Merge pull request #240 from TimKam/238-keep-initial-priorities

Allow keeping belief priorities

README.md

@@ -630,9 +630,22 @@ After applying the belief update, our agent's belief base is as follows:
 
 Note that in detail, the priorities are interpreted as follows:
 
-* If a belief exists in the update, but not in the agent's belief base, this belief is added.
-* If the belief's priority is 0 in the belief base and a belief with the same key exists in the update, the agent's belief is overridden; this behavior is desired for beliefs that are generally defeasible.
-* If a belief's priority in the update is higher than the same belief's priority in the agent's belief base, the agent's belief is overridden.
+  * If a belief exists in the update, but not in the agent's belief base, this belief is added.
+  * If the belief's priority is 0 in the belief base and a belief with the same key exists in the update, the agent's belief is overridden; this behavior is desired for beliefs that are generally defeasible.
+  * If a belief's priority in the update is higher than the same belief's priority in the agent's belief base, the agent's belief is overridden.
+
+A potential issue that the belief revision function we use above does not address is that it essentially requires the *inflation* of priorities in case of regular successful revisions of beliefs with a non-zero priority.
+For example, in order to update ``the belief \verb|propertyValue: { value: 500000, priority: 1 }``, a new ``propertyValue`` belief can only defeat the belief if its priority is ``2`` or higher; the subsequent defeater will then require a priority of ``3``, and so on.    
+We can address this issue by defining whether a particular belief (or beliefs in general) should, when defeated, adopt the priority of their defeater.
+
+When using ``JSson.revisionFunctions.revisePriorityStatic`` as our belief revision function, the priority of the initial beliefs are maintained. Alternatively, we can specify whether or not a belief's priority should be updated, on the level of the individual belief:
+
+```JavaScript
+const beliefBase = {
+      isRaining: Belief('isRaining', true, Infinity, true),
+      temperature: Belief('temperature', 10, Infinity, false)
+    }
+```
 
 ## Messaging
 JS-son agents can send "private" messages to any other JS-son agent, which the environment will then relay to this agent only.

spec/src/agent/Belief.spec.js

@@ -9,8 +9,8 @@ describe('belief()', () => {
     expect(Belief('test', 'test')).toEqual({ test: 'test' })
   })
 
-  it('should create a new belief with the specified key, value (explicitly managed), and priority', () => {
-    expect(Belief('test', 'test', 1)).toEqual({ test: 'test', value: 'test', priority: 1 })
+  it('should create a new belief with the specified key, value (explicitly managed), priority, and priority update spec', () => {
+    expect(Belief('test', 'test', 1)).toEqual({ test: 'test', value: 'test', priority: 1, updatePriority: false })
   })
 
   it('should not throw a warning if belief is of a JSON data type', () => {

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

@@ -3,7 +3,8 @@ const Belief = require('../../../../src/agent/Belief')
 const {
   reviseSimpleNonmonotonic,
   reviseMonotonic,
-  revisePriority } = require('../../../../src/agent/beliefRevision/revisionFunctions')
+  revisePriority,
+  revisePriorityStatic } = require('../../../../src/agent/beliefRevision/revisionFunctions')
 
 const {
   beliefs,
@@ -98,4 +99,38 @@ describe('revisionFunctions', () => {
     expect(newAgent.beliefs.isRaining.value).toBe(true)
     expect(newAgent.beliefs.temperature.value).toEqual(10)
   })
+
+  it('should allow configuring a belief such that its initial priority is kept', () => {
+    const beliefBase = { isRaining: Belief('isRaining', true, 1, false) }
+
+    const update = { isRaining: Belief('isRaining', false, 2) }
+
+    const newAgent = new Agent({
+      id: 'myAgent',
+      beliefs: beliefBase,
+      desires,
+      plans,
+      selfUpdatesPossible: false,
+      reviseBeliefs: revisePriority
+    })
+    newAgent.next(update)
+    expect(newAgent.beliefs.isRaining.priority).toBe(1)
+  })
+
+  it('should allow configuring a priority belief revision function such that the initial priorities of it beliefs are generally kept', () => {
+    const beliefBase = { isRaining: Belief('isRaining', true, 1) }
+
+    const update = { isRaining: Belief('isRaining', false, 2) }
+
+    const newAgent = new Agent({
+      id: 'myAgent',
+      beliefs: beliefBase,
+      desires,
+      plans,
+      selfUpdatesPossible: false,
+      reviseBeliefs: revisePriorityStatic
+    })
+    newAgent.next(update)
+    expect(newAgent.beliefs.isRaining.priority).toBe(1)
+  })
 })

src/agent/Belief.js

@@ -5,14 +5,16 @@ const warning = 'JS-son: Created belief with non-JSON object, non-JSON data type
  * @param {string} id the belief's unique identifier
  * @param {any} value the belief's value
  * @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 belief
  */
-const Belief = (id, value, priority) => {
+const Belief = (id, value, priority, updatePriority=false) => {
   const belief = {}
   belief[id] = value
   if (priority || priority === 0) {
     belief.priority = priority
     belief['value'] = value
+    belief.updatePriority = updatePriority
   }
   try {
     const parsedBelief = JSON.parse(JSON.stringify(belief))

src/agent/beliefRevision/revisionFunctions.js

@@ -26,13 +26,31 @@ const reviseMonotonic = (oldBeliefs, newBeliefs) => ({ ...newBeliefs, ...oldBeli
 const revisePriority = (oldBeliefs, newBeliefs) => Object.fromEntries(
   new Map(
     Object.keys(newBeliefs).map(key =>
-      !key in oldBeliefs || oldBeliefs[key].priority == 0 || oldBeliefs[key].priority < newBeliefs[key].priority ? [key, newBeliefs[key]] : [key, oldBeliefs[key]]
+      !key in oldBeliefs || oldBeliefs[key].priority == 0 || oldBeliefs[key].priority < newBeliefs[key].priority ? oldBeliefs[key].updatePriority ? [key, newBeliefs[key]] : [key, { ...oldBeliefs[key], value: newBeliefs[key].value }] : [key, oldBeliefs[key]]
     )
   )
 )
 
+/**
+ * Revises beliefs by merging old and new beliefs such that an old belief overrides a new one in. Does not update belief priorities.
+ * case of conflict
+ * @param {object} oldBeliefs Old belief base (JSON object of beliefs)
+ * @param {object} newBeliefs New belief base (JSON object of beliefs)
+ * @returns Revised belief base (JSON object of beliefs)
+ */
+
+const revisePriorityStatic = (oldBeliefs, newBeliefs) => revisePriority(Object.fromEntries(
+    new Map(
+      Object.keys(oldBeliefs).map(key => [
+        key,
+        { ...oldBeliefs[key], updatePriority: false }
+      ])
+    )
+  ), newBeliefs)
+
 module.exports = {
   reviseSimpleNonmonotonic,
   reviseMonotonic,
-  revisePriority
+  revisePriority,
+  revisePriorityStatic
 }