1 Introduction to Computational Linguistics Eleni Miltsakaki AUTH Fall 2005-Lecture 4

2 What’s the plan for today? Brief review of Chomsky’s Hierarchy of languages Tree Adjoining Grammar Lexical Functional Grammar Next time: Head-driven Phrase Structure Grammar Readings: –Parsing (very brief) –Parsing in psycholinguistics –Examples of TAG derivations

3 Chomsky hierarchy (review) Containment hierarchy of classes of formal grammars that generate formal languages Type 0: unrestricted, include all formal grammars –Any string of terminals and non-terminals to any string of terminals and non- terminals Type 1: context sensitive –A  any string of terminals and non-terminals Type 2: context free (the theoretical basis for the syntax of most programming languages) –A  a, A  Ba Type 3: regular grammars –A  a

4 Lexicalized grammars Tree adjoining grammar (TAG) –The XTAG project ( ) –The TAG workshops ( Lexical functional grammar (LFG) –LFG resource site ( –LFG conferences ( Head-driven phrase structure grammar (HPSG) –HPSG resource site ( –HPSG conferences (

5 Tree adjoining grammars Introduced by Joshi, Levy & Takahashi (1975) and Joshi (1985) Linguistically motivated –Tree generating grammar (generates tree structures not just strings) Example: I want him to leave, I promised him to leave –Allows factoring recursion from the statement of linguistic constraints (dependencies), thus simplifying linguistic description (Kroch & Joshi 1985) Formally motivated –A (new) class of grammars that describe mildly context sensitive languages (Joshi et al 1991)

6 The TAG formalism Concepts –Locality and recursion –Lexicalization Elementary objects –Initial trees –Auxiliary trees Operations –Substitution –Adjunction

7 Locality and recursion Stretching dependencies –Who do you like t? –Who does John think that you like t? –Who does John think that Mary said that you like t? Also compare: –John tried to please Mary –John seems to like Mary

8 Lexicalization In a lexicalized grammar each elementary structure is associated with a lexical item, called its ‘anchor’

9 Elementary trees

10 Operation 1: Substitution

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13 Tree Families

14 Verbs and particles

15 Wh-extraction

16 Operation 2: Adjunction

17 Operation 2: Adjunction

18 Derived and derivation trees

19 Reference for TAGs Tree adjoining grammars: Formalisms, Linguistic Analysis and Processing by Anne Abeillé and Owen Rambow ( html)

20 Lexical Functional Grammar First introduced by Kaplan & Bresnan (1982) A theory of grammar –Syntax (how words combine to make up sentences) –Semantics (how and why words and combinations of words mean what they mean)

21 Basic components of LFG Two parallel levels of syntactic representation –Constituent structure (c-structure) –Functional structure (f-structure) C-structures have the form of context-free phrase structure trees F-structures are sets of pairs of attributes and values; attributes may be features, such as tense and gender, or functions, such as subject and object.

22 Structure, structural description and correspondence Allowable c-structures for a sentence are defined by context-free rewrite rules The description of an appropriate f-structure is derived from functional annotations on to the c-structure rules To interpret functional annotations Kaplan and Bresnan defined a special instantiation procedure which relies on an implicit c-structure to f-structure correspondence

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24 Head-driven Phrase Structure Grammar Next time!