Overview

For this project, I extended the semantic system to include negation ('not' and 'no'). The existing system was unable to represent the truth values of statements, and therefore could not differentiate between knowing that a statement was false versus not knowing whether a statement was true or false; the inclusion of negation solves this problem. I also fixed/changed several other problems with the system, including determiners, names, prepositions, and passives.

The code (.zip file) can be downloaded here. (Note: requires Python with nltk version 1.2 (later versions aren't compatible with the semantics system).)

The adverb 'not'

Statements and queries are both now allowed to include 'not', indicating the opposite of the positive statement. This was implemented by adding an optional 'Not' production to the beginning of VBARs, and adding a "truth" attribute for statements, which can be set to 'true' or 'false'.

Hello.
> did john see mary
I don't know.
> john did not see mary
Okay.
> did john see mary
No.
> who did john not see
mary
> who did not see mary
john
> john did not give mary fido
Okay.
> did john give fido to mary
No.
> who did john not give mary
fido
> did john not give mary fido
Yes.
> did john not not give mary fido
No.

The determiner 'no'

Any noun phrase - subject, direct object, indirect object, or object of a preposition, in statements and in queries - can begin with 'no'. The meaning of this determiner is not exactly the same as in real English, because that would have required also implementing other determiners like 'every'. Instead, the system behaves as if there is exactly one instance of each NP (this also fits with its treatment of definiteness; see the following section), so quantifying an NP with 'no' is equivalent to negating the sentence.

Hello.
> john gave mary no dog
Okay.
> did john give mary a dog
No.
> what did john not give mary
dog
> who did john not give a dog to
mary
> who did john give no dog to
mary
> no man saw mary
Okay.
> did a man see mary
No.

Definiteness

The idea of definiteness has been removed. In the original system, determiners specified definiteness, but this did not actually have any meaning, and so was not dealt with properly in interpreting statements and queries. A query with a definite NP would not match a statement with an indefinite NP, and vice-versa:

Hello.
> john saw a dog
Okay.
> did john see the dog
No.

Noun matching

In the original system, facts specified different NPs differently (names with a "name" attribute, nouns with a "type" attribute). This caused all names to match against all non-name nouns:

Hello.
> john saw a dog
Okay.
> did john see mary
Yes.

There was also the word 'somebody' in the lexicon, which made just an Object with no attributes, so it would match against any Object:

Hello.
> john saw somebody
Okay.
> did john see mary
Yes.

Multiple answers

In the original system, only a single answer could be given for a query, even if the system knew multiple answers:

Hello.
> john saw fido
Okay.
> john saw mary
Okay.
> who did john see
mary
> did john see fido
Yes.

Hello.
> john saw mary
Okay.
> john saw fido
Okay.
> who did john see
fido and mary

Prepositional phrases

In the original system, prepositional phrases could be handled in a limited number of locations, but not in other locations where it would make sense for them to be allowed:

Hello.
> john gave fido to mary in the park
I don't understand.
> john saw mary in the park
Okay.
> did john see mary in the park
I don't understand.
> where did john see mary
in the park

Passives

In the original system, the words 'is' and 'was' were present in the lexicon, but they were not actually usable. They could only be used to form passives, but passive sentences were incorrectly interpreted as active, and could not be used in queries:

Hello.
> john is a man
I don't understand.
> john is red
I don't understand.
> john is driven
Okay.
> is john driven
I don't understand.
> does john drive
Yes.

Hello.
> john was driven by fido
Okay.
> who drove john
fido
> was john driven by fido
Yes.
> mary does not like john
Okay.
> who is john not liked by
mary