Add section about extensible variant types.

Author: fhammerschmidt

pages/docs/manual/latest/variant.mdx

@@ -356,3 +356,61 @@ The compiler sees the variant, then
 
 1. conceptually turns them into `type animal = 0 | 1 | 2`
 2. compiles `switch` to a constant-time jump table (`O(1)`).
+
+## Extensible Variant Types
+
+Variants can be made extensible by defining a type using `..` and adding new variant constructors using `+=`.
+
+<CodeTab labels={["ReScript", "JS Output"]}>
+
+```res example
+type t = ..
+
+type t += Other
+
+type t +=
+  | Point(float, float)
+  | Line(float, float, float, float)
+```
+```js
+var Caml_exceptions = require("./stdlib/caml_exceptions.js");
+
+var Other = Caml_exceptions.create("Playground.Other");
+
+var Point = Caml_exceptions.create("Playground.Point");
+
+var Line = Caml_exceptions.create("Playground.Line");
+```
+
+</CodeTab>
+
+Pattern-matching is possible the same way as with normal variants but there is one caveat:
+With extensible variants, the possibility to check for exhaustiveness vanishes, you always need a default case `_` here.
+
+
+<CodeTab labels={["ReScript", "JS Output"]}>
+
+
+```res example
+let print = v =>
+  switch v {
+  | Point(x, y) => Js.log2("Point", (x, y))
+  | Line(ax, ay, bx, by) => Js.log2("Line", (ax, ay, bx, by))
+  | Other
+  | _ =>
+    Js.log("Other")
+  }
+```
+```js
+function print(v) {
+  if (v.RE_EXN_ID === Point) {
+    console.log("Point", [v._1, v._2]);
+  } else if (v.RE_EXN_ID === Line) {
+    console.log("Line", [v._1, v._2, v._3, v._4]);
+  } else {
+    console.log("Other");
+  } 
+}
+```
+
+</CodeTab>