nltk.sem.chat80

Overview

Chat-80 was a natural language system which allowed the user to interrogate a Prolog knowledge base in the domain of world geography. It was developed in the early '80s by Warren and Pereira; see http://www.aclweb.org/anthology/J82-3002.pdf for a description and http://www.cis.upenn.edu/~pereira/oldies.html for the source files.

This module contains functions to extract data from the Chat-80 relation files ('the world database'), and convert then into a format that can be incorporated in the FOL models of nltk.sem.evaluate. The code assumes that the Prolog input files are available in the NLTK corpora directory.

The Chat-80 World Database consists of the following files:

world0.pl
rivers.pl
cities.pl
countries.pl
contain.pl
borders.pl

This module uses a slightly modified version of world0.pl, in which a set of Prolog rules have been omitted. The modified file is named world1.pl. Currently, the file rivers.pl is not read in, since it uses a list rather than a string in the second field.

Reading Chat-80 Files

Chat-80 relations are like tables in a relational database. The relation acts as the name of the table; the first argument acts as the 'primary key'; and subsequent arguments are further fields in the table. In general, the name of the table provides a label for a unary predicate whose extension is all the primary keys. For example, relations in cities.pl are of the following form:

'city(athens,greece,1368).'

Here, 'athens' is the key, and will be mapped to a member of the unary predicate city.

The fields in the table are mapped to binary predicates. The first argument of the predicate is the primary key, while the second argument is the data in the relevant field. Thus, in the above example, the third field is mapped to the binary predicate population_of, whose extension is a set of pairs such as '(athens, 1368)'.

An exception to this general framework is required by the relations in the files borders.pl and contains.pl. These contain facts of the following form:

'borders(albania,greece).'

'contains0(africa,central_africa).'

We do not want to form a unary concept out the element in the first field of these records, and we want the label of the binary relation just to be 'border'/'contain' respectively.

In order to drive the extraction process, we use 'relation metadata bundles' which are Python dictionaries such as the following:

city = {'label': 'city',
        'closures': [],
        'schema': ['city', 'country', 'population'],
        'filename': 'cities.pl'}

According to this, the file city['filename'] contains a list of relational tuples (or more accurately, the corresponding strings in Prolog form) whose predicate symbol is city['label'] and whose relational schema is city['schema']. The notion of a closure is discussed in the next section.

Concepts

In order to encapsulate the results of the extraction, a class of Concept objects is introduced. A Concept object has a number of attributes, in particular a prefLabel and extension, which make it easier to inspect the output of the extraction. In addition, the extension can be further processed: in the case of the 'border' relation, we check that the relation is symmetric, and in the case of the 'contain' relation, we carry out the transitive closure. The closure properties associated with a concept is indicated in the relation metadata, as indicated earlier.

The extension of a Concept object is then incorporated into a Valuation object.

Persistence

The functions val_dump and val_load are provided to allow a valuation to be stored in a persistent database and re-loaded, rather than having to be re-computed each time.

Individuals and Lexical Items

As well as deriving relations from the Chat-80 data, we also create a set of individual constants, one for each entity in the domain. The individual constants are string-identical to the entities. For example, given a data item such as 'zloty', we add to the valuation a pair ('zloty', 'zloty'). In order to parse English sentences that refer to these entities, we also create a lexical item such as the following for each individual constant:

PropN[num=sg, sem=<\P.(P zloty)>] -> 'Zloty'

The set of rules is written to the file chat_pnames.cfg in the current directory.