Merge pull request #241 from TimKam/239-beliefs-as-functions

Support functional beliefs

Author: TimKam

README.md

@@ -618,7 +618,7 @@ const update = {
 const agent = new Agent('myAgent', beliefBase, {}, [], undefined, false, revisePriority)
 ```
 
-After applying the belief update, our agent's belief base is as follows:
+After applying the belief update with `agent.next(update)`, our agent's belief base is as follows:
 
 ```JavaScript
 {
@@ -647,6 +647,70 @@ const beliefBase = {
     }
 ```
 
+We may want to specify beliefs that are not simply updated as static objects/properties, but dynamically inferred, based on our current belief base or updates thereof.
+To supports this, JS-son uses the notion of a *functional belief*.
+A functional belief can be specified as follows, for example:
+
+```JavaScript
+const isSlippery = FunctionalBelief(
+      'isSlippery',
+      false,
+      (oldBeliefs, newBeliefs) =>
+        (newBeliefs.isRaining && newBeliefs.isRaining.value) ||
+        (!newBeliefs.isRaining && oldBeliefs.isRaining && oldBeliefs.isRaining.value),
+      1
+    )
+```
+
+The arguments of `FunctionalBelief` have the following meaning:
+
+* `isSlippery` (`id`) is the unique identifier of the (functional) belief,
+* `false` (`value`) is the belief's default/initial value;
+* The function:
+  ```JavaScript
+    (oldBeliefs, newBeliefs) =>
+        (newBeliefs.isRaining && newBeliefs.isRaining.value) ||
+        (!newBeliefs.isRaining && oldBeliefs.isRaining && oldBeliefs.isRaining.value)
+  ```
+ specifies the rule according to which the belief is inferred -- in this simple example, the value of `isSlippery` takes the value of the new belief `isRaining` unless the belief does not exist, in which it will check for the existing (old) belief `isRaining` and return `false` if neither a new nor an old `isRaining` belief exists.
+* `0` (`order`) is the number used for inducing the order in which the functional belief is revised relative to other functional beliefs: e.g., if another functional belief with order `1` is present, then the latter belief is revised later.
+* `2` (`priority`) is the priority that the belief takes when updating the function as well as the default value, analogous to how orders work for non-functional beliefs.
+
+Given this functional belief, we can now demonstrate how functional belief revision works:
+
+1. First, we specify our agent:
+
+```JavaScript
+    const newAgent = new Agent({
+      id: 'myAgent',
+      beliefs: { isRaining: Belief('isRaining', true, 0) },
+      desires,
+      plans
+    })
+    newAgent.next(newBeliefs1)
+    expect(newAgent.beliefs.isSlippery.value).toBe(true)
+    newAgent.next(newBeliefs2)
+    expect(newAgent.beliefs.isSlippery.value).toBe(false)
+```
+
+2. Then, we specify the initial belief base and execute the agent's reasoning loop with a belief base update that merely contains the functional belief:  
+
+```JavaScript
+  newAgent.next({ isSlippery})
+```
+Because ``isRaining`` is not present in the update, our agent infers ``isSlippery`` from its old belief base, i.e., the value of ``isSlippery`` remains ``true``.
+
+3. Finally, we executed the reasoning loop again, with a slightly different belief base update:
+
+```JavaScript
+  newAgent.next({
+      isSlippery,
+      isRaining: Belief('isRaining', false, 0)
+  })
+```
+
+Now, the value of ``isSlippery`` is updated to ``false``, as inferred from the update of ``isRaining``.
+
 ## Messaging
 JS-son agents can send "private" messages to any other JS-son agent, which the environment will then relay to this agent only.
 Agents can send these messages in the same way they register the execution of an action as the result of a plan.

spec/src/agent/Agent.spec.js

@@ -1,4 +1,5 @@
 const Belief = require('../../../src/agent/Belief')
+const FunctionalBelief = require('../../../src/agent/FunctionalBelief')
 const Plan = require('../../../src/agent/Plan')
 const Agent = require('../../../src/agent/Agent')
 
@@ -235,4 +236,68 @@ describe('Agent / next(), configuration object-based', () => {
     })
     expect(newAgent.next({ ...Belief('dogNice', false) })[0].action).toEqual('Good dog, Hasso!')
   })
+
+  it('should correctly revise functional beliefs, given new beliefs', () => {
+    const oldBeliefs = {
