1. Introduction to Computational Linguistics
Eleni Miltsakaki
AUTH
Fall 2005-Lecture 5

2. What’s the plan for today?
Brief review of
Chomsky’s hierarchy of languages
Tree Adjoining Grammar
Lexical Functional Grammar
Head-driven Phrase Structure Grammar

3. 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)

4. 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

5. Tests for phrases/constituents
Substitutability
I met a man. I met the man. I met John.
Permutability
Εχω επισκεφτεί πολλές πόλεις της Ελλάδας
Πολλές πόλεις της Ελλάδας έχω επισκεφτεί
Pronominalizability
I met John. I met him.

7. 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)

8. 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

9. Examples of feature structures

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

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

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

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

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

15. 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

16. A non-linguistic example

17. Head complement schema
Head complement schema + head feature principle

18. A simple example

19. General information about HPSG

20. Applications

21. Acknowledgements
The introduction to HPSG is based on Stefan Muller’s introduction to HPSG at ESSLLI Many thanks to Stefan Muller for making the slides of his tutorial publicly available