A closure that keeps state 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.
Write make_counter(start) that returns a closure; each call adds 1 and returns the new count. The closure changes what it captured, so it is FnMut rather than Fn, and each counter keeps its own count.
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.