CS224N lecture05 笔记

发布于 2020-06-20  158 次阅读


Linguistic Structure: Dependecy Parsing

Slides

  • Two vies of lingustic structure

    • Constituency = phrase structure grammar = context-free grammars(CFGs); Phrase structure organizes words into nested consitituents
    • Deoendency structure shows which words depend on(modify or are arguments of) which other owrds
        Dependency paths identify semantic relations; Dependency Grammar and Structure;
        Dependency syntax postulates that syntactic structure consists of relations between lexical items, normally binary asmmetric relations("arrows") called dependencies , dependencies usually from a tree(connected, acyclic, single-head)

      • The arrows are commonly typed with the name of grammatical relations
      • usually add a fake ROOT, so every word is a dependent of precisely other node
  • The rise of annotated data(Universal Dependencies treebanks)

    • semms a lot slower and less useful than buliding a grammar
    • reusability of the labor; broad coverage, not just a few intuitions; frenqencies and distributional information; a way to evaluate sysytems
  • What are the sources of information for dependency parsing?

    1. Bilexical affinities: [dicussion → issue] is plausiable
    2. Dependency distance: mostly with nearby words
    3. Intervening material: Dependencies rarely span intervening verbs or punctuation
    4. Valency of heads: How many dependents on which side are usual for a head?
  • Dependency Parsing constraints

    • only one word is a depent of ROOT
    • don't want cycles like A → B, B → A
    • this makes the dependencies a tree
    • final issue is whether arrows can cross(non-projective) or not
  • Methods of Dependency Parsing

    1. Dynamic programming
    2. Graph algorithms
    3. Constraint Satisfaction
    4. "Transiton-based parsing" or "deterministic dependency parsing"
  • Neural dependency parser

  • Handing non-projectivity
    not neccessary all the time

    Notes

    • Keyphrases: Dpendency Parsing
    • Two main types of parse trees: constituency structures; dependency strutures
  • Dependecy parsing problems(formally): {input: S = W0W1...Wn, (W0 is the ROOT); output: dependency tree graph G}

  • Two subproblems in dependency parsing

    • Learning: given a training set D of sentences annotated with dependency graphs, induce a parsing model M that can be used to parse new sentences
    • Parsing: given a parsing model M and a sentence S, derive the optimal dependency graph D for S according to M
  • Transition-based dependency parsing
    rely on a state machine to implement mapping function

  • Greedy Deterministic Transition-based parsing
    search strategies?

  • Neural Dependency Parsing
    transition-based; graph methods

Suggested Readings

  1. Incrementality in Deterministic Dependency Parsing
    讨论约束条件对 Deterministic Dependency Parsing的影响, 没细看

  2. A Fast and Accurate Dependecny Parser using Neural Networks
    the model introduced in this lecure; activation function = x^3

  3. Dependency Parsing
    textbook with 127 pages 没看

  4. Globally Normalized Transition-Based Neural Networks
    global normalization(所有的1→n),比local normalization(每一步步进)更好处理label biase(处理早期决策失误);transition-based + feedforward NN VS RNN-based;

  5. Universal Dependencies: A cross-linguistic typology
    a taxonomy of grammatical relations applicable to variety of languages 没细看

  6. Universal Dependencies website
    universal dependecies介绍, 没看


阿克西斯上没有什么重要的东西