A classic implementation of the Huet Zipper for singly-linked lists in MoonBit.

• Immutable, functional list (MList)
• Zipper structure for efficient navigation and modification
• Move focus left/right, update, insert, and delete at focus
• Convert between list and zipper
• Move focus to start or end

pub enum MList[T] {
    Nil
    Cons(T, MList[T])
}

pub struct Zipper[T] {
    left : MList[T]   // elements left of focus, reversed
    focus : T         // current element in focus
    right : MList[T]  // elements right of focus
}

• MList::from_array(arr: Array[T]) -> MList[T]
• MList::reverse(self: MList[T]) -> MList[T]
• Zipper::from_list(list: MList[T]) -> Zipper[T]?
• to_list(self: Zipper[T]) -> MList[T]
• current(self: Zipper[T]) -> T
• move_left(self: Zipper[T]) -> Zipper[T]?
• move_right(self: Zipper[T]) -> Zipper[T]?
• update(self: Zipper[T], new_focus: T) -> Zipper[T]
• insert_left(self: Zipper[T], value: T) -> Zipper[T]
• insert_right(self: Zipper[T], value: T) -> Zipper[T]
• delete(self: Zipper[T]) -> Zipper[T]?
• move_to_start(self: Zipper[T]) -> Zipper[T]
• move_to_end(self: Zipper[T]) -> Zipper[T]

let list = MList::from_array([1, 2, 3, 4])
let zipper = from_list(list).unwrap().move_right().unwrap() // focus on 2
let at_end = zipper.move_to_end() // focus on 4
let at_start = at_end.move_to_start() // focus on 1
let result = at_start.to_list() // [1, 2, 3, 4]

Apache-2.0