# Lua modules

For the getting started guide, see Introduction to `luajr`.

## Introduction

This vignette shows you how to integrate Lua code into your R package, either for internal use within the package or for direct use by your package’s users.

Generally when you want to include Lua code in your package, it means that you want to include one or more functions written in Lua either internally in your package or as an outward-facing function for your users.

In Lua, the standard way to write a “module” – Lua’s equivalent of R’s packages – is to put together a Lua script that returns a Lua table, where the table contains all the functions (and variables) that the module should export. The script can also contain local functions or variables that are accessible only within the module itself. The module is then loaded with Lua’s `require` function. For example, you may have a script `mymodule.lua` containing this Lua code:

    local mymodule = {}

    mymodule.fave_name = "Nick"

    function mymodule.greet(name)
        print("Hello, " .. name .. "!")
        if name == mymodule.fave_name then
            print("Incidentally, that's a great name. Nice one.")
        end
    end

    return mymodule

This script returns a Lua table containing the function `greet`, which in turn makes use of the module variable `fave_name`. In plain Lua, to use this module, you would write something like:

    local mymod = require "mymodule"
    mymod.greet("Janet")

Here, Lua’s `require` looks for a script called `mymodule.lua` in a few places, including the current working directory. For more information on Lua modules, see the Lua wiki and the Lua manual.

You can use a similar technique to load a Lua module into R using luajr as follows. The recommendation is that you put `mymodule.lua` in your package directory under the subdirectory `inst/Lua`. Suppose your package is called `"mypackage"` and your module file is `inst/Lua/mymodule.lua`. Then, somewhere in your R package scripts, declare the module using `lua_module()`:

    # Package module
    mymod <- lua_module("Lua/mymodule.lua", package = "mypackage")

and also declare any functions using `lua_import()`:

    #' Greet the user
    #'
    #' This function gives a cheerful greeting to the user.
    #'
    #' @export
    greet <- function(name) lua_import(mymod, "greet", "native")

You need to have this special function definition to expose the Lua function to your package users and also to specify the argcode (see `lua_func()`) for the function. As shown, you can document and export the function as normal, e.g. with `roxygen2` syntax in the example above.

Note that `lua_import(...)` must be the only statement comprising the function body. Within the call to `lua_import()`, we are supplying (1) the previously-declared module, (2) the name of the Lua function within the module, and (3) an argcode to define how the function parameters will be interpreted as Lua variables (see `lua_func()` for documentation on argcodes).

With the example above, the first time your package user calls `greet()`, the call to `lua_import()` will make sure the module is properly loaded and then will call the Lua function. It will also overwrite the existing body of `greet()` with a direct call to the Lua function so that subsequent calls to `greet()` execute as quickly as possible.

In some cases, you may want to do some processing or checking of function arguments in R before calling the Lua function. You can do that with a “two-step” process like this:

    greet0 <- function(name) lua_import(mymod, "greet", "native")
    greet <- function(name) {
        if (!is.character(name)) {
            stop("greet expects a character string.")
        }
        greet0(name)
    }

You can also manipulate non-function values within your module. For example, if you wanted to change the “special” name recognized by `greet()`, you could write in R:

    mymod["fave_name"] <- "Janet"

For more details, see the documentation for `lua_module()`.

Finally, a word on Lua states. By default, any module is automatically loaded into a newly-created Lua state that is specific to that module. There may be times when you wish to load the module into a specific state, such as the default `NULL` state or anything else. (See `lua_open()` for more details about Lua states.)

To do this in your package, you need to assign this state to `mymod$L` after the module is declared but before any functions or variables from the module have been used. This is easy to do by using `.onLoad()`. Simply include in your package:

    .onLoad <- function(libname, pkgname)
    {
        mymod$L <- NULL
    }

This assigns `NULL` (i.e. the default Lua state shared with the user of your package) to the state used by the module, but you can also supply another state, e.g. one opened by `lua_open()`.