OptionJsonAdapterFactory.kt

import arrow.core.*
import com.squareup.moshi.JsonAdapter
import com.squareup.moshi.JsonReader
import com.squareup.moshi.JsonWriter
import com.squareup.moshi.Moshi
import java.lang.reflect.ParameterizedType
import java.lang.reflect.Type

class OptionJsonAdapterFactory : JsonAdapter.Factory {
    override fun create(type: Type, annotations: MutableSet<out kotlin.Annotation>, moshi: Moshi): JsonAdapter<*>? {
        return if (type is ParameterizedType && Option::class.java == type.rawType) {
            OptionAdapter(moshi, type.actualTypeArguments[0])
        } else null
    }
    private class OptionAdapter(private val moshi: Moshi, private val type: Type) : JsonAdapter<Option<*>>() {
        private var adapter: JsonAdapter<Any>? = null
        override fun toJson(writer: JsonWriter, value: Option<*>?) {
            when (value) {
                is Some -> {
                    if (adapter == null) {
                        adapter = moshi.adapter(type)
                    }
                    adapter!!.toJson(writer, value.t)
                }
                is None -> writer.nullValue()
            }
        }
        override fun fromJson(reader: JsonReader): Option<*> {
            if (JsonReader.Token.NULL === reader.peek()) {
                reader.skipValue()
                return none<Nothing>()
            }
            if (adapter == null) {
                adapter = moshi.adapter(type)
            }
            return adapter!!.fromJson(reader).toOption()
        }
    }
}

 Moshi.Builder()
        .add(OptionJsonAdapterFactory())
        .add(KotlinJsonAdapterFactory())
        .build()