feat: implement .apply REPL dot-command (Approach 1: Minimalist Function Injection)

.changeset/apply-command.md

@@ -0,0 +1,17 @@
+---
+"counterfact": minor
+---
+
+Add `.apply` REPL dot-command (Approach 1: Minimalist Function Injection).
+
+The `.apply` command lets you run scenario scripts from the REPL prompt without leaving the terminal. A scenario is a plain TypeScript (or JavaScript) file that exports named functions. Each function receives an `ApplyContext` object with `{ context, loadContext, routes, route }` and can freely read or mutate state.
+
+**Path resolution:**
+
+| Command | File | Function |
+|---|---|---|
+| `.apply foo` | `scenarios/index.ts` | `foo` |
+| `.apply foo/bar` | `scenarios/foo.ts` | `bar` |
+| `.apply foo/bar/baz` | `scenarios/foo/bar.ts` | `baz` |
+
+The `ApplyContext` type is written to `types/apply-context.ts` during code generation.

docs/usage.md

@@ -542,6 +542,50 @@ await req.send()
 
 See the [Route Builder guide](./route-builder.md) for full documentation.
 
+### Scenario scripts with `.apply`
+
+For more complex setups you can automate REPL interactions by writing _scenario scripts_ — plain TypeScript files that export named functions. Run them with `.apply`:
+
+```
+⬣> .apply soldPets
+```
+
+**Path resolution:** the argument to `.apply` is a slash-separated path. The last segment is the function name; everything before it is the file path, resolved relative to `<basePath>/scenarios/` (with `index.ts` as the default file).
+
+| Command | File | Function |
+|---|---|---|
+| `.apply foo` | `scenarios/index.ts` | `foo` |
+| `.apply foo/bar` | `scenarios/foo.ts` | `bar` |
+| `.apply foo/bar/baz` | `scenarios/foo/bar.ts` | `baz` |
+
+A scenario function receives a single argument with `{ context, loadContext, routes, route }`:
+
+```ts
+// scenarios/index.ts
+import type { Scenario } from "../types/scenario-context.js";
+
+export const soldPets: Scenario = ($) => {
+  // Mutate context directly — same as typing in the REPL
+  $.context.petService.reset();
+  $.context.petService.addPet({ id: 1, status: "sold" });
+  $.context.petService.addPet({ id: 2, status: "available" });
+
+  // Store a pre-configured route builder for later use in the REPL
+  $.routes.findSold = $
+    .route("/pet/findByStatus")
+    .method("get")
+    .query({ status: "sold" });
+}
+```
+
+After the command runs you can immediately use anything stored in `$.routes`:
+
+```js
+⬣> routes.findSold.send()
+```
+
+The `Scenario` type and `ApplyContext` interface are generated automatically into `types/scenario-context.ts` when you run Counterfact with type generation enabled.
+
 ---
 
 ## Proxy 🔀

src/app.ts

@@ -13,8 +13,10 @@ import { loadOpenApiDocument } from "./server/load-openapi-document.js";
 import { ModuleLoader } from "./server/module-loader.js";
 import { OpenApiWatcher } from "./server/openapi-watcher.js";
 import { Registry } from "./server/registry.js";
+import { ScenarioRegistry } from "./server/scenario-registry.js";
 import { Transpiler } from "./server/transpiler.js";
 import { CodeGenerator } from "./typescript-generator/code-generator.js";
+import { writeApplyContextType } from "./typescript-generator/generate.js";
 import { runtimeCanExecuteErasableTs } from "./util/runtime-can-execute-erasable-ts.js";
 
 export { loadOpenApiDocument } from "./server/load-openapi-document.js";
@@ -114,6 +116,8 @@ export async function counterfact(config: Config) {
 
   const contextRegistry = new ContextRegistry();
 