+      isRaining: Belief('isRaining', true, 0),
+    }
+
+    const isSlippery = FunctionalBelief('isSlippery', false, (_, newBeliefs) => newBeliefs.isRaining.value, 1)
+
+    const newBeliefs1 = {
+      isRaining: Belief('isRaining', false, 0),
+      isSlippery
+    }
+
+    const newBeliefs2 = {
+      isRaining: Belief('isRaining', true, 0),
+      isSlippery
+    }
+
+    const newAgent = new Agent({
+      id: 'myAgent',
+      beliefs: oldBeliefs,
+      desires,
+      plans
+    })
+    newAgent.next(newBeliefs1)
+    expect(newAgent.beliefs.isSlippery.value).toBe(false)
+    newAgent.next(newBeliefs2)
+    expect(newAgent.beliefs.isSlippery.value).toBe(true)
+  })
+
+  it('should correctly revise functional beliefs, given new and old beliefs', () => {
+    const oldBeliefs = {
+      isRaining: Belief('isRaining', true, 0),
+    }
+
+    const isSlippery = FunctionalBelief(
+      'isSlippery',
+      false,
+      (oldBeliefs, newBeliefs) =>
+        (newBeliefs.isRaining && newBeliefs.isRaining.value) ||
+        (!newBeliefs.isRaining && oldBeliefs.isRaining && oldBeliefs.isRaining.value),
+      1
+    )
+
+    const newBeliefs1 = {
+      isSlippery
+    }
+
+    const newBeliefs2 = {
+      isSlippery,
+      isRaining: Belief('isRaining', false, 0)
+    }
+
+    const newAgent = new Agent({
+      id: 'myAgent',
+      beliefs: oldBeliefs,
+      desires,
+      plans
+    })
+    newAgent.next(newBeliefs1)
+    expect(newAgent.beliefs.isSlippery.value).toBe(true)
+    newAgent.next(newBeliefs2)
+    expect(newAgent.beliefs.isSlippery.value).toBe(false)
+  })
 })

spec/src/agent/Belief.spec.js

@@ -2,7 +2,7 @@ const Belief = require('../../../src/agent/Belief')
 
 const warning = 'JS-son: Created belief with non-JSON object, non-JSON data type value'
 
-describe('belief()', () => {
+describe('Belief()', () => {
   console.warn = jasmine.createSpy('warn')
 
   it('should create a new belief with the specified key and value', () => {

spec/src/agent/FunctionalBelief.spec.js

@@ -0,0 +1,46 @@
+const FunctionalBelief = require('../../../src/agent/FunctionalBelief')
+
+const beliefWarning = 'JS-son: Created belief with non-JSON object, non-JSON data type value'
+const functionalBeliefWarning = 'JS-son: functional belief without proper two-argument function'
+
+describe('FunctionalBelief()', () => {
+  console.warn = jasmine.createSpy('warn')
+
+  it('should create a new functional belief with the specified key, value, rule, and order', () => {
+    const functionBelief = FunctionalBelief('isSlippery', false, (_, newBeliefs) => newBeliefs.isRaining.value, 0)
+    expect(functionBelief.isSlippery).toEqual(false)
+    expect(functionBelief.value).toEqual(false)
+    expect(functionBelief.rule.toString()).toEqual(((_, newBeliefs) => newBeliefs.isRaining.value).toString())
+    expect(functionBelief.order).toEqual(0)
+  })
+
+  it('should create a new functional belief with the specified key, value, rule, order, priority, and priority update spec', () => {
+    const functionBelief = FunctionalBelief('isSlippery', false, (_, newBeliefs) => newBeliefs.isRaining.value, 0, 2)
+    expect(functionBelief.isSlippery).toEqual(false)
+    expect(functionBelief.value).toEqual(false)
+    expect(functionBelief.rule.toString()).toEqual(((_, newBeliefs) => newBeliefs.isRaining.value).toString())
+    expect(functionBelief.order).toEqual(0)
+    expect(functionBelief.priority).toEqual(2)
+  })
+
+  it('should throw warning if base belief is not JSON.stringify-able & not of a JSON data type', () => {
+    console.warn.calls.reset()
+    // eslint-disable-next-line no-unused-vars
+    const functionBelief = FunctionalBelief('function', () => {}, (_, newBeliefs) => newBeliefs.isRaining, 0, 2)
+    expect(console.warn).toHaveBeenCalledWith(beliefWarning)
+  })
+
+  it('should throw warning if rule is not a function', () => {
+    console.warn.calls.reset()
+    // eslint-disable-next-line no-unused-vars
+    const functionBelief = FunctionalBelief('isSlippery', false, true, 0, 2)
+    expect(console.warn).toHaveBeenCalledWith(functionalBeliefWarning)
+  })
+
+  it('should throw warning if rule is a function that does not take exactly two arguments', () => {
+    console.warn.calls.reset()
+    // eslint-disable-next-line no-unused-vars
