Scryer Prolog

Scryer Prolog aims to become to ISO Prolog what GHC is to Haskell: an open source industrial strength production environment that is also a testbed for bleeding edge research in logic and constraint programming, which is itself written in a high-level language.

Phase 1

Produce 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 Scryer Prolog implements in some form all of the WAM book, including lists, cuts, Debray allocation, first argument indexing, last call optimization and conjunctive queries.

Phase 2

Extend Scryer Prolog to include the following, among other features:

• [x] call/N as a built-in meta-predicate.
• [x] ISO Prolog compliant throw/catch.
• [x] Built-in and user-defined operators of all fixities, with custom associativity and precedence.
• [x] Bignum, rational number and floating point arithmetic.
• [x] Built-in control operators (,, ;, ->, etc.).
• [x] A revised, not-terrible module system.
• [x] Built-in predicates for list processing and top-level declarative control (setup_call_cleanup/3, call_with_inference_limit/3, etc.)
• [x] ~~Default representation of strings as lists of characters, using a packed internal representation.~~
• [x] term_expansion/2 and goal_expansion/2.
• [x] Definite Clause Grammars.
• [x] Attributed variables using the SICStus Prolog interface and semantics. Adding coroutines like dif/2, freeze/2, etc. is straightforward with attributed variables.
  • [x] Support for verify_attributes/3
  • [x] Support for attribute_goals/2 and project_attributes/2
  • [x] call_residue_vars/2
• [x] if_ and related predicates, following the developments of the paper "Indexing dif/2".
• [x] All-solutions predicates (findall/{3,4}, bagof/3, setof/3, forall/2).
• [x] Clause creation and destruction (asserta/1, assertz/1, retract/1, abolish/1) with logical update semantics.
• [x] Backtrackable and non-backtrackable global variables via bb_get/2 bb_put/2 (non-backtrackable) and bb_b_put/2 (backtrackable).
• [x] Delimited continuations based on reset/3, shift/1 (documented in "Delimited Continuations for Prolog").
• [x] Tabling library based on delimited continuations (documented in "Tabling as a Library with Delimited Control").
• [x] A redone representation of strings as difference lists of characters, using a packed internal representation.
• [x] clp(B) and clp(ℤ) as builtin libraries.
• [ ] Streams and predicates for stream control (in progress).
• [ ] A compacting garbage collector satisfying the five properties of "Precise Garbage Collection in Prolog."
• [ ] Mode declarations.

Phase 3

Use the WAM code produced by the completed code generator to get JIT-compiled and -executed Prolog programs. The question of how to get assembly from WAM code is something I'm still considering.

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

Nice to have features

There are no current plans to implement any of these, but they might be nice to have in the future. They'd make a good project for anyone wanting to contribute code to Scryer Prolog.

1. Implement the global analysis techniques described in Peter van Roy's thesis, "Can Logic Programming Execute as Fast as Imperative Programming?"

2. Add unum representation and arithmetic, using either an existing unum implementation or an ad hoc one. Unums are described in Gustafson's book "The End of Error."

3. Add concurrent tables to manage shared references to atoms and strings.

4. Add some form of JIT predicate indexing.

Installing Scryer Prolog

First, install the latest stable version of Rust using your preferred method. Scryer tends to use features from newer Rust releases, whereas Rust packages in Linux distributions, Macports, etc. tend to lag behind. rustup will keep your Rust updated to the latest stable release; any existing Rust distribution should be uninstalled from your system before rustup is used.

Scryer Prolog can be installed with cargo, like so:

$> cargo install scryer-prolog

cargo will download and install the libraries Scryer Prolog uses automatically from crates.io. You can find the scryer-prolog executable in ~/.cargo/bin.

Publishing Rust crates to crates.io and pushing to git are entirely distinct, independent processes, so to be sure you have the latest commit, it is recommended to clone directly from this git repository, which can be done as follows:

$> git clone https://github.com/mthom/scryer-prolog $> cd scryer-prolog $> cargo run [--release]

The optional --release flag will perform various optimizations, producing a faster executable.

Tutorial

To enter a multi-clause predicate, the directive "[user]" is used.

For example, ?- [user]. (type Enter + Ctrl-D to terminate the stream when finished) p(f(f(X)), h(W), Y) :- g(W), h(W), f(X). p(X, Y, Z) :- h(Y), z(Z). ?- [user]. (type Enter + Ctrl-D to terminate the stream when finished) h(x). h(y). h(z). In the example, Enter + Ctrl-D is used to terminate the standard input stream. The instructive message is always printed.

Queries are issued as ?- p(X, Y, Z).

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:

?- [user]. (type Enter + Ctrl-D to terminate the stream when finished) member(X, [X|_]). member(X, [_|Xs]) :- member(X, Xs). ?- member(X, [a, b, c]). X = a ; X = b ; X = c ; false. and so do conjunctive queries: ?- [user]. (type Enter + Ctrl-D to terminate the stream when finished) f(X) :- g(X). g(x). g(y). g(z). h(call(f, X)). ?- h(X), X. 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).

