feat(kernel): Add Error as a possible params type of CommandReply.

packages/extension/src/kernel-worker.ts

@@ -172,6 +172,17 @@ async function main(): Promise<void> {
     postMessage({ method, params });
   };
 
+  /**
+   * Cast an unknown problem to an Error object.
+   *
+   * @param problem - Whatever was caught.
+   * @returns The problem if it is an Error, or a new Error with the problem as the cause.
+   */
+  const asError = (problem: unknown): Error =>
+    problem instanceof Error
+      ? problem
+      : new Error('Unknown', { cause: problem });
+
   // Handle messages from the console service worker
   globalThis.onmessage = async (event: MessageEvent<unknown>) => {
     if (!isCommand(event.data)) {
@@ -213,7 +224,7 @@ async function main(): Promise<void> {
           const result = kvGet(params);
           reply(CommandMethod.KVGet, result);
         } catch (problem) {
-          reply(CommandMethod.KVGet, problem as string); // cast is a lie, it really is an Error
+          reply(CommandMethod.KVGet, asError(problem));
         }
         break;
       }

packages/kernel/src/command.ts

@@ -31,6 +31,15 @@ type CommandLike<Method extends CommandMethod, Data extends CommandParams> = {
   params: Data;
 };
 
+type CommandReplyLike<
+  Method extends CommandMethod,
+  Data extends CommandParams,
+  ErrorType extends Error = never,
+> = {
+  method: Method;
+  params: Data | ErrorType;
+};
+
 const isCommandLike = (
   value: unknown,
 ): value is {
@@ -67,22 +76,24 @@ export type CommandFunction<Return = void | Promise<void>> = {
 };
 
 export type CommandReply =
-  | CommandLike<CommandMethod.Ping, 'pong'>
-  | CommandLike<CommandMethod.Evaluate, string>
-  | CommandLike<CommandMethod.CapTpInit, string>
-  | CommandLike<CommandMethod.CapTpCall, string>
-  | CommandLike<CommandMethod.KVGet, string>
-  | CommandLike<CommandMethod.KVSet, string>;
+  | CommandReplyLike<CommandMethod.Ping, 'pong'>
+  | CommandReplyLike<CommandMethod.Evaluate, string>
+  | CommandReplyLike<CommandMethod.CapTpInit, string>
+  | CommandReplyLike<CommandMethod.CapTpCall, string>
+  | CommandReplyLike<CommandMethod.KVGet, string, Error>
+  | CommandReplyLike<CommandMethod.KVSet, string>;
 
 export const isCommandReply = (value: unknown): value is CommandReply =>
-  isCommandLike(value) && typeof value.params === 'string';
+  isCommandLike(value) &&
+  (typeof value.params === 'string' || value.params instanceof Error);
 
 type UnionMinus<Union, Minus> = Union extends Minus ? never : Union;
 
 export type CommandReplyFunction<Return = void> = {
   (method: CommandMethod.Ping, params: 'pong'): Return;
+  (method: CommandMethod.KVGet, params: string | Error): Return;
   (
-    method: UnionMinus<CommandMethod, CommandMethod.Ping>,
+    method: UnionMinus<CommandMethod, CommandMethod.Ping | CommandMethod.KVGet>,
     params: string,
   ): Return;
 };