+    const functionBelief = FunctionalBelief('isSlippery', false, newBeliefs => newBeliefs.isRaining, 0)
+    expect(console.warn).toHaveBeenCalledWith(functionalBeliefWarning)
+  })
+})

spec/src/agent/beliefRevision/revisionFunctions.spec.js

@@ -1,10 +1,13 @@
 const Agent = require('../../../../src/agent/Agent')
 const Belief = require('../../../../src/agent/Belief')
+const FunctionalBelief = require('../../../../src/agent/FunctionalBelief')
 const {
   reviseSimpleNonmonotonic,
   reviseMonotonic,
   revisePriority,
-  revisePriorityStatic } = require('../../../../src/agent/beliefRevision/revisionFunctions')
+  revisePriorityStatic,
+  getNonFunctionalBeliefs,
+  preprocessFunctionalBeliefs } = require('../../../../src/agent/beliefRevision/revisionFunctions')
 
 const {
   beliefs,
@@ -134,3 +137,23 @@ describe('revisionFunctions', () => {
     expect(newAgent.beliefs.isRaining.priority).toBe(1)
   })
 })
+
+describe('functional belief revision functions', () => {
+  const isRaining = Belief('isRaining', true, 1, false)
+  const isSlippery = FunctionalBelief(
+    'isSlippery', false, (_, newBeliefs) => newBeliefs.isRaining, 0
+  )
+
+  it('(getNonFunctionalBeliefs) should filter out functional beliefs, given a belief base', () => {
+    const nonFunctionalBeliefs = getNonFunctionalBeliefs({isRaining, isSlippery})
+    expect(Object.keys(nonFunctionalBeliefs).length).toEqual(1)
+    expect(nonFunctionalBeliefs.isRaining.value).toEqual(true)
+  })
+
+  it('(preprocessFunctionalBeliefs) should filter out non-functional beliefs, given a belief base', () => {
+    const functionalBeliefs = preprocessFunctionalBeliefs({isRaining, isSlippery})
+    expect(functionalBeliefs.length).toEqual(1)
+    expect(functionalBeliefs[0].value).toEqual(false)
+  })
+
+})

src/agent/Agent.js

@@ -1,5 +1,11 @@
 const Intentions = require('./Intentions')
 const defaultBeliefRevisionFunction = require('./beliefRevision/revisionFunctions').reviseSimpleNonmonotonic
+const {
+  preprocessFunctionalBeliefs,
+  getNonFunctionalBeliefs,
+  getFunctionalBeliefs,
+  processFunctionalBeliefs
+} = require('./beliefRevision/revisionFunctions')
 
 const defaultPreferenceFunction = (beliefs, desires) => desireKey => desires[desireKey](beliefs)
 const defaultGoalRevisionFunction = (beliefs, goals) => goals
@@ -82,7 +88,19 @@ function Agent (
   }
   this.isActive = true
   this.next = function (beliefs) {
-    this.beliefs = this.reviseBeliefs(this.beliefs, beliefs)
+    // revision of non-functional beliefs
+    const oldBeliefs = getNonFunctionalBeliefs(this.beliefs)
+    this.beliefs = this.reviseBeliefs(this.beliefs, getNonFunctionalBeliefs(beliefs))
+    // revision of functional beliefs
+    const newFunctionalBeliefs = processFunctionalBeliefs(
+      getFunctionalBeliefs(this.beliefs),
+      getFunctionalBeliefs(beliefs),
+      oldBeliefs,
+      beliefs,
+      reviseBeliefs
+    )
+    this.beliefs = { ...this.beliefs, ...newFunctionalBeliefs }
+    // end: revision of functional beliefs
     this.goals = this.reviseGoals(this.beliefs, this.goals)
     if (this.isActive) {
       if (Object.keys(this.desires).length === 0) {

src/agent/FunctionalBelief.js

@@ -0,0 +1,21 @@
+const Belief = require('./Belief')
+
+const warning = 'JS-son: functional belief without proper two-argument function'
+
+/**
+ * JS-son agent belief generator
+ * @param {string} id the belief's unique identifier
+ * @param {any} value the belief's default value
+ * @param {function} rule the function that is used to infer the belief, given the agent's current beliefs and the belief update
+ * @param {number} order the belief's order when belief functions are evaluated
+ * @param {number} priority the belief's priority in case of belief revision; optional
+ * @param {boolean} updatePriority whether in case of a belief update, the priority of the defeating belief should be adopted; optional, defaults to true
+ * @returns {object} JS-son agent functional belief
+ */
+const FunctionalBelief = (id, value, rule, order, priority, updatePriority=false) => {
+  if (typeof rule !== 'function' || rule.length !== 2) console.warn(warning)
+  const baseBelief = Belief(id, value, priority, updatePriority=false)
+  return { ...baseBelief, rule, order, value }
+}
+
+module.exports = FunctionalBelief