Very unclear "Nesting Problem" error

[foxfriends]

Though admittedly the reason an exception was thrown was my own fault, there was not actually a problem with nesting as the message would have me believe, but rather I had implemented a JsonAdapter that did not actually write any value, and then KotlinJsonAdapter continued to try and write other entries.

Here is some code that causes this situation.

sealed class MyType {
  data class MyInt(val int: Int): MyType()
  data class MyString(val string: String): MyType()
}

data class MyContainer(
  val value: MyType?,
  val otherValue: Int
)

object MyAdapter: JsonAdapter<MyType>() {
  @ToJson
  override fun toJson(writer: JsonWriter, request: MyType?) {
    when (request) {
      is MyType.MyInt -> writer.value(request.int)
      is MyType.MyString -> writer.value(request.string)
      // whoops I forgot to handle the null case!
    }
  }

  @FromJson
  override fun fromJson(reader: JsonReader): MyType? {
    TODO("Not relevant")
  }
}

val converter = Moshi.Builder()
  .add(MyAdapter)
  .add(KotlinJsonAdapterFactory())
  .build()

val container = MyContainer(null, 3)

val json = converter
  .adapter(MyContainer::class.java)
  .toJson(container) // Uh oh... Nesting problem!

I think this case is one where it would be preferable to fail earlier, and show a more meaningful error message to the user, such as the "Dangling name: __" one you would get if the MyContainer type did not have a second val in it. This could save someone else in this situation lot of unneeded debugging!

Here is KotlinJsonAdapter where the problem is encountered.

  override fun toJson(writer: JsonWriter, value: T?) {
    if (value == null) throw NullPointerException("value == null")

    writer.beginObject()
    for (binding in bindings) {
      if (binding == null) continue // Skip constructor parameters that aren't properties.

      writer.name(binding.name)
      binding.adapter.toJson(writer, binding.get(value))
      // Suggestion: detect if a value as not actually been written around here
      //   and throw an exception!
      // Alternatively, just use a better error message in the Writer implementation
      //   when deferredName is not null
    }
    writer.endObject()
  }