Attolog

Minimal Prolog interpreter.

Description

This is mostly a learning project to know more about F# and Prolog.

• The implementation is rather basic. It does not contain cuts, arithmetic, lists, equality and so on — only basic Horn clauses.
• Besides the interpreter, there's a simple hand-rolled parser combinator library. Zero dependencies, yay!

Prerequisites

You pretty much need only .NET SDK 8.0.

CLI

Available options:

• -f, --file A file to load before starting a REPL.

REPL

Simply run the project:

$ dotnet run --project Attolog

With arguments:

$ dotnet run --project Attolog -- -f examples/sw.pl

rlwrap

REPL can also be used with rlwrap:

$ rlwrap -a -N -t dumb dotnet run --project Attolog

With arguments:

$ rlwrap -a -N -t dumb dotnet run --project Attolog -- -f examples/sw.pl

just

If you have just installed, you can use the following recipes:

$ just --list

Available recipes:
    default
    rl *args=''  # Run Attolog with rlwrap for a slightly better experience in REPL.
    run *args='' # Run Attolog with the default REPL.

Arguments can be passed as well:

$ just run -f examples/sw.pl

Language

The language currently consists of:

• Constants: x, y, constant
• Variables: X, Y, Variable
• Clauses: son(abel, adam), less(X, plus(X, one))
• Assertions: son(X, Y) :- male(X), child(X, Y).
• Queries: ?- child(X, vader).

Sample program:

% Persons.
female(leia).
male(vader).
male(luke).
male(kylo).

% Relations.
child(luke, vader).
child(leia, vader).
child(kylo, leia).

% Assertions.
son(X, Y) :- male(X), child(X, Y).
daughter(X, Y) :- female(X), child(X, Y).
grandchild(X, Z) :- child(X, Y), child(Y, Z).

% Queries (goals) can't be part of a saved program.
% ?- son(X, vader).
% ?- daughter(X, vader).
% ?- grandchild(X, vader).

Peano numbers (using terms to represent unary numbers, e.g. 0, s(0), s(s(0)), s(s(s(0))), etc.):

add(0, N, N).
add(s(N), M, s(R)) :- add(N, M, R).

% ?- add(s(0), s(0), X).
% X = s(s(0))

% ?- add(s(0), X, s(s(0))).
% X = s(0)

TODO

• Get rid of exceptions, embrace errors as values.
• Refactor solver to produce values instead of effects (direct printing, reading input, etc).

License

MIT.