Prolog.rs

This is a minimal Prolog interpreter implemented in Rust. The implementation covers only a subset of Prolog features, is not intended to be fast, or optimal in any sense. It is a learning project that helped me to understand Prolog better.

The implementation is tested using 350+ unit tests, including running some code solutions for the "99 Prolog problems", and unit tests checking for ISO Prolog consistency. There are some differences from other Prolog implementations though, as described below.

If you are looking for a mature Prolog implementation in Rust, check rather the Scryer Prolog.

Usage

To run the Prolog interpreter you can use the Justfile commands:

$ just test      # runs tests
$ just build     # builds the standalone binary ./prolog
$ just run FILE  # evaluate the FILE and start REPL
$ just repl      # start REPL

To install it, move the prolog binary together with the lib/ directory to some directory of your choice.

Calling ./prolog -e FILE from the command line would evaluate the FILE, run the main/0 goal (if available), and exit.

Data types

The units of data in Prolog are called terms.

• Atoms are name-only datatypes, for example, foo. Their names need to start with a lowercase letter.
• Integers are the only supported numerical type.
• Variables have no specific value, but they can be initialized with a value during unification (see below). Their names need to start with uppercase letters or _.
• There exist also compound terms:
  • Structures like foo(a, b) have name foo and arguments (c and b).
  • Lists like [1, 2, 3] can contain multiple terms.

In Prolog everything is a struct, so atom foo is the same as foo(), operation like 2 + 2 is +(2, 2), the "and" operator in a , b is ,(a, b) (don't be confused with the comma separating the arguments), etc.

Lists in Prolog are also structs, so [1, 2, 3] is represented as .(1, .(2, .(3, []))), where [] stands for an empty list. Prolog allows you to create dotted pairs (in lisp terms), for example [1 | 2] is represented as .(1, 2).

There are no booleans. Terms are evaluated by unification. The term fail always fails the unification.

?- 1=1.
yes
?- \+ 1=1.
no
?- fail.
no
?- \+ fail.
yes

Facts, rules, and questions

Prolog programs are defined in terms of three kinds of expressions:

• Facts, like foo. or bar(a,b,32). state what is "true".
• Rules, like mortal(Who) :- person(Who). define logical implications.
• Questions, like ?- mortal(socrates). validate if the question is true.

A very simple Prolog program may solve the classical logical question:

% fact
person(socrates).

% rule
mortal(Who) :-
    person(Who).

% question
?- mortal(socrates).

Unification

Prolog extensively uses pattern matching. When you ask a question, it searches its database if any of the recorded facts and rules that match the goals in the question. This is nicely explained in the Adventure in Prolog book by Dennis Merritt:

Prolog's pattern matching is called unification. In the case where the logicbase contains only facts, unification succeeds if the following three conditions hold.

• The predicate named in the goal and logicbase are the same.
• Both predicates have the same arity.
• All of the arguments are the same.

When variable is unified with the value, the variable becomes equivalent to the value. If two free variables are unified, they become each other's aliases.

There are also procedures using special evaluation rules instead of unification, for example:

?- X is 1+2.
X = 3
?- 1+2 < 5.
yes
?- writeln('hello, world!').
hello, world!
yes

Features

By design, this interpreter covers only a subset of Prolog's features. Those include:

• fail is a goal that always fails.

• a , b means that we want to satisfy both a and b, while a ; b means a or b.

• \+ can be used to negate a goal.

• ! is the cut operator. It prevents backtracking for the goals preceding it.

• -> is the if-else operator, Cond -> Then tries to satisfy Cond, if it succeeds, then attempts to satisfy Then, otherwise it fails. Underneath, it is a syntactic sugar for expressing Cond, !, Then.

• = is a unification operator, it is equivalent to =(A, A).

• _ is a wildcard variable that unifies with anything but never holds any value.

• is operator, as in X is 2 + 2, evaluates the right-hand-side and if left-hand-side is a free variable, assigns the result to it, otherwise compares the result to it's value (so 4 is 2 + 2 evaluates to "yes").

• The supported mathematical operators and functions are:

  • unary operators +, -,
  • binary operators +, -, *, /, // (last two are synonyms), and rem,
  • div and mod (using i32::div_euclid and i32::rem_euclid),
  • abs and sign functions.

  Those operators can be used together with procedures with special evaluation rules like is, =:=, <, etc. Outside of those procedures, they will create structs, for example 2 + 3 would become +(2,3).

• =:=, < are the numerical comparison operators that evaluate both sides and compare them, e.g. 2 + 1 < 7 - 2. The operators =\=, =<, >, >= are available through lib/stdlib.pl.

• ==, @< are comparison operators checking the standard order of terms (see below). The operators \==, @=<, @>, @>= are available through lib/stdlib.pl.

• consult('path/to/file.pl') loads and evaluates the file. If the file contains a question (?-) which cannot be satisfied, it will fail with an error. It takes as an argument an atom with path to the file, or a list of such atoms.

• write(X) prints X and nl prints a newline.

• The trace and untrace commands can be used to turn the tracing logging on and off.

• {a, b, c} is a syntactic sugar for writing {}(,(a, ,(b, c))). It has no special meaning.

• atom(X), integer(X), number(X) are type checkers. var(X) checks if X is a free variable.

More functionalities are implemented in the standard library available through lib/stdlib.pl.

Limitations and differences from other implementations

• Only a subset of Prolog's functionalities are implemented. Features such as strings or floats types, DCG's, defining custom operators, etc are not available.
• The precedence of ; and , operators is reversed, so a , b ; c, d is parsed the same as a , (b ; c) , d. Use brackets to assure the correct precedence.
• In quoted atom names only a subset of escape characters are allowed, including: \n, \t, \s, \\, \'. \", or \NEWLINE.
• Under the standard order of terms variables should be sorted by their memory addresses. Since in this implementation variables don't get memory addresses until they are initialized, such ordering is not possible.
• In Prolog ,/2 is an operator such that a , b tries to satisfy a and b. In this implementation instead of using linked lists, structs are based on Rust Vec's of any size, so a , b , c would become ,(a, b, c) rather than ,(a, ,(b, c)).
• Arithmetic div and mod use Rust's i32::div_euclid and i32::rem_euclid which are defined differently to Prolog, but meet the requirement of being consistent with each other.
• Since _ does not bind, the query like ?- L = [_, _], L = [1, _], L = [2, _] would give a logically inconsistent answer "yes". The same query would work correctly in SWI Prolog, but not in Tau prolog (e.g. prolog.run).
• The implementation is not tail-call optimized, so can easily overflow when satisfying complex goals.
• Tracing is simplified and limited as compared to the other implementations.