json_to_prolog

json_to_prolog converts structured JSON descriptions of facts, queries, and rules into Prolog source code. It is published as both a Rust crate (for native use) and a Wasm package (for npm), so you can validate domain data on the server or in the browser with the same logic.

[!WARNING] The schema is still evolving. Expect breaking changes before 1.0.0.

Features

JSON Schema validation for each claim type (facts, rules, queries) before conversion.
Deterministic Prolog output (fields sorted alphabetically, consistent quoting, and wildcard handling).
Support for logical combinators (and, or, not) when defining rule bodies.
Wasm bindings generated with wasm-bindgen, making the same API available on npm.

Installation

Rust (crates.io)

[dependencies]
json_to_prolog = "0.1.6"

JavaScript/TypeScript (npm)

npm install json_to_prolog
# or
yarn add json_to_prolog

The npm package ships pre-built Wasm bindings in pkg/. You can import the helper functions from json_to_prolog or use the generated JS glue directly.

Quick Start

Converting a fact (Rust)

use json_to_prolog::convert_fact_to_prolog;
use serde_json::json;

let fact = json!({
	"predicate": "person",
	"updateView": "assert",
	"name": "Alice",
	"age": 30,
	"active": true,
	"nickname": null // converted to wildcard `_`
});

let prolog = convert_fact_to_prolog(&fact)?;
assert_eq!(prolog, "assert(person(true, 30, 'Alice', _)).");

JavaScript + SWI-Prolog (Node)

import init, {
  convert_fact_to_prolog_wasm,
  convert_query_to_prolog_wasm,
} from "json_to_prolog";
import SWIPL from "swipl-wasm";

const main = async () => {
  await init();
  const swipl = await SWIPL({ arguments: ["-q"] });

  const fact = {
    predicate: "person",
    name: "Alice",
    age: 20,
    updateView: "assert",
  };

  const query = {
    predicate: "person",
    name: "Alice",
    age: { var: "Age" },
  };

  const factClause = convert_fact_to_prolog_wasm(fact);
  const queryClause = convert_query_to_prolog_wasm(query);

  await swipl.prolog.call(factClause); // assert fact once
  const results = await swipl.prolog.query(queryClause);
  console.log(results.once()); // -> { Age: "20" }
};

main();

Converting a rule

use json_to_prolog::convert_rule_to_prolog;
use serde_json::json;

let rule = json!({
	"name": "grandparent",
	"variables": ["X", "Z"],
	"returns": "boolean",
	"evaluate": {
		"and": [
			{ "predicate": "parent", "args": ["X", "Y"] },
			{ "predicate": "parent", "args": ["Y", "Z"] }
		]
	}
});

let prolog = convert_rule_to_prolog(&rule)?;
assert_eq!(prolog, "assert(grandparent('X', 'Z') :- (parent('X', 'Y'), parent('Y', 'Z'))).");

Converting a query

use json_to_prolog::convert_query_to_prolog;
use serde_json::json;

let query = json!({
	"predicate": "person",
	"name": { "var": "Name" },
	"age": { "var": "Age" }
});

let prolog = convert_query_to_prolog(&query)?;
assert_eq!(prolog, "person(Age, Name)");

JSON Schema Overview

Each converter validates its input using jsonschema before emitting Prolog:

Facts – must provide predicate and updateView. Additional keys become arguments and are sorted alphabetically for deterministic output.
Rules – enforce a head name, declared variables, and an evaluate tree composed of logic nodes (predicate, and, or, not). Only boolean rules are currently supported.
Queries – require a predicate; any other properties become arguments. Objects of the form { "var": "VarName" } produce bare Prolog variables.

Null JSON values translate to the wildcard _. Arrays become Prolog lists, and strings are single-quoted to match atom syntax.

API Surface

Rust functions:

convert_fact_to_prolog(value: &Value) -> Result<String, String>
convert_rule_to_prolog(value: &Value) -> Result<String, String>
convert_query_to_prolog(value: &Value) -> Result<String, String>

Wasm bindings expose the same conversions but accept/return JsValue:

convert_fact_to_prolog_wasm(value: &JsValue) -> Result<JsValue, JsValue>
convert_rule_to_prolog_wasm(value: &JsValue) -> Result<JsValue, JsValue>
convert_query_to_prolog_wasm(value: &JsValue) -> Result<JsValue, JsValue>

All functions return descriptive error messages when validation fails.

Development

Prerequisites:

Rust toolchain (stable)
wasm-pack (for rebuilding the npm package)

Helpful commands:

cargo test           # Run the unit test suite
wasm-pack build --target bundler  # Rebuild Wasm bindings

Before publishing to crates.io or npm, ensure tests pass and review the schema contracts. Because the project is still experimental, contributions that extend validation or add new claim types are welcome—open an issue or PR describing your idea.

License

MIT. See Cargo.toml for the authoritative declaration.

json_to_prolog 0.1.6