The Infernal Engine is a 1-order logic forward chaining inference engine. Inference engine are used to build expert systems to modelise human experience using rules. The goal of the engine is to maintain its internal fact base concistent with its rules base. Each change to the fact or rule base triggers the inference. It is possible to control how and when the inference is fired using models.

InfernalEngine class Reference

See Philippe Morignot order logic explanations to know more about different inference engine order logics.

var InfernalEngine = require("infernal-engine");
let engine = new InfernalEngine();

await engine.def("count5", async function(i) {
    if (typeof i !== "undefined" && i < 5) {
        return { "i": i + 1 };
    }
});

await engine.assert("i", 1);

let final_i = await engine.peek("i");

console.log(final_i); // displays 5

Some things to consider for the above example:

1. All InfernalEngine methods must be called with await.

2. Rules must be async function! the parser do not recognize lambda parameters pattern yet.

3. Defining a rule using the #def method triggers the inference for that rule. Therefore rules code must be resilient to undefined facts unless implemented within a model that has pre-initialized facts.

4. Rules must either return nothing or an object that maps a relative or absolute fact reference. Those concepts are explained in details below.

5. Asserting a fact is done using the #assert method. Asserting new facts triggers the inference as well. To retract a fact, use #retract.

6. If you want to know a fact value, call the #peek method.

This simple example shows all the atomic inference methods of the InfernalEngine. You can go from there and have fun building a complex web of facts and rules already. But sincerely, your expert system programming life would be pretty miserable without the model.

Rules are simple Javascript functions. A rule is defined within a context and can reference facts within that context that match the function parameter names. For example, the rule at path "/speed/maxSpeedReached" is inside the context "/speed/". Given the following model:

{
  engine: "V6",
  speed: {
    max: 150,
    value: 0,
    maxReached: false,

    maxSpeedReached: async function(value, max) {
      return { "maxReached": value === max};
    },

    user: {
      input: ""
    }
  }
}

The rule parameters value and max will respectively reference "/speed/value" and "/speed/max". But how can we reference a fact that is outside the rule context? Simple, by using the parameter annotation comment like this:

{
  engine: "V6",
  speed: {
    max: 150,
    value: 0,
    maxReached: false,

    maxSpeedReached: async function(value, max) {
      return { "maxReached": value === max};
    },

    translate: function(/*@ user/input */ input) {
      return {"value": Number(input)}
    }

    user: {
      input: "",
    }
  }
}

Annotation comments must be placed in front of the annotated parameter.

Rules must return an object or nothing. The simple case is to return an object where the key is the fact to update and the value being the valu to assign to that fact. You have plenty of examples above.

What we just explained is a shorthand for the more general case though. The equivalent general case would be to return the command '#assert' instead of the factpath-value pair. This is the command list available to be returned by a rule:

1. "#assert": {path:str, value:any}

2. "#retract": {path:str}

3. "#def": {path:str, value:AsyncFunction}

4. "#undef": {path:str}

5. "#import": {path:str, value:object}

Each command expects an object with one or two properties which are 'path' and 'value'. Example:

await engine.def("count5", async function(i) {
    if (typeof i !== "undefined" && i < 5) {
      return {
        "#assert": {
          path: "i",
          value: i + 1
        }
      };
    }
});

The shorthand structure works with scalar values and arrays (#assert), functions (#def) and objects (#import). Thus returning the following structure:

// This rule will execute only once when defined.
await engine.def("assert_def_import", async function() {
    let model = require("./models/someInferenceModel");
    return {
      "/input/quantity": "23.5", // #assert
      "/input/isQuantityNumeric": async function(/*@ /input/quantity */ quantity) { // #def
        if (Number(quantity) === NaN) {
          return {
            message: "Invalid input quantity."
          };
        }
      },
      "/submodel": model // #import
    };
});

