self-referential function to implement state machine. [reference](https://commandcenter.blogspot.com/2014/01/self-referential-functions-and-design.html)

self-referential.go

package main

import "fmt"

//
// https://commandcenter.blogspot.com/2014/01/self-referential-functions-and-design.html
//
// self-referential definition
// recursive definition
// it's like linked list
//
// type Node struct {
// 	next *Node
// }
//
type node func() node

func addOne(val, threshold int) node {
	val += 1
	fmt.Println(val)

	return func() node {
		if val >= threshold {
			return nil
		}
		return addOne(val, threshold)
	}
}

func main() {
	// get the head, start from 0, end at 10
	node := addOne(0, 10)
	for node != nil {
		// next node
		node = node()
	}
}

self_referential.rs

// https://users.rust-lang.org/t/self-referential-functions/60161?u=jarvi-izana

struct State {
    val: i32,
    end: i32,
}

pub struct StateFn(Box<dyn FnOnce(&mut State) -> Option<StateFn>>);

fn add_one() -> StateFn {
    StateFn(Box::new(|state| {
        println!("{}", state.val);

        if state.val >= state.end {
            return None;
        }

        state.val += 1;
        Some(add_one())
    }))
}

fn main() {
    let mut state = State { val: 0, end: 10 };

    let mut state_fn = add_one();
    while let Some(next_state_fn) = state_fn.0(&mut state) {
        state_fn = next_state_fn
    }
}