rusty-wam

Phase 1

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

Phase 1 has been completed, in that rusty-wam implements in some form all of the WAM book, including lists, cuts, Debray allocation, first argument indexing, and conjunctive queries.

Phase 2

Extend rusty-wam to include the following, among other features:

call/N as a built-in meta-predicate (done).
ISO Prolog compliant throw/catch (done).
Built-in and user-defined operators of all fixities, with custom associativity and precedence.
Bignum and floating point arithmetic.
Built-in control operators (;, ->, etc.).
Attributed variables using the SICStus Prolog interface and semantics. Adding coroutines like dif/2, freeze/2, etc. is straightforward with attributed variables.
An occurs check.
Built-in predicates for list processing and top-level declarative control (setup_call_control/3, call_with_inference_limit/3, etc.)
Mode declarations.
Extensions for clp(FD).
if_ and related predicates, following the developments of the paper "Indexing dif/2".
Strings, blobs, and other data types.

Phase 3

Use the WAM code produced by the completed code generator to target LLVM IR to get JIT-compiled and -executed Prolog programs.

It's my hope to use rusty-wam as the logic engine of a low level (and ideally, very fast) Shen implementation.

Built-in predicates

The following predicates are built-in to rusty-wam.

atomic/1
call/N (1 <= N <= 63)
catch/3
duplicate_term/2
false/0
not/1
throw/1
var/1

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).
true
Y = x
X = _0
Z = _2

Pressing SPACE 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]).      
true
X = a ;
X = b ;
X = c ;
false.

and so do conjunctive queries:

prolog> f(X) :- g(X).
prolog> g(x). g(y). g(z).
prolog> h(call(f, X)).
prolog> ?- h(X), X.
true .
X = call(f, x) ;
X = call(f, y) ;
X = call(f, z).

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 in an example above).

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