Compose boxed closures in Rust
Rust interview exercise on Closures & Iterators, difficulty 2 of 3. Solve it in the editor beside a worked example, or cold in Interview mode with the example hidden; the tests run the moment you do.
Every closure has its own type, so a Vec of them needs Box<dyn Fn>. Write compose_all(fns) that returns a function applying fns right to left: compose_all(vec![f, g, h])(x) is f(g(h(x))). An empty Vec gives the identity function.
Each exercise pairs a worked example with a twin problem to solve in a real editor; the tests run the moment you do. Interview mode hides the example: solve it from memory, without the hint or the solution, and it counts as solved cold. Progress is saved in your browser, and in your account if you sign in.