209 lines
5.2 KiB
Markdown
209 lines
5.2 KiB
Markdown
# rusty-wam
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rusty-wam aims to become to Prolog what GHC is to Haskell: an open
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source industrial strength production environment that is also a
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testbed for bleeding edge research in logic and constraint
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programming, which is itself written in a high-level language.
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## Phase 1
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Produce 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).
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Phase 1 has been completed in that rusty-wam implements in some form
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all of the WAM book, including lists, cuts, Debray allocation, first
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argument indexing, last call optimization and conjunctive queries.
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## Phase 2
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Extend rusty-wam to include the following, among other features:
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* call/N as a built-in meta-predicate (_done_).
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* ISO Prolog compliant throw/catch (_done_).
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* Built-in and user-defined operators of all fixities, with custom
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associativity and precedence (_done_).
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* Bignum, rational number and floating point arithmetic (_done_).
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* Built-in control operators (`,`, `;`, `->`, etc.) (_done_).
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* Built-in predicates for list processing and top-level declarative
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control (`setup_call_control/3`, `call_with_inference_limit/3`,
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etc.) (_done_).
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* Add a rudimentary module system.
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* Attributed variables using the SICStus Prolog interface and
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semantics. Adding coroutines like `dif/2`, `freeze/2`, etc.
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is straightforward with attributed variables.
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* An occurs check.
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* Mode declarations.
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* Extensions for clp(FD).
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* `if_` and related predicates, following the developments of the
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paper "Indexing `dif/2`".
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* Strings, blobs, and other data types.
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## Phase 3
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Use the WAM code produced by the completed code generator to get
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JIT-compiled and -executed Prolog programs. The question of how to get
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assembly from WAM code is something I'm still considering.
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It's my hope to use rusty-wam as the logic engine of a low level (and
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ideally, very fast) [Shen](http://shenlanguage.org) implementation.
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## Nice to have features
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There are no current plans to implement any of these, but they might be
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nice to have in the future. They'd make a good project for anyone wanting
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to contribute code to rusty-wam.
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1. Implement the global analysis techniques described in Peter van
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Roy's thesis, "Can Logic Programming Execute as Fast as Imperative
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Programming?"
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2. Add unum representation and arithmetic, using either an existing
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unum implementation or an ad hoc one. Unums are described in
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Gustafson's book "The End of Error."
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3. Add support for shift/reset delimited continuations, see "Delimited
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Continuations for Prolog."
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4. Add an incremental compacting garbage collector for the heap.
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5. Add concurrent tables to manage shared references to atoms and
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strings.
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6. Add optional SLG resolution for fast memoization of predicates.
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7. Add some form of JIT predicate indexing.
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## Built-in predicates
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The following predicates are built-in to rusty-wam.
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* Arithmetic support:
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* `is/2` works for `(+)/2`, `(-)/{1,2}`, `(*)/2`, `(//)/2`, `(div)/2`, `(/)/2`, `(rdiv)/2`,
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`(xor)/2`, `(rem)/2`, `(mod)/2`, `(/\)/2`, `(\/)/2`, `(>>)/2`, `(<<)/2`.
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* Comparison operators: `>`, `<`, `=<`, `>=`, `=:=`, `=\=`.
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* `(@>)/2`
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* `(@>=)/2`
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* `(@=<)/2`
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* `(@<)/2`
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* `(=@=)/2`
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* `(\=@=)/2`
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* `(\+)/1`
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* `(==)/2`
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* `(\==)/2`
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* `(=)/2`
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* `(\=)/2`
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* `(=..)/2`
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* `(->)/2`
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* `(;)/2`
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* `append/3`
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* `arg/3`
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* `atomic/1`
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* `between/3`
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* `call/1..63`
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* `call_with_inference_limit/3`
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* `catch/3`
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* `compare/3`
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* `compound/1`
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* `display/1`
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* `duplicate_term/2`
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* `false/0`
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* `float/1`
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* `functor/3`
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* `ground/1`
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* `integer/1`
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* `length/2`
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* `maplist/1..8`
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* `member/2`
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* `memberchk/2`
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* `nonvar/1`
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* `once/1`
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* `rational/1`
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* `reverse/2`
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* `select/3`
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* `setup_call_cleanup/3`
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* `string/1`
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* `throw/1`
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* `true/0`
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* `var/1`
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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 `SPACE` 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> f(X) :- g(X).
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prolog> g(x). g(y). g(z).
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prolog> h(call(f, X)).
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prolog> ?- h(X), X.
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true .
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X = call(f, x) ;
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X = call(f, y) ;
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X = call(f, z).
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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` in an example above).
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Lastly, rusty-wam supports dynamic operators. Using the built-in
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arithmetic operators with the usual precedences,
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```
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prolog> ?- display(-5 + 3 - (2 * 4) // 8).
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'-'('+'('-'(5), 3), '//'('*'(2, 4), 8))
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true.
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``` |