(det ((cat det)(root "the"))))
(*O-MAN (REFERENCE DEFINITE))

    • Structural changes
      "gallon of water"
((cat n)(root "bucket")
 (pp ((cat p)(root "of")
      (obj ((cat n)(root "gallon"))))))
(*O-WATER (QUANTITY *O-BUCKET))

    • What about ordering information?

      • Feature structures inherently unordered
      • Surface elements explicitly ordered
      • Some constructions are sensitive to ordering
        (e.g., coordination)
        "Insert the key and start the engine."
"Attach the cable to the harness and the terminal."

  • Montague Semantics
    (Dowty, Wall and Peters (1985). Introduction to Montague Semantics. Dordrecht: Reidel)
    (Rich and Knight, Section 15.3.4)

    • At every step in the syntactic parsing process, there is a corresponding step in semantic interpretation

    • Semantic rules associated with semantic rules

    • Strictly compositional treatment of NL sentences using lamba expressions and quantifiers in formal logic

    • Some success (e.g., Hirst's ABSITY [Hirst, 1987])

    • Ambiguity: quantifier scoping

      1. "Every student who hadn't declared a major took an English class"
        • There is a major such that evey student who had not declared it took an English class
        • There is an English class such that every student who had not declared some major took it

      2. "John only eats meat on Friday"
        • It is always Friday when John eats meat
        • Meat is the only thing John eats on Friday

  • Logical Form
    (Allen, Chapter 8)

    • Another formal approach, based on first-order predicate calculus

    • Adds generalized quantifiers (each, every, some, most, several, etc.)

    • Adds modal operators (believe, hope; tenses, etc.)

    • Operations on predicates (e.g., operator for plurals: from predicate P to sets of individuals with property P)

    • Extension to allow ambiguity "in place" (more than one sense where a single sense is allowed).

    • Event and state variables on predicates (efficiently represent adverbial meaning)

20-Nov-96 by ehn@cs.cmu.edu