By default, functions are visible outside of their module. You may restrict the visibility of a function by using the local keyword:

module Foo

local function a = {
  return 666
}

function b = {
  return a()
}

Here, b is visible while a can only be invoked from within the Foo module. Given another module called Bogus, the following would fail at runtime:

module Bogus

function i_will_crash = {
  return Foo.a()
}