diff --git a/advent-of-ts/day-17.ts b/advent-of-ts/day-17.ts
new file mode 100644
index 0000000..c931a92
--- /dev/null
+++ b/advent-of-ts/day-17.ts
@@ -0,0 +1,52 @@
+const compose =
+  <A extends string, B, C, D>(f: (a: A) => B, g: (b: B) => C, h: (c: C) => D) =>
+  (a: A): D =>
+    h(g(f(a)));
+
+type FirstChar<S extends string> = S extends `${infer First}${string}`
+  ? First
+  : never;
+type FirstItem<Items extends readonly unknown[]> = Items extends readonly [
+  infer First,
+  ...unknown[]
+]
+  ? First
+  : never;
+
+const upperCase: <S extends string>(s: S) => Uppercase<S> = (x) =>
+  x.toUpperCase() as Uppercase<typeof x>;
+const lowerCase: <S extends string>(s: S) => Lowercase<S> = (x) =>
+  x.toLowerCase() as Lowercase<typeof x>;
+const firstChar = <S extends string>(x: S): FirstChar<S> =>
+  x[0] as FirstChar<S>;
+const firstItem = <T extends readonly unknown[]>(x: T): FirstItem<T> =>
+  x[0] as FirstItem<T>;
+const makeTuple = <T extends string>(x: T): [T] => [x] as [T];
+const makeBox = <T>(value: T): { value: T } => ({ value });
+
+// const tuple = makeTuple("hello!" as const); // Type: readonly ["hello!"]
+// const item = firstItem(tuple); // Should resolve to type "hello!"
+// const box = makeBox(item); // Should be { value: "hello!" }
+
+// // Check intermediate inferred types explicitly:
+// type T1_Step1 = ReturnType<typeof makeTuple>; // Expect readonly ["hello!"]
+// type T1_Step2 = ReturnType<typeof firstItem>; // Expect "hello!"
+// type T1_Step3 = ReturnType<typeof makeBox>;
+
+import type { Equal, Expect } from "npm:type-testing";
+
+const t0 = compose(upperCase, makeTuple, makeBox)("hello!").value[0];
+//    ^?
+type t0_actual = typeof t0; // =>
+type t0_expected = "HELLO!"; // =>
+type t0_test = Expect<Equal<t0_actual, t0_expected>>;
+
+const t1 = compose(makeTuple, firstItem, makeBox)("hello!" as const).value;
+type t1_actual = typeof t1; // =>
+type t1_expected = "hello!"; // =>
+type t1_test = Expect<Equal<t1_actual, t1_expected>>;
+
+const t2 = compose(upperCase, firstChar, lowerCase)("hello!");
+type t2_actual = typeof t2; // =>
+type t2_expected = "h"; // =>
+type t2_test = Expect<Equal<t2_actual, t2_expected>>;