Prolog can be a key technology for the Digital Humanities: It makes the processing of knowledge accessible to the researcher by providing a smooth transition from data representation to processing, querying, data manipulation, data-centered programming and logic-based knowledge processing. For the KBSET environment, Prolog, and in particular SWI-Prolog, is used in many roles:
This is discussed in more detail in Section 4 of KBSET – Knowledge-Based Support for Scholarly Editing and Text Processing with Declarative LaTeX Markup and a Core Written in SWI-Prolog, Jana Kittelmann and Christoph Wernhard. In: Declarative Programming and Knowledge Management (DECLARE 2019), Revised Selected Papers LNCS (LNAI), 2020, to appear.
A scholarly edition project involves, more or less explicitly, the creation of data. In accord with identifying Prolog as a potential key technology for the Digital Humanities, KBSET provides support for making these data accessible primarily as Prolog fact bases that can be accessed via Web URLs. This page gives introductory examples for using such documents.
Tau Prolog is a small Prolog system that runs directly in the Web browser and is currently under active development. It permits to load documents with Prolog programs (with fact bases a special case) in a Semantic Web fashion by “consulting” (as loading is called in Prolog) them via an HTTP URL. Here is an example session.
Open the Tau Prolog Sandbox. Paste the following tiny program into the program editor on the left side:
init :- consult('http://cs.christophwernhard.com/kbset/downloads/kbset/examples/letters_mini/outputs/metadata_export.pl').
Press Consult program (or Reconsult program, respectively).
The following query when entered in the goal field on the
right (with Text “Type a Prolog goal in here and press
ENTER”) now effects that the example fact
is loaded from the KBSET site directly into the Tau Prolog sandbox in
your browser. (Note: Queries are displayed here always with the Prolog
?- ”, but should be inserted without
the prompt into Tau Prolog's goal field.)
You can now evaluate queries against the loaded fact base by entering them into the query field. Pressing Enter repeatedly effects that alternative solutions are enumerated. Here are some examples (a brief explanation of Prolog's syntax is below).
Enumerating all instances of the letter relation:
?- letter(L, A, V).
Enumerating all attributes and values a specific letter:
?- letter('b:sv:1767-03-27', A, V).
Enumerating all letters whose date is in a given year:
?- letter(L, date_year, 1767).
Enumerating all letters whose date is in a given year along with the ID of their author:
?- letter(L, date_year, 1767), letter(L, from, P).
Enumerating all letters whose date is in a given year along with the ID and name string of their author:
?- letter(L, date_year, 1767), letter(L, from, P), defperson(P, N).
For a more complex query, it can be useful to define a dedicated predicate:
wrote_letter_in_year(Y, N) :- letter(L, date_year, Y), letter(L, from, P), defperson(P, N).
The predicate definition can be entered into the program editor on the left side and is activated by pressing Reconsult program. The newly defined predicate can now be used in queries. For example, to enumerate all its instances:
?- wrote_letter_in_year(Y, N).
Or to enumerate name strings of persons who wrote a letter in a given year:
?- wrote_letter_in_year(1767, N).
Prolog enumerates solutions in a certain order corresponding to its
underlying evaluation regime, possibly returning the same answer
multiple times. This may or may not be sufficient for applications.
Standard predicates that allow to write advanced queries and to
control the handling of multiple solutions include the aggregation
setof/3 as well as the sorting
Symbols starting with a capital letter are considered
as logical variables by Prolog. Examples
Non-variable terms include atoms and numbers. An atom
can contain arbitrary characters, but, if the first character is not a
lowercase letter or if it contains a character that is not
alphanumeric (with exception of the underscore “_”), then
it needs to be placed in single quotes. Examples are
Solutions of a Prolog query are represented by variable bindings, that is, variable substitutions under which the query follows from the program, which, in our case, is the imported fact base. The Prolog interpreter enumerates these substitutions and prints them, one by one, as long as the user asks for more solutions.
consultwith an HTTP URL as in the example is implemented in Tau Prolog with an
XMLHttpRequestobject. This is subject to the Cross-Origin Resource Sharing (CORS) mechanism, which requires that the Web server that provides the Prolog document is configured to deliver it together with the response
library(http/http_load). In the sandboxed mode of SWISH, however, this form of consulting is not possible.
Prolog, as a general purpose programming language, associates a specific top-down processing model with logic formulas. Other approaches to logic programming proceed in a bottom-up fashion. Programming is understood there rather in the sense of linear programming. Many of these systems, including model computation systems, answer set programming systems and deductive database systems can process Prolog fact bases. Here are some examples: