fix exercise 2 and add exercise 3

src/ch08/exercises.ts

@@ -57,7 +57,7 @@ function createProtocol<P extends Protocol>(script: string) {
     new Promise<P[K]['out']>((resolve, reject) => {
       let worker = new Worker(script)
       worker.onerror = reject
-      worker.onmessage = event => resolve(event.data.data)
+      worker.onmessage = event => resolve(event.data)
       worker.postMessage({command, args})
     })
 }
@@ -73,32 +73,59 @@ parallelDeterminant([[1, 2], [3, 4]]).then(
 
 // WorkerScript.ts
 
-let handlers: {
-  [C in keyof MatrixProtocol]: (
-    ...args: MatrixProtocol[C]['in']
-  ) => MatrixProtocol[C]['out']
-} = {
-  determinant(matrix) {
-    return determinant(matrix)
-  },
-  ['dot-product'](a, b) {
-    return dotProduct(a, b)
-  },
-  invert(matrix) {
-    return invert(matrix)
+type Data<
+  P extends Protocol,
+  C extends keyof P = keyof P
+> = C extends C
+  ? {command: C; args: P[C]['in']}
+  : never
+
+function handle(
+  data: Data<MatrixProtocol>
+): MatrixProtocol[typeof data.command]['out'] {
+  switch (data.command) {
+    case 'determinant':
+      return determinant(...data.args)
+    case 'dot-product':
+      return dotProduct(...data.args)
+    case 'invert':
+      return invert(...data.args)
   }
 }
 
-onmessage = <C extends keyof MatrixProtocol>({
-  data: {command, args}
-}: {
-  data: {command: C; args: MatrixProtocol[C]['in']}
-}) => {
-  let handler = handlers[command]
-  let result = handler(...args)
-  postMessage(result)
-}
+onmessage = ({data}) => postMessage(handle(data))
 
 declare function determinant(matrix: Matrix): number
 declare function dotProduct(matrixA: Matrix, matrixB: Matrix): Matrix
 declare function invert(matrix: Matrix): Matrix
+
+// 3. Use a mapped type (as in 「8.6.1 In the Browser: With Web Workers★」) to implement a typesafe message-passing protocol for NodeJS's `child_process`.
+
+// MainThread.ts
+import {fork} from 'child_process'
+
+function createProtocolCP<P extends Protocol>(script: string) {
+  return <K extends keyof P>(command: K) => (...args: P[K]['in']) =>
+    new Promise<P[K]['out']>((resolve, reject) => {
+      let child = fork(script)
+      child.on('error', reject)
+      child.on('message', resolve)
+      child.send({command, args})
+    })
+}
+
+let runWithMatrixProtocolCP = createProtocolCP<MatrixProtocol>(
+  './ChildThread.js'
+)
+let parallelDeterminantCP = runWithMatrixProtocolCP('determinant')
+
+parallelDeterminantCP([[1, 2], [3, 4]]).then(
+  determinant => console.log(determinant) // -2
+)
+
+// ChildThread.ts
+
+// type Data ...
+// function handle ...
+
+process.on('message', data => process.send!(handle(data)))