fix: proper option fromjson (#1027)

* fix: proper option fromjson

* fix(json): use Array[val] and null for `to_json` and `from_json`

builtin/json.mbt

@@ -125,7 +125,7 @@ pub fn Map::to_json[K : Show, V : ToJson](self : Map[K, V]) -> Json {
 pub fn Option::to_json[T : ToJson](self : T?) -> Json {
   match self {
     None => Null
-    Some(value) => value.to_json()
+    Some(value) => [value.to_json()]
   }
 }
 

json/from_json.mbt

@@ -145,7 +145,8 @@ pub impl[V : FromJson] FromJson for Map[String, V] with from_json(json, path) {
 pub impl[T : FromJson] FromJson for T? with from_json(json, path) {
   match json {
     Null => None
-    _ => Some(FromJson::from_json!(json, path))
+    Array([value]) => Some(FromJson::from_json!(value, path.add_index(0)))
+    _ => decode_error!(path, "Option::from_json: expected array or null")
   }
 }
 

json/from_json_test.mbt

@@ -314,6 +314,12 @@ test "Option" {
   let u = Some(1).to_json()
   let v : Int? = @json.from_json!(u)
   inspect!(v, content="Some(1)")
+  let nested : Json = (Some(None) : Unit??).to_json()
+  let v : Unit?? = @json.from_json!(nested)
+  inspect!(v, content="Some(None)")
+  let nested : Json = (Some(Some(None)) : Unit???).to_json()
+  let v : Unit??? = @json.from_json!(nested)
+  inspect!(v, content="Some(Some(None))")
 }
 
 test "Result" {

json/to_json_test.mbt

@@ -129,9 +129,40 @@ test "Map::to_json" {
   )
 }
 
+struct Opt {
+  x : Int?
+  t : Int??
+} derive(ToJson)
+
 test "Option::to_json" {
-  inspect!((Some(42) : Int?).to_json(), content="Number(42)")
+  inspect!((Some(42) : Int?).to_json(), content="Array([Number(42)])")
   inspect!((None : Int?).to_json(), content="Null")
+  inspect!((Some(None) : Unit??).to_json(), content="Array([Null])")
+  inspect!(
+    (Some(Some(None)) : Unit???).to_json(),
+    content="Array([Array([Null])])",
+  )
+  let opt = { x: Some(42), t: Some(Some(42)) }
+  inspect!(
+    opt.to_json(),
+    content=
+      #|Object({"x": Array([Number(42)]), "t": Array([Array([Number(42)])])})
+    ,
+  )
+  let opt = { x: None, t: None }
+  inspect!(
+    opt.to_json(),
+    content=
+      #|Object({"x": Null, "t": Null})
+    ,
+  )
+  let opt = { x: None, t: Some(None) }
+  inspect!(
+    opt.to_json(),
+    content=
+      #|Object({"x": Null, "t": Array([Null])})
+    ,
+  )
 }
 
 test "Tuple::to_json" {