Copy Update

Model updating an object-style value by calculating a new scalar state.

copy_update.lua

local stock = 
local sold = 3
local remaining = ({stock = stock}).stock - sold
local status = "available"
if remaining <= 0 then
    status = "empty"
end
print("stock=" .. stock)
print("remaining=" .. remaining)
print("status=" .. status)

local stock = 
local sold = 3
local remaining = ({stock = stock}).stock - sold
local status = "available"
if remaining <= 0 then
    status = "empty"
end
print("stock=" .. stock)
print("remaining=" .. remaining)
print("status=" .. status)

local stock = 
local sold = 3
local remaining = ({stock = stock}).stock - sold
local status = "available"
if remaining <= 0 then
    status = "empty"
end
print("stock=" .. stock)
print("remaining=" .. remaining)
print("status=" .. status)

copy update Instead of mutating a table in place, calculate the next value from the current field and keep the result explicit.