To quit scryer-prolog, type ?- halt.

Dynamic operators

Scryer supports dynamic operators. Using the built-in arithmetic operators with the usual precedences,

?- write_canonical(-5 + 3 - (2 * 4) // 8), nl. -(+(-5,3),//(*(2,4),8)) true.

New operators can be defined using the op declaration.

Strings and partial strings

In Scryer Prolog, the default value of the Prolog flag double_quotes is chars, which is also the recommended setting. This means that double-quoted strings are interpreted as lists of characters, in the tradition of Marseille Prolog.

For example, the following query succeeds:

?- "abc" = [a,b,c]. true.

Internally, strings are represented very compactly in packed UTF-8 encoding. A naive representation of strings as lists of characters would use one memory cell per character, one memory cell per list constructor, and one memory cell for each tail that occurs in the list. Since one memory cell takes 8 bytes on 64-bit machines, the packed representation used by Scryer Prolog yields an up to 24-fold reduction of memory usage, and corresponding reduction of memory accesses when creating and processing strings.

Scryer Prolog uses the same efficient encoding for partial strings, which appear to Prolog code as partial lists of characters. The predicate partial_string/3 from library(iso_ext) lets you construct partial strings explicitly. For example:

?- partial_string("abc", Ls0, Ls). Ls0 = [a,b,c|Ls].

In this case, and as the answer illustrates, Ls0 is indistinguishable from a partial list with tail Ls, while the efficient packed representation is used internally.

An important design goal of Scryer Prolog is to automatically use the efficient string representation whenever possible. Therefore, it is only very rarely necessary to use partial_string/3 explicitly. In the above example, posting Ls0 = [a,b,c|Ls] yields the exact same internal representation, and has the advantage that only the standard predicate (=)/2 is used.

Definite clause grammars as provided by library(dcgs) are ideally suited for reasoning about strings.

Modules

Scryer has a simple predicate-based module system. It provides a way to separate units of code into distinct namespaces, for both predicates and operators. See the files src/prolog/lib/*.pl for examples.

At the time of this writing, many predicates reside in their own modules that need to be imported before they can be used. The modules that ship with Scryer Prolog are also called library modules or libraries, and include:

• lists providing length/2, member/2, select/3, append/[2,3], foldl/[4,5], maplist/[2-9], same_length/2, transpose/2 etc.
• dcgs Definite Clause Grammars (DCGs), a built-in grammar mechanism that uses the operator (-->)/2 to define grammar rules, and the predicates phrase/[2,3] to invoke them.
• dif The predicate dif/2 provides declarative disequality: It is true if and only if its arguments are different, and delays the test until a sound decision can be made.
• reif providing if_/3, tfilter/3 and related predicates as described in Indexing dif/2.
• clpz CLP(ℤ): Constraint Logic Programming over Integers, providing declarative integer arithmetic via (#=)/2, (#\=)/2, (#>=)/2 etc., and various global constraints and enumeration predicates for solving combinatorial tasks.
• pairs By convention, pairs are Prolog terms with principal functor (-)/2, written as Key-Value. This library provides pairs_keys_values/3, pairs_keys/2, and other predicates to reason about pairs.
• si The predicates atom_si/1, integer_si/1, atomic_si/1 and list_si/1 implement sound type checks. They raise instantiation errors if no decision can be made. They are declarative replacements for logically flawed lower-level type tests. For instance, instead of integer(X), write integer_si(X) to ensure soundness of your programs. "si" stands for sufficiently instantiated, and also for sound inference.
• error must_be/2 and can_be/2 complement the type checks provided by library(si), and are especially useful for Prolog library authors.
• tabling The operator (table)/1 is used in directives that prepare predicates for tabled execution (SLG resolution).
• format The nonterminal format_//2 is used to describe formatted output, arranging arguments according to a given format string. The predicate format/2 is provided for impure output.
• assoc providing empty_assoc/1, get_assoc/3, put_assoc/4 etc. to manage elements in AVL trees which ensure O(log(N)) access.
• clpb CLP(B): Constraint Logic Programming over Boolean variables, a BDD-based SAT solver provided via the predicates sat/1, taut/2, labeling/1 etc.

To use predicates provided by the lists library, write:

?- use_module(library(lists)).

To load modules contained in files, the library functor can be omitted, prompting Scryer to search for the file (specified as an atom) from its working directory:

?- use_module('file.pl').

use_module directives can be qualified by adding a list of imports:

?- use_module(library(lists), [member/2]).

A qualified use_module can be used to remove imports from the toplevel by calling it with an empty import list.

The (:)/2 operator resolves calls to predicates that might not be imported to the current working namespace:

?- lists:member(X, Xs).

The [user] prompt can also be used to define modules inline at the REPL:

``` ?- [user]. (type Enter + Ctrl-D to terminate the stream when finished) :- module(test, [localmember/2]). :- usemodule(library(lists)).

local_member(X, Xs) :- member(X, Xs). ```

The user listing can also be terminated by placing end_of_file. at the end of the stream.