Introduction to Computational Linguistics

Introduction to Computational Linguistics

Introduction to Computational Linguistics Eleni Miltsakaki AUTH Fall 2005-Lecture 5 1 Whats the plan for today? Brief review of

Chomskys hierarchy of languages Tree Adjoining Grammar Lexical Functional Grammar Head-driven Phrase Structure Grammar 2 Main points from last lecture

Chomsky hierarchy of languages Tree adjoining grammar Lexicalization Locality and recursion Substitution and adjunction Lexical Functional Grammar Constituent structure (c-structure)

Functional structure (f-structure) 3 A little more background on syntax and phrase structure Language has a finite set of symbols but gives the possibility of producing infinitely many sentences Our brains have finite capacity we need structure

patterns Phrase structure is basic to all grammars Words, phrases, sentences We obtain the meaning of an utterance from the meaning of its parts Syntax defines the way the combination takes place, i.e., it provides structure

4 Tests for phrases/constituents Substitutability I met a man. I met the man. I met John. Permutability

Pronominalizability I met John. I met him. 5 6

Features We need to specify that the subject and the verb need to agree in person and number If we put all this information in the rules, the rules become to complex Instead, we use features S NP(Per, Num) VP (Per, Num)

S NP(Agr) VP (Agr) 7 Feature structure in HPSG A feature structure is a set of pairs of the form [ATTRIBUTE value] ATTRIBUTE: an element of the set of features named ATT in the grammar (e.g., case, person

etc) value can be atomic (a string) or another feature structure 8 Examples of feature structures 9

Feature types Feature structures are of a certain type, written in italics Features are organized in hierarchies

10 Identity in HPSG Identity of values is marked with boxes 11

Valence and grammar rules Complements are specified as complex categories in the lexical representation There are also specific rules for head complement combinations

12 Representation of valence in feature descriptions A lexical entry consists of: 13

Head feature principle In a headed structure, the head features of the mother are identical to the head features of the head daughter

14 Linguistic generalizations in the type hierarchy Types are arranged in a hierarchy The most general type is at the top Information about properties of an object of a certain type are specified in the definition of the type

Subtypes inherit these properties Like an encyclopedic entry The upper part of the hierarchy is relevant to all languages (universal grammar) More specific types maybe specific for classes of languages or just one language 15

A non-linguistic example 16 Head complement schema Head complement schema + head feature principle

17 A simple example 18 General information about HPSG

19 Applications 20 Acknowledgements The introduction to HPSG is based on

Stefan Mullers introduction to HPSG at ESSLLI 2001. Many thanks to Stefan Muller for making the slides of his tutorial publicly available 21

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