First stab at attenuation

actor.ts

@@ -6,13 +6,13 @@ export type ExitReason = null | { ok: true } | { ok: false, err: Error };
 export type LocalAction = (t: Turn) => void;
 
 export interface Entity {
-    assert?(turn: Turn, assertion: Assertion, handle: Handle): void;
-    retract?(turn: Turn, handle: Handle): void;
-    message?(turn: Turn, body: Assertion): void;
-    sync?(turn: Turn, peer: Ref): void;
+    assert(turn: Turn, assertion: Assertion, handle: Handle): void;
+    retract(turn: Turn, handle: Handle): void;
+    message(turn: Turn, body: Assertion): void;
+    sync(turn: Turn, peer: Ref): void;
 }
 
-export interface Ref { readonly relay: Actor, readonly target: Entity };
+export interface Ref { readonly relay: Actor, readonly target: Partial<Entity> };
 
 export class Actor {
     readonly outbound: Map<Handle, Ref>;
@@ -43,6 +43,10 @@ export class Actor {
 
 let nextHandle = 0;
 
+export function _sync(turn: Turn, e: Partial<Entity>, peer: Ref): void {
+    e.sync ? e.sync!(turn, peer) : turn.message(peer, true);
+}
+
 export class Turn {
     readonly actor: Actor;
     readonly queues = new Map<Actor, LocalAction[]>();
@@ -57,7 +61,7 @@ export class Turn {
         this.actor = actor;
     }
 
-    ref(e: Entity): Ref {
+    ref(e: Partial<Entity>): Ref {
         return { relay: this.actor, target: e };
     }
 
@@ -104,11 +108,8 @@ export class Turn {
     }
 
     sync(ref: Ref): Promise<Turn> {
-        return new Promise(resolve => {
-            const k = this.ref({ message: resolve });
-            this.enqueue(ref.relay, t =>
-                ref.target.sync ? ref.target.sync!(t, k) : t.message(k, true));
-        });
+        return new Promise(resolve => this.enqueue(ref.relay, t =>
+            _sync(t, ref.target, this.ref({ message: resolve }))));
     }
 
     message(ref: Ref, assertion: Assertion): void {
@@ -119,3 +120,38 @@ export class Turn {
         this.queues.get(relay)?.push(a) ?? this.queues.set(relay, [a]);
     }
 }
+
+export type AttenuationFilter = (assertion: Assertion) => Assertion | null;
+
+export class Attenuation implements Entity {
+    readonly target: Partial<Entity>;
+    readonly filter: AttenuationFilter;
+
+    constructor(target: Partial<Entity>, filter: AttenuationFilter) {
+        this.target = target;
+        this.filter = filter;
+    }
+
+    assert(turn: Turn, assertion: Assertion, handle: Handle): void {
+        const filtered = this.filter(assertion);
+        if (filtered !== null) this.target.assert?.(turn, filtered, handle);
+    }
+
+    retract(turn: Turn, handle: Handle): void {
+        // TODO: consider whether we want targets to even see blocked handles
+        this.target.retract?.(turn, handle);
+    }
+
+    message(turn: Turn, body: Assertion): void {
+        const filtered = this.filter(body);
+        if (filtered !== null) this.target.message?.(turn, filtered);
+    }
+
+    sync(turn: Turn, peer: Ref): void {
+        _sync(turn, this.target, peer);
+    }
+}
+
+export function attenuate(ref: Ref, filter: AttenuationFilter): Ref {
+    return { relay: ref.relay, target: new Attenuation(ref.target, filter) };
+}

main.ts

@@ -1,10 +1,10 @@
-import { Actor, Assertion, Entity, Handle, Ref, Turn } from './actor.js';
-import { Dictionary, IdentityMap, is, Record } from 'preserves';
+import { Actor, Assertion, attenuate, Entity, Handle, Ref, Turn } from './actor.js';
+import { Dictionary, IdentityMap, is, Record, RecordConstructor } from 'preserves';
 import { Bag, ChangeDescription } from './bag';
 
 const Observe = Record.makeConstructor<Ref>('Observe', ['label', 'observer']);
 
-class Dataspace implements Entity {
+class Dataspace implements Partial<Entity> {
     readonly handleMap: IdentityMap<Handle, Record<Ref>> = new IdentityMap();
     readonly assertions = new Bag<Ref>();
     readonly subscriptions: Dictionary<Map<Ref, Dictionary<Handle>>> = new Dictionary();
@@ -52,16 +52,16 @@ class Dataspace implements Entity {
     }
 }
 
+//---------------------------------------------------------------------------
+
 const BoxState = Record.makeConstructor<Ref>('BoxState', ['value']);
 const SetBox = Record.makeConstructor<Ref>('SetBox', ['newValue']);
 
 let startTime = Date.now();
 let prevValue = 0;
 const LIMIT = 500000;
-Turn.for(new Actor(), async (t: Turn) => {
-    const ds = t.ref(new Dataspace());
 
-    // Box
+function spawnBox(t: Turn, ds: Ref) {
     t.spawn(t => {
         console.log('Spawning Box');
         let valueHandle: Handle | undefined;
@@ -85,8 +85,9 @@ Turn.for(new Actor(), async (t: Turn) => {
             }
         })));
     });
+}
 
-    // Client
+function spawnClient(t: Turn, ds: Ref) {
     t.spawn(t => {
         console.log('Spawning Client');
         let count = 0;
@@ -111,4 +112,22 @@ Turn.for(new Actor(), async (t: Turn) => {
             },
         })));
     });
+}
+
+function isObserverOf(a: Assertion, ctor: RecordConstructor<Ref>): boolean {
+    return Observe.isClassOf(a) && is(Observe._.label(a), ctor.constructorInfo.label);
+}
+
+Turn.for(new Actor(), async (t: Turn) => {
+    const ds = t.ref(new Dataspace());
+    spawnBox(t, attenuate(ds, a => {
+        if (BoxState.isClassOf(a)) return a;
+        if (isObserverOf(a, SetBox)) return a;
+        return null;
+    }));
+    spawnClient(t, attenuate(ds, a => {
+        if (SetBox.isClassOf(a)) return a;
+        if (isObserverOf(a, BoxState)) return a;
+        return null;
+    }));
 });