+  const scenarioRegistry = new ScenarioRegistry();
+
   const codeGenerator = new CodeGenerator(
     config.openApiPath,
     config.basePath,
@@ -142,8 +146,14 @@ export async function counterfact(config: Config) {
     compiledPathsDirectory,
     registry,
     contextRegistry,
+    nodePath.join(modulesPath, "scenarios").replaceAll("\\", "/"),
+    scenarioRegistry,
   );
 
+  contextRegistry.addEventListener("context-changed", () => {
+    void writeApplyContextType(modulesPath);
+  });
+
   const middleware = koaMiddleware(dispatcher, config);
 
   const koaApp = createKoaApp(registry, middleware, config, contextRegistry);
@@ -209,6 +219,7 @@ export async function counterfact(config: Config) {
         config,
         undefined, // use the default print function (stdout)
         openApiDocument,
+        scenarioRegistry,
       ),
   };
 }

src/repl/repl.ts

@@ -4,6 +4,7 @@ import type { Config } from "../server/config.js";
 import type { ContextRegistry } from "../server/context-registry.js";
 import type { OpenApiDocument } from "../server/dispatcher.js";
 import type { Registry } from "../server/registry.js";
+import type { ScenarioRegistry } from "../server/scenario-registry.js";
 
 import { RawHttpClient } from "./raw-http-client.js";
 import { createRouteFunction } from "./route-builder.js";
@@ -29,11 +30,61 @@ const ROUTE_BUILDER_METHODS = [
   "send(",
 ];
 
+/**
+ * Creates a tab-completion function for the REPL.
+ *
+ * @param registry - The route registry used to complete path arguments for `route()` and `client.*()` calls.
+ * @param fallback - Optional fallback completer (e.g. the Node.js built-in completer) invoked when no custom completion matches.
+ * @param scenarioRegistry - When provided, enables tab completion for `.apply` commands by enumerating
+ *   exported function names and file-key prefixes from the loaded scenario modules.
+ */
 export function createCompleter(
   registry: Registry,
   fallback?: (line: string, callback: CompleterCallback) => void,
+  scenarioRegistry?: ScenarioRegistry,
 ) {
   return (line: string, callback: CompleterCallback): void => {
+    // Check for .apply completion: .apply <partial>
+    const applyMatch = line.match(/^\.apply\s+(?<partial>\S*)$/u);
+
+    if (applyMatch) {
+      const partial = applyMatch.groups?.["partial"] ?? "";
+
+      if (scenarioRegistry !== undefined) {
+        const slashIdx = partial.lastIndexOf("/");
+
+        if (slashIdx === -1) {
+          // No slash: complete exports from "index" key + top-level file prefixes
+          const indexFunctions =
+            scenarioRegistry.getExportedFunctionNames("index");
+          const fileKeys = scenarioRegistry
+            .getFileKeys()
+            .filter((k) => k !== "index");
+          const topLevelPrefixes = [
+            ...new Set(fileKeys.map((k) => k.split("/")[0] + "/")),
+          ];
+          const allOptions = [...indexFunctions, ...topLevelPrefixes];
+          const matches = allOptions.filter((c) => c.startsWith(partial));
+
+          callback(null, [matches, partial]);
+        } else {
+          // Has slash: complete exports from the named file key
+          const fileKey = partial.slice(0, slashIdx);
+          const funcPartial = partial.slice(slashIdx + 1);
+          const functions = scenarioRegistry.getExportedFunctionNames(fileKey);
+          const matches = functions
+            .filter((e) => e.startsWith(funcPartial))
+            .map((e) => `${fileKey}/${e}`);
+
+          callback(null, [matches, partial]);
+        }
+      } else {
+        callback(null, [[], partial]);
+      }
+
+      return;
+    }
+
     // Check for RouteBuilder method completion: route("..."). or chained calls
     const builderMatch = line.match(/route\(.*\)\.(?<partial>[a-zA-Z]*)$/u);
 
@@ -76,6 +127,7 @@ export function startRepl(
   config: Config,
   print = printToStdout,
   openApiDocument?: OpenApiDocument,
+  scenarioRegistry?: ScenarioRegistry,
 ) {
   function printProxyStatus() {
     if (config.proxyUrl === "") {
@@ -148,7 +200,11 @@ export function startRepl(
 
