Syntax lookup: Include

Author: kevanstannard

misc_docs/syntax/language_include.mdx

@@ -0,0 +1,100 @@
+---
+id: "include"
+keywords: ["include", "module"]
+name: "include"
+summary: "This is the `include` keyword."
+category: "languageconstructs"
+---
+
+Using `include` in a module statically "spreads" another module's content into the module, thus often fulfills the role of "inheritance" or "mixin".
+
+### Include module example
+
+<CodeTab labels={["ReScript", "JS Output"]}>
+
+```res
+module Message = {
+  let greeting = "Hello"
+}
+
+module Greeter = {
+  include Message
+  let greet = name => greeting ++ " " ++ name
+}
+```
+
+```js
+var greeting = "Hello";
+
+var Message = {
+  greeting: greeting,
+};
+
+function greet(name) {
+  return "Hello " + name;
+}
+
+var Greeter = {
+  greeting: greeting,
+  greet: greet,
+};
+```
+
+</CodeTab>
+
+Similarly, module signatures can also be extended by other module signatures.
+
+### Include signature example
+
+<CodeTab labels={["ReScript", "JS Output"]}>
+
+```res
+module type Message = {
+  let greeting: string
+}
+
+module type Greeter = {
+  include Message
+  let greet: string => string
+}
+```
+
+```js
+// No output
+```
+
+</CodeTab>
+
+Lastly, you can also extract module types from existing modules using `module type of`.
+
+### Include "module type of" example
+
+<CodeTab labels={["ReScript", "JS Output"]}>
+
+```res
+module Message = {
+  let greeting = "Hello"
+}
+
+module type Greeter = {
+  // Includes the type definitions of Message
+  include module type of Message
+  let greet: string => string
+}
+```
+
+```js
+var Message = {
+  greeting: "Hello"
+};
+```
+
+</CodeTab>
+
+> Note that use of `include` is heavily discouraged and should only be used as a last resort.
+
+### References
+
+* [Extending modules](/docs/manual/latest/module#extending-modules)
+* [Extending module signatures](/module#extending-module-signatures)
+