How to extend a Lua library with access to it's local functions?

Question

How would one extend a library in Lua such that:

• don't touch the file you're extending
• want to add another method to the class
• want to have your own custom name for the library class (i.e. not just "M")
• KEY: have the ability for the function you add to have access to the library's local functions. So it's as if you put the function directly into the 3rd party Library, but in fact it's not, you've separated it out into another file (your own) to avoid issues when the library file is updated
• for your info: using Corona SDK in this case

Example below: Here there is an error: "Attempt to call global 'getANumber' (a nil value)". Any better ways to do it welcomed too in terms of approach.

main.lua

local myLibrary = require("library")
local myLibraryExtension = require("myLibraryExtension")
myLibraryExtension:extend(myLibrary)

myLibrary:doX()
myLibrary:doY()   -- <=== ERROR HERE

library.lua

local M = {}

local function getANumber()
    return 55
end

function M:doX()
    print("X", getANumber())
end

return M

myLibraryExtension.lua

local M = {}

local function doY()
    print("X", getANumber())
end

function M:extend(sourceClass)
    sourceClass.doY = doY
end

return M

Answer 1

You can not call the getANumber function from your myLibraryExtension.lua file. The local function exists only in the library.lua scope and can't be used outside for the reason as explained in comments by hugomg. If you want access to private (read local) function, you can modify scripts like this:

library.lua

Add the following statement before return M

M.getANumber = getANumber

myLibraryExtension.lua

Modify the function doY a little:

local function doY(this)
    print("X", this.getANumber())
end

---

What happens is, when you execute the statement

myLibrary:doY()

lua executes it as follows:

myLibrary.doY( myLibrary ) -- yes, myLibrary is passed as an argument too

You can use the passed reference to the library to your benefit by extending the doY function to accept an argument as well (this in my case).

This object can then have the access to the desired local function.