pub type MyFn[T] = fn (mut my_state T)

fn higher_order_fn[T](initial_state T, callback MyFn[T]) T {
    mut my_state := initial_state

    callback[T](mut my_state)
    callback[T](mut my_state)

    return my_state
}

pub struct Stateless {}

pub fn higher_order_fn_stateless(callback MyFn[Stateless]) {
    initial_state := Stateless{}
    higher_order_fn[Stateless](initial_state, callback)
}

pub struct State {
mut:
    x int
}

fn handler(mut my_state State) {
    my_state.x += 1
}

fn test_main() {
    initial_state := State{}
    new_state := higher_order_fn[State](initial_state, handler)
    assert new_state.x == 2
}