-- This function takes a string (a value type) and a table (reference type).
-- The value and the reference are copied to their respective parameters.
local function f(s, t)
    assert(type(s) == "string", "First parameter must be a string.")
    assert(type(t) == "table",  "Second parameter must be a table.")
    -- Add the first argument to the table.
    t:insert(s)  -- the original 't' will reflect this
    -- Mutate the first argument by reversing it.
    s = s:reverse()  -- the original 's' will not reflect this
end

local s = "pluto"
local t = {"uranus", "neptune"}
f(s, t)
print(t:concat(", "))
print(s)