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This adds SPDX license headers to all source code files, following the REUSE[1] specification. This does not include any asset files (such as generated JSON files, or textures). While REUSE does support doing so with ".license" files, for now we define these licences using the .reuse/dep5 file. [1]: https://reuse.software/
90 lines
3.9 KiB
Markdown
90 lines
3.9 KiB
Markdown
---
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module: [kind=guide] using_require
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---
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<!--
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SPDX-FileCopyrightText: 2021 The CC: Tweaked Developers
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SPDX-License-Identifier: MPL-2.0
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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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