doc: update trait object syntax

Close #358

next/language/methods.md

@@ -135,7 +135,7 @@ MoonBit features a structural trait system for overloading/ad-hoc polymorphism.
 
 In the body of a trait definition, a special type `Self` is used to refer to the type that implements the trait.
 
-### Super trait
+### Extending traits
 
 A trait can depend on other traits, for example:
 
@@ -239,7 +239,7 @@ Here's an example of calling trait `impl` with dot syntax:
 MoonBit supports runtime polymorphism via trait objects.
 If `t` is of type `T`, which implements trait `I`,
 one can pack the methods of `T` that implements `I`, together with `t`,
-into a runtime object via `t as I`.
+into a runtime object via `t as &I`.
 Trait object erases the concrete type of a value,
 so objects created from different concrete types can be put in the same data structure and handled uniformly:
 

next/sources/language/src/trait/top.mbt

@@ -144,10 +144,10 @@ test {
   let duck1 = Duck::make("duck1")
   let duck2 = Duck::make("duck2")
   let fox1 = Fox::make("fox1")
-  let animals : Array[Animal] = [
-    duck1 as Animal,
-    duck2 as Animal,
-    fox1 as Animal,
+  let animals : Array[&Animal] = [
+    duck1 as &Animal,
+    duck2 as &Animal,
+    fox1 as &Animal,
   ]
   inspect!(
     animals.map(fn(animal) { animal.speak() }),
@@ -164,10 +164,10 @@ trait Logger {
 }
 
 trait CanLog {
-  log(Self, Logger) -> Unit
+  log(Self, &Logger) -> Unit
 }
 
-fn Logger::write_object[Obj : CanLog](self : Logger, obj : Obj) -> Unit {
+fn &Logger::write_object[Obj : CanLog](self : &Logger, obj : Obj) -> Unit {
   obj.log(self)
 }