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84 lines
3.8 KiB
Markdown
84 lines
3.8 KiB
Markdown
---
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module: [kind=guide] using_require
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---
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# Reusing code with require
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A library is a collection of useful functions and other definitions which is stored separately to your main program. You
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might want to create a library because you have some functions which are used in multiple programs, or just to split
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your program into multiple more modular files.
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Let's say we want to create a small library to make working with the @{term|terminal} a little easier. We'll provide two
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functions: `reset`, which clears the terminal and sets the cursor to (1, 1), and `write_center`, which prints some text
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in the middle of the screen.
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Start off by creating a file called `more_term.lua`:
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```lua {data-snippet=more_term}
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local function reset()
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term.clear()
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term.setCursorPos(1, 1)
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end
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local function write_center(text)
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local x, y = term.getCursorPos()
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local width, height = term.getSize()
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term.setCursorPos(math.floor((width - #text) / 2) + 1, y)
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term.write(text)
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end
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return { reset = reset, write_center = write_center }
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```
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Now, what's going on here? We define our two functions as one might expect, and then at the bottom return a table with
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the two functions. When we require this library, this table is what is returned. With that, we can then call the
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original functions. Now create a new file, with the following:
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```lua {data-mount=more_term:more_term.lua}
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local more_term = require("more_term")
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more_term.reset()
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more_term.write_center("Hello, world!")
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```
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When run, this'll clear the screen and print some text in the middle of the first line.
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## require in depth
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While the previous section is a good introduction to how @{require} operates, there are a couple of remaining points
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which are worth mentioning for more advanced usage.
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### Libraries can return anything
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In our above example, we return a table containing the functions we want to expose. However, it's worth pointing out
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that you can return ''anything'' from your library - a table, a function or even just a string! @{require} treats them
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all the same, and just returns whatever your library provides.
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### Module resolution and the package path
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In the above examples, we defined our library in a file, and @{require} read from it. While this is what you'll do most
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of the time, it is possible to make @{require} look elsewhere for your library, such as downloading from a website or
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loading from an in-memory library store.
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As a result, the *module name* you pass to @{require} doesn't correspond to a file path. One common mistake is to load
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code from a sub-directory using `require("folder/library")` or even `require("folder/library.lua")`, neither of which
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will do quite what you expect.
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When loading libraries (also referred to as *modules*) from files, @{require} searches along the *@{package.path|module
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path}*. By default, this looks something like:
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* `?.lua`
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* `?/init.lua`
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* `/rom/modules/main/?.lua`
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* etc...
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When you call `require("my_library")`, @{require} replaces the `?` in each element of the path with your module name, and
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checks if the file exists. In this case, we'd look for `my_library.lua`, `my_library/init.lua`,
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`/rom/modules/main/my_library.lua` and so on. Note that this works *relative to the current program*, so if your
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program is actually called `folder/program`, then we'll look for `folder/my_library.lua`, etc...
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One other caveat is loading libraries from sub-directories. For instance, say we have a file
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`my/fancy/library.lua`. This can be loaded by using `require("my.fancy.library")` - the '.'s are replaced with '/'
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before we start looking for the library.
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## External links
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There are several external resources which go into require in a little more detail:
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- The [Lua Module tutorial](http://lua-users.org/wiki/ModulesTutorial) on the Lua wiki.
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- [Lua's manual section on @{require}](https://www.lua.org/manual/5.1/manual.html#pdf-require).
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