In Rust these specifically take `FnMut`, a function which can update internal/borrowed state, rather than `Fn` which can't easily. In `map` or `filter` you shouldn't rely on the iteration order so that's not often useful – maybe something 'logically' stateless but which needs a mutable connection/threadpool/cache, or eg a counter which is really an ancillary reduction. There's even `inspect` which is explicitly for such side effects. In `fold`, the order is guaranteed and you could use it for a state machine, a fiddly `zip` with other mutable iterators, etc – something you need to perform the reduction, but which isn't really an output, I think you could reasonably write either

    .fold(init, move |acc, x| {…})  // or
    .fold((state, init), |(state, acc), x| {…}).1