add casting of event and element types

gleam.toml

···
 name = "plinth"
-version = "0.4.13"
+version = "0.4.14"
 description = "Bindings to Node.js and browser platform APIs"
 target = "javascript"
 

src/element_ffi.mjs

···
 import { Ok, Error } from "./gleam.mjs";
 
+export function cast(raw) {
+  if (raw instanceof Element) {
+    return new Ok(raw)
+  } else {
+    return new Error()
+  }
+}
+
 export function getAttribute(element, name) {
   let attribute = element.getAttribute(name);
   if (attribute) {

src/event_ffi.mjs

···
+import { Ok, Error } from "./gleam.mjs";
+
+export function cast(raw) {
+  if (raw instanceof Event) {
+    return new Ok(raw)
+  } else {
+    return new Error()
+  }
+}
+
 export function target(event) {
   return event.target;
 }

src/plinth/browser/element.gleam

···
+import gleam/dynamic.{type Dynamic, DecodeError}
 import gleam/javascript/promise.{type Promise}
 import plinth/browser/event.{type Event}
 
 pub type Element
+
+@external(javascript, "../../element_ffi.mjs", "cast")
+fn do_cast(raw: Dynamic) -> Result(Element, Nil)
+
+pub fn cast(raw) {
+  case do_cast(raw) {
+    Ok(element) -> Ok(element)
+    Error(Nil) -> Error(DecodeError("Element", dynamic.classify(raw), []))
+  }
+}
 
 @external(javascript, "../../element_ffi.mjs", "addEventListener")
 pub fn add_event_listener(

src/plinth/browser/event.gleam

···
-import gleam/dynamic.{type Dynamic}
+import gleam/dynamic.{type Dynamic, DecodeError}
 
 pub type Event
+
+@external(javascript, "../../event_ffi.mjs", "cast")
+fn do_cast(raw: Dynamic) -> Result(Event, Nil)
+
+pub fn cast(raw) {
+  case do_cast(raw) {
+    Ok(event) -> Ok(event)
+    Error(Nil) -> Error(DecodeError("Event", dynamic.classify(raw), []))
+  }
+}
 
 // event only knows about event target not Element/Document etc
 @external(javascript, "../../event_ffi.mjs", "target")