# 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](http://wambook.sourceforge.net/wambook.pdf).

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.
* 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](http://shenlanguage.org) implementation.

## 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 ; 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> ?- member([X,X],[a,b,c,[d,d],[e,d]]), member(X, [a,b,c,d,e,f,g]), member(Y, [X, a, b, c, d]).
true
Y = d
X = d ;
Y = a
X = d ;
Y = b
X = d ;
Y = c
X = d ;
Y = d
X = d ;
false.
prolog>
```

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).