107 lines
2.1 KiB
Markdown
107 lines
2.1 KiB
Markdown
# rusty-wam
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An implementation of the Warren Abstract Machine in Rust, done
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according to the progression of languages in [Warren's Abstract
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Machine: A Tutorial
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Reconstruction](http://wambook.sourceforge.net/wambook.pdf), ending in
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pure Prolog.
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## Progress
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Prolog is implemented as a simple REPL. It is without meta- or
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extra-logical operators, or side effects of any kind, with the lone
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exception of cut. In terms of the tutorial pacing, the work covers in
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some form all of the WAM book, including lists, cuts, Debray
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allocation, indexing, and conjunctive queries.
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## Tutorial
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To enter a multi-clause predicate, the brackets ":{" and "}:" are used
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as delimiters. They must be contained entirely within their own lines.
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For example,
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```
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prolog> :{
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p(f(f(X)), h(W), Y) :- g(W), h(W), f(X).
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p(X, Y, Z) :- h(Y), z(Z).
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}:
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prolog> :{
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h(x).
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h(y).
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h(z).
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}:
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```
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Single clause predicates can be entered without brackets, as in
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```
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prolog> p(X) :- q(X).
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prolog> f(s).
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prolog> z(Z).
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```
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Queries are issued as
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```
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prolog> ?- p(X, Y, Z).
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```
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Given the above work, the result of the query will be
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```
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prolog> ?- p(X, Y, Z).
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true
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Y = x
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X = _0
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Z = _2
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```
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Pressing ; will backtrack through other possible answers, if any exist.
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Pressing . will abort the search and return to the prompt.
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Wildcards work as well:
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```
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prolog> :{
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member(X, [X|_]).
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member(X, [_|Xs]) :- member(X, Xs).
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}:
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prolog> ?- member(X, [a, b, c]).
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true
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X = a ;
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X = b ;
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X = c ;
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false.
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```
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and so do conjunctive queries:
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```
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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]).
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true
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Y = d
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X = d ;
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Y = a
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X = d ;
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Y = b
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X = d ;
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Y = c
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X = d ;
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Y = d
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X = d ;
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false.
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prolog>
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```
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Note that the values of variables belonging to successful queries are
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printed out, on one line each. Uninstantiated variables are denoted by
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a number preceded by an underscore (`X = _0` is an example in the
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above).
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## Occurs check
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There's no occurs check, but there soon will be. Currently, attempting
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unification on a cyclic term succeeds, and the attempt to write the
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term to a string results in an infinite loop, ie.
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```
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prolog> p(W, W).
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prolog> ?- p(f(f(W)), W).
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true
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*loops to infinity*
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``` |