diff --git a/07-typestate-ts/main.ts b/07-typestate-ts/main.ts
new file mode 100644
index 0000000..bf032e8
--- /dev/null
+++ b/07-typestate-ts/main.ts
@@ -0,0 +1,30 @@
+type State = "locked" | "unlocked";
+
+class CredentialManager<S extends State> {
+  private readonly state?: S;
+
+  unlock(this: CredentialManager<"locked">, master_key: string) {
+    return this as unknown as CredentialManager<"unlocked">;
+  }
+
+  set(this: CredentialManager<"unlocked">, key: string, value: any) {
+    // ...
+  }
+
+  get(this: CredentialManager<"unlocked">, key: string): string | undefined {
+    // ...
+    return undefined;
+  }
+
+  lock(this: CredentialManager<"unlocked">): CredentialManager<"locked"> {
+    return this as unknown as CredentialManager<"locked">;
+  }
+}
+
+function main2() {
+  let pw_locked = new CredentialManager<"locked">();
+  let pw_unlocked = pw_locked.unlock("psst");
+  pw_unlocked.set("foo", "bar");
+  let val = pw_unlocked.get("foo");
+  pw_locked = pw_unlocked.lock();
+}
diff --git a/07-type-mapping/main.ts b/08-type-mapping/main.ts
similarity index 96%
rename from 07-type-mapping/main.ts
rename to 08-type-mapping/main.ts
index 90030b5..f5dccb4 100644
--- a/07-type-mapping/main.ts
+++ b/08-type-mapping/main.ts
@@ -30,7 +30,7 @@ class WebSocketClient {
   }
 }
 
-function main_ws() {
+function main1() {
   const client = new WebSocketClient();
   client.send(OpCode.PING, {});
 }
diff --git a/08-type-programming/main.ts b/09-type-programming/main.ts
similarity index 98%
rename from 08-type-programming/main.ts
rename to 09-type-programming/main.ts
index 4ada33a..17a5415 100644
--- a/08-type-programming/main.ts
+++ b/09-type-programming/main.ts
@@ -23,7 +23,7 @@ function navigate<P extends string>(path: P, params: ParseUrlParams<P>) {
   // ...
 }
 
-function main() {
+function main3() {
   navigate("/hello", {});
   navigate("/hello/{name}", { name: "foo" });
   navigate("/book/{author}/{id}", { author: "foo", id: "bar" });
diff --git a/Cargo.toml b/Cargo.toml
index 222e172..935cde4 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,2 +1,3 @@
 [workspace]
 members = ["01-problem", "02-runtime-check", "03-builder-pattern", "04-typestate"]
+resolver = "3"