A model is a way to define rules and facts using a simple Javascript object. The following code block translates the Wikipedia article example for forward chaining inference into an InfernalEngine model representation.

var critterModel = {

  name: "Fritz",
  
  sound: "",
  eats: "",
  sings: false,

  color: "unknown",
  species: "unknown",
  
  isFrog: async function(sound, eats){
    let species = "unknown";
    if (sound === "croaks" && eats === "flies") {
      species = "frog";
    }
    return {"species": species};
  },

  isCanary: async function(sound, sings) {
    if ( sings && sound === "chirps" ) {
      return {"species": "canary"};
    }
  },
  
  isGreen: async function(species) {
    if (species === "frog") {
      return {"color": "green"};
    }
  },
  
  isYellow: async function(species) {
    if (species === "canary") {
      return {"color": "yellow"};
    }
  }

};

A model is a javascript object with scalar values or arrays and methods. Facts and rules paths are built based on their location in the object. The internal representation of a model is explained in the next section.

The following example displays the critter model after setting two facts at the same time:

let InfernalEngine = require("infernal-engine");
let engine = new InfernalEngine();

let critterModel = require("./models/critterModel");

await engine.import(critterModel);
await engine.import({
    eats: "flies",
    sound: "croaks"
});

let state = await engine.export();
console.log(JSON.stringify(state, null, "  "));

{
  "name": "Fritz",
  "sound": "croaks",
  "eats": "flies",
  "species": "frog",
  "color": "green"
}

In the above example, the inference runs twice: once after the model is imported and another time after the facts eats and sound are imported.

Given the set of rules, the inference that runs after importing the critterModel should not change anything to the internal fact base. On another hand, inferring after importing eats and sound facts will update the internal fact base.

This example shows how to set multiple facts at once and have the inferrence being triggered only once. Using the #assert method for each fact would have worked as well but it would have launched the inference one more time.

It is also possible to import a model into a different path if the import method path parameter was given. For example:

await engine.import(critterModel, "/some/other/path");

Doing so would have resulted in the following JSON output:

{
  "some": {
    "other": {
      "path" : {
        "name": "Fritz",
        "sound": "croaks",
        "eats": "flies",
        "species": "frog",
        "color": "green"
      }
    }
  }
}

Finally, the export method also supports the path parameter. When provided, the exported model only contains facts that are under it. Given the previous import example with a path, exporting the content of "/some/other/path" would print the original output. Example:

let state = await engine.export("/some/other/path");

Back to the original result:

{
  "name": "Fritz",
  "sound": "croaks",
  "eats": "flies",
  "species": "frog",
  "color": "green"
}

The import method crawls the model object structure and creates the matching fact and rule mapings. This is how the fact and rules are mapped in the engine based on the following model:

let carModel = {

  name: "Minivan",

  speed: {
    input: "0",
    limit: 140,
    value: 0,

    inputIsValidInteger: async function(input) {
      let isInt = /^-?\d+$/.test(input);
      if (!isInt) {
        return { "../message": `Error: '${input}' is not a valid integer.` }
      }
      return { 
        "../message": undefined,
        value: Number(input) 
      };
    },
    
    valueIsUnderLimit: async function(value, limit) {
      if (value > limit) {
        return {
          value: limit,
          "/message": `WARN: The speed input can not exceed the speed limit of ${limit}.`
        }
      }
    }

  }
}

By default rules parameters are fetching facts that have the same name as the given parameter from the same rule context. To reach a fact somewhere else in the fact tree, a parameter can be prefixed with a special comment that we are calling a Parameter Annotation. The parameter annotation lets you set the exact fact path that shall be set to the following parameter. The syntax is:

/*@ {path_to_fact} */

path_to_fact can be either a relative or an absolute path.

A path is the full name of a fact or a rule. A path always begin with a "/" and each segment of the path is separated by "/". Examples:

• /name

• /speed/input

• /speed/inputIsValidInteger

Are three valid paths.