   // completer is typed as readonly in @types/node but is writable at runtime
   // eslint-disable-next-line @typescript-eslint/no-explicit-any
-  (replServer as any).completer = createCompleter(registry, builtinCompleter);
+  (replServer as any).completer = createCompleter(
+    registry,
+    builtinCompleter,
+    scenarioRegistry,
+  );
 
   replServer.defineCommand("counterfact", {
     action() {
@@ -214,5 +270,75 @@ export function startRepl(
     openApiDocument,
   );
 
+  replServer.context.routes = {};
+
+  replServer.defineCommand("apply", {
+    async action(text: string) {
+      const parts = text.trim().split("/").filter(Boolean);
+
+      if (parts.length === 0) {
+        print("usage: .apply <path>");
+        this.clearBufferedCommand();
+        this.displayPrompt();
+        return;
+      }
+
+      if (parts.some((part) => part === ".." || part === ".")) {
+        print("Error: Path must not contain '.' or '..' segments");
+        this.clearBufferedCommand();
+        this.displayPrompt();
+        return;
+      }
+
+      const functionName = parts[parts.length - 1] ?? "";
+      const fileKey =
+        parts.length === 1 ? "index" : parts.slice(0, -1).join("/");
+
+      const module = scenarioRegistry?.getModule(fileKey);
+
+      if (module === undefined) {
+        print(`Error: Could not find scenario file "${fileKey}"`);
+        this.clearBufferedCommand();
+        this.displayPrompt();
+        return;
+      }
+
+      const fn = module[functionName];
+
+      if (typeof fn !== "function") {
+        print(
+          `Error: "${functionName}" is not a function exported from "${fileKey}"`,
+        );
+        this.clearBufferedCommand();
+        this.displayPrompt();
+        return;
+      }
+
+      try {
+        const applyContext = {
+          context: replServer.context["context"] as Record<string, unknown>,
+          loadContext: replServer.context["loadContext"] as (
+            path: string,
+          ) => Record<string, unknown>,
+          route: replServer.context["route"] as (path: string) => unknown,
+          routes: replServer.context["routes"] as Record<string, unknown>,
+        };
+
+        await (fn as (ctx: typeof applyContext) => Promise<void> | void)(
+          applyContext,
+        );
+
+        print(`Applied ${text.trim()}`);
+      } catch (error) {
+        print(`Error: ${String(error)}`);
+      }
+
+      this.clearBufferedCommand();
+      this.displayPrompt();
+    },
+
+    help: 'apply a scenario script (".apply <path>" calls the named export from scenarios/)',
+  });
+
   return replServer;
 }

src/server/context-registry.ts

@@ -40,14 +40,15 @@ function cloneForCache(value: unknown): unknown {
   return clone;
 }
 
-export class ContextRegistry {
+export class ContextRegistry extends EventTarget {
   private readonly entries = new Map<string, Context>();
 
   private readonly cache = new Map<string, Context>();
 
   private readonly seen = new Set<string>();
 
   public constructor() {
+    super();
     this.add("/", {});
   }
 
@@ -67,6 +68,26 @@ export class ContextRegistry {
     this.entries.set(path, context);
 
     this.cache.set(path, cloneForCache(context) as Context);
+
+    this.dispatchEvent(new Event("context-changed"));
+  }
+
+  /**
+   * Removes the context entry for the given path and dispatches a
+   * "context-changed" event so that listeners (e.g. the scenario-context type
+   * generator) can regenerate type files in response to the removal.
+   *
+   * @param path - The route path whose context entry should be deleted
+   *   (e.g. "/pets").
+   */
+  public remove(path: string): void {
+    this.entries.delete(path);
+
+    this.cache.delete(path);
+
+    this.seen.delete(path);
+
+    this.dispatchEvent(new Event("context-changed"));
   }
 
   public find(path: string): Context {