CC-Tweaked/doc/guides/using_require.md

module: [kind=guide] using_require

Reusing code with require

A library is a collection of useful functions and other definitions which is stored separately to your main program. You might want to create a library because you have some functions which are used in multiple programs, or just to split your program into multiple more modular files.

Let's say we want to create a small library to make working with the @{term|terminal} a little easier. We'll provide two functions: reset, which clears the terminal and sets the cursor to (1, 1), and write_center, which prints some text in the middle of the screen.

Start off by creating a file called more_term.lua:

local function reset()
  term.clear()
  term.setCursorPos(1, 1)
end

local function write_center(text)
  local x, y = term.getCursorPos()
  local width, height = term.getSize()
  term.setCursorPos(math.floor((width - #text) / 2) + 1, y)
  term.write(text)
end

return { reset = reset, write_center = write_center }

Now, what's going on here? We define our two functions as one might expect, and then at the bottom return a table with the two functions. When we require this library, this table is what is returned. With that, we can then call the original functions. Now create a new file, with the following:

local more_term = require("more_term")
more_term.reset()
more_term.write_center("Hello, world!")

When run, this'll clear the screen and print some text in the middle of the first line.

require in depth

While the previous section is a good introduction to how @{require} operates, there are a couple of remaining points which are worth mentioning for more advanced usage.

Libraries can return anything

In our above example, we return a table containing the functions we want to expose. However, it's worth pointing out that you can return ''anything'' from your library - a table, a function or even just a string! @{require} treats them all the same, and just returns whatever your library provides.

Module resolution and the package path

In the above examples, we defined our library in a file, and @{require} read from it. While this is what you'll do most of the time, it is possible to make @{require} look elsewhere for your library, such as downloading from a website or loading from an in-memory library store.

As a result, the module name you pass to @{require} doesn't correspond to a file path. One common mistake is to load code from a sub-directory using require("folder/library") or even require("folder/library.lua"), neither of which will do quite what you expect.

When loading libraries (also referred to as modules) from files, @{require} searches along the @{package.path|module path}. By default, this looks something like:

• ?.lua
• ?/init.lua
• /rom/modules/main/?.lua
• etc...

When you call require("my_library"), @{require} replaces the ? in each element of the path with your module name, and checks if the file exists. In this case, we'd look for my_library.lua, my_library/init.lua, /rom/modules/main/my_library.lua and so on. Note that this works relative to the current program, so if your program is actually called folder/program, then we'll look for folder/my_library.lua, etc...

One other caveat is loading libraries from sub-directories. For instance, say we have a file my/fancy/library.lua. This can be loaded by using require("my.fancy.library") - the '.'s are replaced with '/' before we start looking for the library.

External links

There are several external resources which go into require in a little more detail:

• The Lua Module tutorial on the Lua wiki.
• Lua's manual section on @{require}.