The name of the rule or the fact is the last string after the last "/" character. For example, the name in "/speed/input" is "input".

The context is the path less the name. For example, the context in "/speed/input" is "/speed/". For "/name" the context is simply "/" (aslo called the root context).

Absolute fact reference involves using a fact full name by including the leading "/". As a convention, a fact can be contextualized using a directory like notation. Given the above minivan speed example, lets add a message telling the user that the spped limit of the engine is reached:

let carModel = {

  name: "Minivan",
  message: "",

  speed: {
    input: "0",
    limit: 140,
    value: 0,

    inputIsValidInteger: async function(input) {
      let isInt = /^-?\d+$/.test(input);
      if (!isInt) {
        return { "../message": `Error: '${input}' is not a valid integer.` }
      }
      return { 
        "../message": undefined,
        value: Number(input) 
      };
    },
    
    valueIsUnderLimit: async function(value, limit) {
      if (value > limit) {
        return {
          value: limit,
          "/message": `WARN: The speed input can not exceed the speed limit of ${limit}.`
        }
      }
    }

    // This rule is in the '/speed' context. To reach the /name fact,
    // it is possible to use the parameter annotation to set the modelName
    // parameter to the /name absolute fact. The same goes for the returned
    // /message2 fact.
    updateMessage: function(value, limit, /*@ /name */ modelName) {
      if (value === limit) {
        return {"/message2": `${modelName} ludicrous speed, GO!`};
      }
      return {"/message2": undefined};
    }
  }
}

In the above example, the relative reference "../message" and the absolute reference "/message" reference the same "message" fact. Since the "message" fact is directly in the root context and the "input" fact is under the "/speed/" context, it is easy to see that poping up one directory from "/speed/" takes the reference to the root context.

Just like directory reference in a file system, facts can be referenced using their relative path from the context of the executing rule. This happens when the path does not begin with "/". When referencing a fact, "../" does pop up one path element. When the path starts with "/", it references an absolute path from the root context. To dig down inside the fact tree from the current context, just write the path elements without a leading "/" like: "child/of/the/current/context".

There is a special kind of facts that lies within the engine. Meta facts can be referenced in a rule to know about the context of the current rule execution. Meta facts cannot trigger rules that reference them. They are to be injected into rules that have been triggered by any other fact change. Metafacts are in the "/$/" context. For now there is only two internal metafacts:

1. /$/maxDepth contains the value passed to the InfernalEngine constructor of the same name. It tells how many agenda can be generated in one inference run before failing.

2. /$/depth is the current agenda generation. Its value starts at 1 and is always smaller or equal to /$/maxDepth.

You can add metafacts as well, they will not trigger any rule either. You can even overwrite the aforementionned metafacts, but that change will last one inference run or one agenda generation only. Obviously not a good idea but I don't care since the engine is providing that information to you for your benefits only. These two metafacts do not affect the actual internal state of the InfernalEngine.

To trace what happens internally, provide a tracing function with one parameter to the InfernalEngine constructor's second parameter. The following code gives an example of that:

const InfernalEngine = require("infernal-engine");
const model = require("../models/critterModel");

(async () => {
    let engine = new InfernalEngine(null, 
        msg => console.log("-> ", JSON.stringify(msg)));
    
    console.log("Importing the critterModel:");
    await engine.import(model);
    
    console.log("Initial model:")
    let initialModel = await engine.export();
    console.log(JSON.stringify(initialModel, null, "  "));
    
    console.log("Importing two facts to be asserted:");
    await engine.import({
        sound: "croaks",
        eats: "flies"
    })
    
    console.log("Inferred model:")
    let inferredModel = await engine.export();
    console.log(JSON.stringify(inferredModel, null, "  "));
})();

