The error() Function

Sometimes your code needs to stop immediately when something goes wrong. Lua provides the error() function to handle these situations by raising an error and halting script execution.

The error() function takes a message as its argument and stops the program, displaying that message to indicate what went wrong. Here's the basic syntax:

error("Something went wrong!")

This is particularly useful when validating function inputs. For example, if a function expects a positive number but receives a negative one, you can use error() to prevent further execution:

function calculateSquareRoot(num)
    if num < 0 then
        error("Cannot calculate square root of a negative number")
    end
    return math.sqrt(num)
end

When error() is called, the program stops immediately at that point and displays your custom error message. This helps you catch problems early and communicate clearly what went wrong, making your code more reliable and easier to debug.

error() is for real mistakes, such as a function being called with the wrong kind of value. For failures a caller should expect, like a missing item or bad user input, Lua code usually doesn't raise an error. Instead, the function returns nil and a message, and the caller checks the result:

function findItem(inventory, name)
    if inventory[name] == nil then
        return nil, "No item called " .. name
    end
    return inventory[name]
end

local count, err = findItem({potion = 3}, "sword")
if count then
    print("You have " .. count)
else
    print(err)  -- No item called sword
end

Lua's own functions work the same way: tonumber("abc") returns nil, and io.open() returns nil and a message when it can't open a file.