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A copy of a list never changes when the original does, and copying is cheap

A List, a Map and a string are values, like a struct. saved := cart is an undo point: pushing onto cart afterwards leaves saved alone, and a function that is given a cart can build its own changed cart without touching the caller's. Copies share their storage until one of them changes, and the change copies it first, so keeping a history of a thousand carts costs one copy per edit, not per snapshot. To change the caller's list on purpose, pass &mut cart.items, as a struct field is a place like any other, and write the &mut at the call.

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fn with_item(cart: Cart, item: string) -> Cart: let mut next = cart # ← the function's own copy next.items.push(item) nextfn add_gift(items: &mut List[string]) -> void: items.push("gift card")pub fn main() -> void $ Console: let mut cart = Cart { items: ["lamp"] } saved := cart # ← an undo point cart.items.push("bulb") println("${saved.items.len()} ${cart.items.len()}") updated := with_item(cart, "shade") println("${cart.items.len()} ${updated.items.len()}") add_gift(&mut cart.items) # ← the caller's list changes, and the call says so println(cart.items.len())# Delete `&mut ` from the `add_gift` call and Run. A list is a# value, so passing one hands over a copy:# type-mismatch: in argument: expected &mut List[string], found List[string]# ── plumbing ──struct Cart: items: List[string]