scryer-prolog/README.md

rusty-wam

An implementation of the Warren Abstract Machine in Rust, done according to the progression of languages in Warren's Abstract Machine: A Tutorial Reconstruction, ending in pure Prolog.

Progress

Prolog is implemented as a simple REPL. It is without without meta- or extra-logical operators, or side effects of any kind, with the lone exception of cut. In terms of the tutorial pacing, the work covers in some form all of the WAM book, including lists, cuts, Debray allocation, and indexing.

Tutorial

To enter a multi-clause predicate, the brackets ":{" and "}:" are used as delimiters. They must be contained entirely within their own lines.

For example,

prolog> :{
p(f(f(X)), h(W), Y) :- g(W), h(W), f(X).
p(X, Y, Z) :- h(Y), z(Z).
}:
prolog> :{
h(x).
h(y).
h(z).
}:

Single clause predicates can be entered without brackets, as in

prolog> p(X) :- q(X).
prolog> f(s).
prolog> z(Z).

Queries are issued as

prolog> ?- p(X, Y, Z).

Given the above work, the result of the query will be

prolog> ?- p(X, Y, Z).
yes
X = _0
Y = x
Z = _2
Press ; to continue or . to abort.

Pressing ; will backtrack through other possible answers, if any exist. Pressing . will abort the search and return to the prompt.

Wildcards work as well:

prolog> :{
member(X, [X|_]).
member(X, [_|Xs]) :- member(X, Xs).
}:
prolog> ?- member(X, [a, b, c]).
yes
X = a
Press ; to continue or . to abort.
;
X = b
Press ; to continue or . to abort.
;
X = c
Press ; to continue or . to abort.
;
no

Note that the values of variables belonging to successful queries are printed out, on one line each. Uninstantiated variables are denoted by a number preceded by an underscore (X = _0 is an example in the above).

Occurs check

There's no occurs check, but there soon will be. Currently, attempting unification on a cyclic term succeeds, and the attempt to write the term to a string results in an infinite loop, ie.

prolog> p(W, W).
prolog> ?- p(f(f(W)), W).
yes
*loops to infinity*