#haskell #logic #prolog #testbed

app scryer-prolog

A modern Prolog implementation written mostly in Rust

114 releases

✓ Uses Rust 2018 edition

0.8.117 Feb 4, 2020
0.8.116 Nov 16, 2019
0.8.115 Oct 20, 2019
0.8.96 Sep 29, 2019
0.8.33 Mar 31, 2019
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25K SLoC

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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:

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

Note on compatibility: Scryer Prolog should work on Linux, Mac OS X, and BSD variants on which Rust runs. Windows support hinges on rustyline and Termion being functional in that environment, which to my knowledge is not presently the case.

Built-in predicates

The following predicates are built-in to Scryer.

  • Arithmetic support:
    • is/2 works for (+)/{1,2}, (-)/{1,2}, (*)/2, (//)/2, (**)/2, (^)/2, (div)/2, (/)/2, (rdiv)/2, (xor)/2, (rem)/2, (mod)/2, (/\)/2, (\/)/2, (>>)/2,(<<)/2, (\)/1, abs/1, sin/1, cos/1, tan/1, asin/1, acos/1, atan/1, atan2/2, log/1, exp/1, sqrt/1, float/1, truncate/1, round/1, floor/1, ceiling/1, pi/0, min/1, max/1, gcd/2, sign/1
    • Comparison operators: >, <, =<, >=, =:=, =\=.
  • (:)/2
  • (@>)/2
  • (@>=)/2
  • (@=<)/2
  • (@<)/2
  • (\+)/1
  • (==)/2
  • (\==)/2
  • (=)/2
  • (\=)/2
  • (=..)/2
  • (->)/2
  • (;)/2
  • abolish/1
  • acyclic_term/2
  • append/3
  • arg/3
  • asserta/1
  • assertz/1
  • atom/1
  • atomic/1
  • atom_chars/2
  • atom_codes/2
  • atom_concat/3
  • atom_length/2
  • bagof/3
  • bb_b_put/2
  • bb_get/2
  • bb_put/2
  • between/3
  • call/1..62
  • call_cleanup/2
  • call_with_inference_limit/3
  • call_residue_vars/2
  • can_be/2
  • catch/3
  • clause/2
  • compare/3
  • compound/1
  • copy_term/{2,3}
  • current_predicate/1
  • current_op/3
  • cyclic_term/1
  • dif/2
  • expand_goal/2
  • expand_term/2
  • fail/0
  • false/0
  • findall/{3,4}
  • float/1
  • foldl/{4,5}
  • forall/2
  • freeze/2
  • functor/3
  • gen_int/1
  • gen_nat/1
  • get_char/1
  • goal_expansion/2
  • ground/1
  • halt/0
  • integer/1
  • is_list/1
  • is_partial_string/1
  • keysort/2
  • length/2
  • maplist/2..9
  • member/2
  • memberchk/2
  • must_be/2
  • nl/0
  • nonvar/1
  • number_chars/2
  • number_codes/2
  • numbervars/2
  • numlist/{2,3}
  • once/1
  • op/3
  • partial_string/2
  • phrase/{2,3}
  • rational/1
  • read/1
  • repeat/{0,1}
  • retract/1
  • reverse/2
  • same_length/2
  • select/3
  • setof/3
  • setup_call_cleanup/3
  • sort/2
  • string/1
  • sub_atom/5
  • subsumes_term/2
  • sumlist/2
  • term_expansion/2
  • term_variables/2
  • throw/1
  • true/0
  • unify_with_occurs_check/2
  • use_module/{1,2}
  • user:goal_expansion/2
  • user:term_expansion/2
  • var/1
  • variant/2
  • wam_instructions/2
  • write/1
  • write_canonical/1
  • writeq/1
  • write_term/2


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).

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]).
true .
X = a ;
X = b ;
X = c ;

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.
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).

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).
-(+(-(5), 3), //(*(2, 4), 8))

New operators can be defined using the op declaration.

Partial strings

Scryer has two specialized, non-ISO builtin predicates for handling so-called "partial strings". Partial strings imitate difference lists of characters, but are much more space efficient. This efficiency comes at the cost of full generality -- you cannot unify the tail variables of two distinct partial strings, because their buffers will always be distinct.

If X is a free variable, the query

?- partial_string("abc", X), X = [a, b, c | Y], is_partial_string(X), is_partial_string(Y).

will succeed. Further, if Y a free variable, unifying Y against another string, "def" in this case, produces the equations

X = [a, b, c, d, e, f], Y = [d, e, f].


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, several control and list processing operators and predicates are hidden in their own modules that have not been exported to the toplevel. To export them, 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, [local_member/2]).
:- use_module(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.


~119K SLoC