Values never change

Every value is immutable. Passing a value to a function or returning it never lets anyone else change it. A local declared with var mut can be pointed at a new value of the same type, but fields cannot be assigned: to change an object, build a new one.

type Product = Object {    name: ProductName    stock: Stock}function restock(product: Product, stock: Stock) -> Product {    return Product {        name: product.name        stock: stock    }}