Importing the critterModel:
->  {"action":"import","object":{"name":"Fritz","sound":"","eats":"","sings":false,"color":"unknown","species":"unknown"}}
->  {"action":"assert","fact":"/name","newValue":"Fritz"}
->  {"action":"assert","fact":"/sound","newValue":""}
->  {"action":"assert","fact":"/eats","newValue":""}
->  {"action":"assert","fact":"/sings","newValue":false}
->  {"action":"assert","fact":"/color","newValue":"unknown"}
->  {"action":"assert","fact":"/species","newValue":"unknown"}
->  {"action":"def","rule":"/isFrog","inputFacts":["/sound","/eats"]}
->  {"action":"addToAgenda","rule":"/isFrog"}
->  {"action":"def","rule":"/isCanary","inputFacts":["/sound","/sings"]}
->  {"action":"addToAgenda","rule":"/isCanary"}
->  {"action":"def","rule":"/isGreen","inputFacts":["/species"]}
->  {"action":"addToAgenda","rule":"/isGreen"}
->  {"action":"def","rule":"/isYellow","inputFacts":["/species"]}
->  {"action":"addToAgenda","rule":"/isYellow"}
->  {"action":"infer","maxGen":50}
->  {"action":"executeAgenda","gen":1,"ruleCount":4}
->  {"action":"execute","rule":"/isFrog","inputs":["",""]}
->  {"action":"execute","rule":"/isCanary","inputs":["",false]}
->  {"action":"execute","rule":"/isGreen","inputs":["unknown"]}
->  {"action":"execute","rule":"/isYellow","inputs":["unknown"]}
Initial facts:
{
  "name": "Fritz",
  "sound": "",
  "eats": "",
  "sings": false,
  "color": "unknown",
  "species": "unknown",
  "$": {
    "maxGen": 50,
    "gen": 1
  }
}
Importing two facts to be asserted:
->  {"action":"import","object":{"sound":"croaks","eats":"flies"}}
->  {"action":"assert","fact":"/sound","oldValue":"","newValue":"croaks"}
->  {"action":"addToAgenda","rule":"/isFrog"}
->  {"action":"addToAgenda","rule":"/isCanary"}
->  {"action":"assert","fact":"/eats","oldValue":"","newValue":"flies"}
->  {"action":"addToAgenda","rule":"/isFrog"}
->  {"action":"infer","maxGen":50}
->  {"action":"executeAgenda","gen":1,"ruleCount":2}
->  {"action":"execute","rule":"/isFrog","inputs":["croaks","flies"]}
->  {"action":"assert","fact":"/species","oldValue":"unknown","newValue":"frog"}
->  {"action":"addToAgenda","rule":"/isGreen"}
->  {"action":"addToAgenda","rule":"/isYellow"}
->  {"action":"execute","rule":"/isCanary","inputs":["croaks",false]}
->  {"action":"executeAgenda","gen":2,"ruleCount":2}
->  {"action":"execute","rule":"/isGreen","inputs":["frog"]}
->  {"action":"assert","fact":"/color","oldValue":"unknown","newValue":"green"}
->  {"action":"execute","rule":"/isYellow","inputs":["frog"]}
Inferred facts:
{
  "name": "Fritz",
  "sound": "croaks",
  "eats": "flies",
  "sings": false,
  "color": "green",
  "species": "frog",
  "$": {
    "maxGen": 50,
    "gen": 2
  }
}

• Fix an issue when returning a date or an array from a rule.

• Deprecate InfernalEngine#defRule and replaced it by InfernalEngine#def
• Deprecate InfernalEngine#undefRule and replaced it by InfernalEngine#undef
• Fix some documentation sentences

• Add the InfernalEngine#assertAll method.
• Add the InfernalEngine#fact static method to create facts to be consumed by assertAll.
• Improve code documentation.

• Complete rewrite of the engine with ECMAScript 2016.
• Method names have been changed to align with other inference engines (namely by CLIPS)
• Add the possibility to retract facts and undefine rules with wildcards.
• Improve tracing.