Use variant for IntersectionObserver root

Author: stephanoskomnenos

src/DOMAPI/Document.js

@@ -458,3 +458,5 @@ external hasStorageAccess: document => promise<bool> = "hasStorageAccess"
 */
 @send
 external requestStorageAccess: document => promise<unit> = "requestStorageAccess"
+
+let isInstanceOf = (_: 't): bool => %raw(`param instanceof Document`)

src/DOMAPI/Document.res

@@ -498,3 +498,5 @@ Returns true if qualifiedName is now present, and false otherwise.
 include Impl({
   type t = element
 })
+
+let isInstanceOf = (_: 't): bool => %raw(`param instanceof Element`)

src/DOMAPI/Element.js

@@ -2,6 +2,9 @@
 
 open DOMAPI
 
+@editor.completeFrom(IntersectionObserverRoot)
+type root
+
 /**
 provides a way to asynchronously observe changes in the intersection of a target element with an ancestor element or with a top-level document's viewport.
 [See IntersectionObserver on MDN](https://developer.mozilla.org/docs/Web/API/IntersectionObserver)
@@ -11,7 +14,7 @@ type intersectionObserver = {
   /**
     [Read more on MDN](https://developer.mozilla.org/docs/Web/API/IntersectionObserver/root)
     */
-  root: Null.t<unknown>,
+  root: root,
   /**
     [Read more on MDN](https://developer.mozilla.org/docs/Web/API/IntersectionObserver/rootMargin)
     */
@@ -58,7 +61,7 @@ type intersectionObserverEntry = {
 }
 
 type intersectionObserverInit = {
-  mutable root?: Null.t<unknown>,
+  mutable root?: root,
   mutable rootMargin?: string,
   mutable threshold?: array<float>,
 }

src/DOMAPI/Element.res

@@ -0,0 +1,22 @@
+open DOMAPI
+open IntersectionObserverAPI
+
+external fromDocument: document => root = "%identity"
+external fromElement: element => root = "%identity"
+external fromNull: root = "null"
+
+type decoded =
+  | Element(element)
+  | Document(document)
+  | Null
+
+let decode = (t: root): decoded => {
+  open Prelude
+  if Element.isInstanceOf(t) {
+    Element(t->unsafeConversation)
+  } else if Document.isInstanceOf(t) {
+    Document(t->unsafeConversation)
+  } else {
+    Null
+  }
+}

src/IntersectionObserverAPI.res

@@ -2,11 +2,21 @@ let observer = IntersectionObserver.make(~callback=(entry, observer) => {
   Console.log2(entry, observer)
 })
 
+let root = Global.document->Document.querySelector("#root")->Null.getUnsafe
+
 let observer2 = IntersectionObserver.make(~callback=(entry, observer) => {
   Console.log2(entry, observer)
-}, ~options={rootMargin: "10px", threshold: [0.1]})
+}, ~options={
+  root: root->IntersectionObserverRoot.fromElement,
+  rootMargin: "10px",
+  threshold: [0.1],
+})
 
-let root2 = observer2.root
+switch observer2.root->IntersectionObserverRoot.decode {
+| Element(_) => Console.log("Element")
+| Document(_) => Console.log("Document")
+| Null => Console.log("Null")
+}
 let rootMargin2 = observer2.rootMargin
 
 let targetElement = Global.document->Document.querySelector("#targetElement")->Null.toOption