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

Pure Prolog is implemented as a simple REPL. "Pure Prolog" is Prolog without cut, meta- or extra-logical operators, or side effects of any kind. In terms of the tutorial pacing, the work has progressed to the to the end of section 5.7, skipping past 5.4. Atoms and lists are the only two data types currently supported.

While proper environment trimming code is emitted by the code generator, it has no effect on the bytecode WAM, which lacks fine-grained control over the alignment and allocation of stack frames.

Tutorial

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