Copy and paste the wiki guide on require

I wrote the original, so I don't need to feel guilty :)

Closes #565.

doc/guides/using_require.md

@@ -0,0 +1,83 @@
+---
+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`:
+
+```lua {data-snippet=more_term}
+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:
+
+```lua {data-mount=more_term:more_term.lua}
+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](http://lua-users.org/wiki/ModulesTutorial) on the Lua wiki.
+ - [Lua's manual section on @{require}](https://www.lua.org/manual/5.1/manual.html#pdf-require).

src/main/resources/data/computercraft/lua/rom/modules/main/cc/require.lua

@@ -1,22 +1,24 @@
---- This provides a pure Lua implementation of the builtin @{require} function
--- and @{package} library.
---
--- Generally you do not need to use this module - it is injected into the
--- every program's environment. However, it may be useful when building a
--- custom shell or when running programs yourself.
---
--- @module cc.require
--- @since 1.88.0
--- @usage Construct the package and require function, and insert them into a
--- custom environment.
---
---     local r = require "cc.require"
---     local env = setmetatable({}, { __index = _ENV })
---     env.require, env.package = r.make(env, "/")
---
---     -- Now we have our own require function, separate to the original.
---     local r2 = env.require "cc.require"
---     print(r, r2)
+--[[- This provides a pure Lua implementation of the builtin @{require} function
+and @{package} library.
+
+Generally you do not need to use this module - it is injected into the every
+program's environment. However, it may be useful when building a custom shell or
+when running programs yourself.
+
+@module cc.require
+@since 1.88.0
+@see using_require For an introduction on how to use @{require}.
+@usage Construct the package and require function, and insert them into a
+custom environment.
+
+    local r = require "cc.require"
+    local env = setmetatable({}, { __index = _ENV })
+    env.require, env.package = r.make(env, "/")
+
+    -- Now we have our own require function, separate to the original.
+    local r2 = env.require "cc.require"
+    print(r, r2)
+]]
 
 local expect = require and require("cc.expect") or dofile("rom/modules/main/cc/expect.lua")
 local expect = expect.expect

src/web/index.tsx

@@ -26,13 +26,14 @@ const Click = (options: { run: () => void }) =>
 
 type WindowProps = {};
 
-type WindowState = {
-    visible: boolean,
-
-    example: string,
-    exampleIdx: number,
+type Example = {
+    files: { [file: string]: string },
 }
 
+type WindowState = {
+    exampleIdx: number,
+} & ({ visible: false, example: null } | { visible: true, example: Example })
+
 type Touch = { clientX: number, clientY: number };
 
 class Window extends Component<WindowProps, WindowState> {
@@ -41,12 +42,14 @@ class Window extends Component<WindowProps, WindowState> {
     private top: number = 0;
     private dragging?: { downX: number, downY: number, initialX: number, initialY: number };
 
+    private snippets: { [file: string]: string } = {};
+
     constructor(props: WindowProps, context: unknown) {
         super(props, context);
 
         this.state = {
             visible: false,
-            example: "",
+            example: null,
             exampleIdx: 0,
         }
     }
@@ -57,10 +60,16 @@ class Window extends Component<WindowProps, WindowState> {
             const element = elements[i] as HTMLElement;
 
             let example = element.innerText;
+
+            const snippet = element.getAttribute("data-snippet");
+            if (snippet) this.snippets[snippet] = example;
+
             if (element.getAttribute("data-lua-kind") == "expr") {
                 example = exprTemplate.replace("__expr__", example);
             }
-            render(<Click run={this.runExample(example)} />, element);
+
+            const mount = element.getAttribute("data-mount");
+            render(<Click run={this.runExample(example, mount)} />, element);
         }
     }
 
@@ -76,13 +85,13 @@ class Window extends Component<WindowProps, WindowState> {
             </div>
             <div class="computer-container">
                 <Computer key={exampleIdx} files={{
-                    "example.lua": example, ...defaultFiles
+                    ...example!.files, ...defaultFiles
                 }} />
             </div>
         </div> : <div class="example-window example-window-hidden" />;
     }
 
-    private runExample(example: string): () => void {
+    private runExample(example: string, mount: string | null): () => void {
         return () => {
             if (!this.positioned) {
                 this.positioned = true;
@@ -90,9 +99,17 @@ class Window extends Component<WindowProps, WindowState> {
                 this.top = 20;
             }
 
+            const files: { [file: string]: string } = { "example.lua": example };
+            if (mount !== null) {
+                for (const toMount of mount.split(",")) {
+                    const [name, path] = toMount.split(":", 2);
+                    files[path] = this.snippets[name] || "";
+                }
+            }
+
             this.setState(({ exampleIdx }: WindowState) => ({
                 visible: true,
-                example: example,
+                example: { files },
                 exampleIdx: exampleIdx + 1,
             }));
         }