1. Natural language processing
2. Generation
3. Understanding
4. Syntactic Analysis
5. What is a language?
6. What should a grammar tell us?
7. A more complex grammar
8. Components of a syntactic analysis program
9. Additional grammar issues
10. Results of syntactic analysis
11. Semantic Analysis
12. Reference
13. Lexical Meaning
14. Relationship among entities

Natural language processing (NLP)

• to make interfaces easier to use (e.g., database queries)
• to summarize large volumes of text
• to translate text
• to do better information retrieval
• to enable alternative interfaces (e.g., speech)

• understanding
• generation

Generation

Need to:

1. decide that a speech act is called for
2. decide which speech act is the right one
3. decide what information you want to convey
4. make choices about vocabulary
5. create language that is appropriate (i.e., syntactically correct and unambiguous)

• inform
• query
• answer
• request or command
• promise
• acknowledge
• share

Understanding

Need to:

1. understand syntax - i.e., the structure of the text.
2. understand semantics - i.e., a partial representation of the meaning of the text
3. pragmatics - i.e., the complete meaning of the text, determined by using contextual information.

Syntactic Analysis

Because it helps us understand the roles played by different words in a body of text. The words themselves are not enough. Consider

Innocent peacefully children sleep little vs

Innocent little children sleep peacefully

There is some evidence that human understanding of language is, in part, based on structural analysis. Consider

"Twas brillig and the slithy toves did gyre and gimble in the wabe." [Lewis Carroll]

Here we can understand the sentence on some level, even though most of the words make no sense to us.

Colorless green ideas sleep furiously makes more sense to us than

Ideas green furiously colorless sleep

Finally, consider the sentence

The old dog the footsteps of the young.

In reading this sentence, you might have found yourself having to re-examine the first part. In all likelihood, you thought that the word "dog" was being used as a noun, when, in fact, it is a verb.

What is a language?

Words are formed into sentences. A sentence, then, is a well-formed sequence of words.

The language is the set of all sentences that are well-formed, i.e., that follow a set of rules.

This set of rules is called a grammar.

To do syntactic analysis, we build a parser, i.e., a software systems that checks whether the rules are followed and that provides an analysis based on the grammar.

What should a grammar tell us?

For example, the grammar might tell us that some legal sentence structures are as follows:

1. noun verb noun (i.e., a noun followed by a verb followed by a noun), as in

   Dogs chase cats.

2. determiner noun verb determiner noun, as in

   The cat ate the fish.

3. det noun prep adj noun verb prep det noun, as in

   The dog with one eye ran from the cat.

We want a grammar to give us more structural information. So, for example, some better grammar rules for sentences might be:

1. Sentence -> NP VP (i.e., a sentence can be formed from a noun phrase followed by a verb phrase).
2. NP -> noun (a noun phrase can be just a single noun)
3. NP -> det noun (a noun phrase can be a determiner followed by a noun)
4. VP -> verb (a verb phrase can be just a single verb)
5. VP -> verb NP (a verb phrase can be a verb followed by a noun phrase)

The cat ate the fish

our grammar could help us determine the following structure, which we call a parse tree:

This might be represented textually as:

(Sentence
   (NP
      (det  the)
      (noun cat))
   (VP
      (verb ate)
      (NP
         (det  the)
         (noun fish))))

A more complex grammar

1. Sentence -> NP VP
2. NP -> noun
3. NP -> det noun
4. NP -> adj noun
5. NP -> det noun PP (where PP means "prepositional phrase")
6. VP -> verb
7. VP -> verb NP
8. VP -> verb PP
9. PP -> prep NP

The dog with one eye ran from the cat., a parser could produce the following parse tree:

Components of a syntactic analysis program

1. a parser - i.e., a program that takes as input a sentence and produces the analysis.
2. a grammar - i.e., a set of rules that the parser can use.
3. a lexicon - i.e., a dictionary of legal words and their parts of speech

• It relies on the grammar and lexicon -- if there are rules in the language that are not specified in the grammar, certain sentences will not be analyzed. If there are words that are not listed in the lexicon, they will not be recognized as legal words.
• It will produces analyses of syntactically correct but meaningless sentences.

Additional grammar issues

• subject-verb agreement
• distinguishing between transitive and intransitive verbs (i.e., those that require a direct object and those that don't.)

Results of syntactic analysis

1. clues on word meaning:

   The nurses hand the doctors the scalpels.

   The nurse's hand was bandaged.

2. clues on overall phrase meaning:

   Flying planes is dangerous.

   Flying planes are dangerous.

Semantic Analysis

• problems of reference
• lexical meaning
• relationships (modifier attachment, etc.)

Reference

Determining the type of thing referred to. For example, noun phrases can refer to:

• entities (e.g., semantics)
• objects (e.g., the desk)
• events (e.g., the lecture)

• events (e.g., I gave a talk in Vancouver.)
• states (e.g., It was hot on Tuesday.)
• relations e.g., I own a pet lovebird.

Your parser should be able to handle 10 sentences. (indefinite)

Your parser should be able to handle the 10 sentences I gave you. (definite)

Generic vs instance. For example,

Of all the breeds of dogs, the dalmation is my favorite.

My friend Jane has two dogs. My favorite is the dalmation.

Anaphora, i.e., pronouns and definite reference. For example,

The lizard's tail fell off and three days later it had grown a new one.

Quantification. For example,

Jane bought every girl a yellow T-shirt.

Lexical Meaning

Kathy was so busy she ate her lunch on the fly.

Kathy was distracted and put her sandwich on the fly.

How can we do lexical disambiguation?

• Can use knowledge of context.

  The waiter served the lasagna.

  Venus Williams served first.

• One syntactic form can restrict another (called selectional restriction).

  The gasoline killed all the flies.

  If we have a rule that says that the object of the verb "kills" must be animate, then we can determine the meaning of the noun "flies".

• Pure syntactic cues.

     to keep
        (1) meaning: continue to be
            object: adjective phrase
  
        (2) meaning: maintain
            object: noun phrase
  

  I kept calm. vs I keep a diary.
• Semantic association with nearby words, as in The dog's bark woke me up. (not talking about tree bark here)
• Higher level inferences, as in I swung a hammer and the head flew off.

Relationship among entities

• agent - carries out the action
• goal - entity that undergoes the action
• beneficiary - something that benefits from the action
• instrument - by which the action is done
• from-loc - location of origin
• to-loc - location to which an object goes

Syntax can sometimes signal a role.