#384 About Prolog
A quick introduction to Prolog and a test drive with GNU Prolog on macOS.
Notes
Prolog is a logic programming language that has was created around 1972 by Alain Colmerauer with Philippe Roussel, from the Artificial Intelligence Group of the Faculty of Sciences of Luminy of Aix-Marseille II University of France.
It perhaps garnered some fresh attentino when it earned a chapter in Bruce Tate’s Seven Languages in Seven Weeks.
Prolog In a Nutshell
Prolog is…
- one of the first logic programming languages
- a declarative programming language
Prolog has..
- a single data type: the term.
- Terms are either atoms, numbers, variables or compound terms (lists, strings)
- clauses: Prolog programs describe relations, defined by means of clauses
- Pure Prolog is restricted to Horn clauses
- Two types of Horn clauses are used to define Prolog programs: rules and facts
- A rule is of the form:
Head :- Body.- Negation can be used in the rule
- A fact is a clauses with an empty body.
- Predicates and programs
- A predicate (or procedure definition) is a collection of clauses whose heads have the same name and arity.
- A logic program is a set of predicates
- Loops and recursion
- Execution
- Execution of a Prolog program is initiated by the user’s posting of a single goal, called the query.
Prolog applications..
- mainly:
- theorem proving
- expert systems
- term rewriting
- type systems
- automated planning
- question answering
- natural language processing (its original intended use)
- most applications are small by industrial standards
- widely used in research and education, though more common in Europe (Americans favor Lisp).
- however, Prolog and other logic programming languages have not had a significant impact on the computer industry in general.
Prolog is governed by..
- The International Organization for Standardization (ISO) Prolog technical standard
Prolog implementations include..
- Amzi! Prolog - now open source as the maintainer is retiring
- B-Prolog - commercial, no longer maintained
- Ciao
- https://ciao-lang.org/
- Ciao is a modern Prolog implementation that builds up from a logic-based simple kernel designed to be portable, extensible, and modular.
- Ciao can install natively in your operating system or run in the browser without installation.
- ECLiPSe
- https://sourceforge.net/projects/eclipse-clp/
- open source, still maintained
- GNU Prolog
- Poplog
- now open sourced https://getpoplog.github.io/
- P#
- P# compiles a linear logic extension of Prolog to C# source code
- Quintus Prolog
- proprietary
- final release in 2003 but still maintained by SICS
- Scryer Prolog
- SICStus
- state-of-the-art, ISO standard compliant, Prolog
- commercial
- appears to supersede Quintus Prolog also maintained by SICS
- Strawberry - Windows, free “light” version
- SWI-Prolog - free implementation, commonly used for teaching and semantic web applications.
- Tau Prolog - An open source Prolog interpreter in JavaScript
- tuProlog
- WIN-PROLOG 8.1
- XSB
- YAP
Test drive: GNU Prolog on macOS
As I’m on macOS right now, I’ll use the Homebrew Formulae for gnu-prolog to install:
brew install gnu-prolog
Let’s test it with an example from Seven Languages in Seven Weeks. See friend.pl:
likes(wallace, cheese).
likes(grommit, cheese).
likes(wendolene, sheep).
friend(X, Y) :- \+(X = Y), likes(X, Z), likes(Y, Z).
We have three facts about who likes cheese.
The friend rule with three clauses:
\+(X = Y)means you can’t friend yourself (\+indicates negation)likes(X, Z), likes(Y, Z)means X and Y both have to share a like for the same thing
Testing it out, querying for likes and friend conditions:
$ gprolog --consult-file friend.pl
GNU Prolog 1.5.0 (64 bits)
Compiled Jul 8 2021, 09:35:47 with clang
Copyright (C) 1999-2024 Daniel Diaz
compiling ./friend.pl for byte code...
./friend.pl compiled, 5 lines read - 1029 bytes written, 6 ms
| ?- likes(wallace, cheese).
yes
| ?- likes(grommit, cheese).
yes
| ?- likes(wendolene, cheese).
no
| ?- friend(wallace, grommit).
yes
| ?- friend(grommit, wallace).
yes
| ?- friend(wallace, wendolene).
no
| ?- likes(wallace, wallace).
no