Hello guys, trying to convert `Retrofit` responses...

# arrow

[rcd27]

Hello guys, trying to convert Retrofit responses to arrow-kt Either monad:

class JSONRpcResponseEnvelopeConverter<T>(
    private val delegate: Converter<ResponseBody, JSONRpcResponseEnvelope<Any>>
) : Converter<ResponseBody, Either<Throwable, T>> {

    override fun convert(value: ResponseBody): Either<Throwable, T> {
        val response = delegate.convert(value)
        val error = response?.error
        return if (error == null) {
            Either.Right(
                response?.result as T
            )
        } else {
            Either.Left(
                InternalServerException(response.error)
            )
        }
    }
}

But it fails with:

java.lang.RuntimeException: Failed to invoke private arrow.core.Either() with no args
	at com.google.gson.internal.ConstructorConstructor$3.construct(ConstructorConstructor.java:113)
	at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:212)
	at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.read(ReflectiveTypeAdapterFactory.java:131)
	at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:222)
	at retrofit2.converter.gson.GsonResponseBodyConverter.convert(GsonResponseBodyConverter.java:40)
	at retrofit2.converter.gson.GsonResponseBodyConverter.convert(GsonResponseBodyConverter.java:27)
	at com.tochka.bank.npd.core.network.JSONRpcResponseEnvelopeConverter.convert(JSONRpcResponseEnvelopeConverter.kt:12)
	at com.tochka.bank.npd.core.network.JSONRpcResponseEnvelopeConverter.convert(JSONRpcResponseEnvelopeConverter.kt:7)
	at retrofit2.OkHttpCall.parseResponse(OkHttpCall.java:243)
	at retrofit2.OkHttpCall$1.onResponse(OkHttpCall.java:153)
	at okhttp3.internal.connection.RealCall$AsyncCall.run(RealCall.kt:519)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641)
	at java.lang.Thread.run(Thread.java:923)
Caused by: java.lang.InstantiationException: Can't instantiate abstract class arrow.core.Either
	at java.lang.reflect.Constructor.newInstance0(Native Method)
	at java.lang.reflect.Constructor.newInstance(Constructor.java:343)
	at com.google.gson.internal.ConstructorConstructor$3.construct(ConstructorConstructor.java:110)
	... 13 more

Maybe there are already proper converters for retrofit2 to arrow-kt?

[raulraja]

Hi @rcd27, There is arrow-core-retrofit in 0.13.2 which includes an adapter for Either https://github.com/arrow-kt/arrow/blob/5a3037b4563289ec3402bfe0006b3affcb82799a/ar[…]otlin/arrow/retrofit/adapter/either/EitherCallAdapterFactory.kt

[raulraja]

You can find more file and classes related to the adapter in the same folder

[rcd27]

@raulraja thanks! This is about call adapter factory, that's good, but I'm trying to extract Either from my Json object, which either has error field or data field...

[rcd27]

I use standard CouroutinesAdapterFactory for retrofit calls -> suspend functions

[raulraja]

I see, sorry I thought this was related to the call adapter. Not sure why it’s failing but it seems by the trace is the actual Gson adapter. You may need a gson type adapter but I don’t use Gson, just a shot in the air 🙂

[raulraja]

Maybe like this but for Either https://stackoverflow.com/questions/53124280/kotlin-arrow-gson-none

[rcd27]

@raulraja since I wanted to unwrap JSONRpc envelope, which looks like

data class JSONRpcResponseEnvelope<T>(
    val id: String,
    val jsonrpc: String,
    val result: T?,
    val error: ApiError?
)

Into Either<ApiError, T> (so the retorfit's interface functions look like fun getUser(...): Either<ApiError, User>), the solution was neither take EitherCallAdapter or implementing GSON deserialization to Either. I added ConverterFactory with Gson delegate. And just checked type parameters. Solution looks like:

class JSONRpcEnvelopeConverterFactory : Converter.Factory() {

    private val gsonConverterFactory = GsonConverterFactory.create(
        GsonBuilder()
            .setFieldNamingPolicy(FieldNamingPolicy.LOWER_CASE_WITH_UNDERSCORES)
            .create()
    )

    override fun responseBodyConverter(
        type: Type,
        annotations: Array<Annotation>,
        retrofit: Retrofit
    ): Converter<ResponseBody, Either<ApiError, Any>> {

        val typeOfInterest =
            (type as ParameterizedType).actualTypeArguments[1] // We take second parameter, because first one is always `ApiError`

        val wrappedType = object : ParameterizedType {
            override fun getRawType(): Type = JSONRpcResponseEnvelope::class.java
            override fun getOwnerType(): Type? = null
            override fun getActualTypeArguments(): Array<Type> = arrayOf(typeOfInterest)
        }

        // Delegate deserializing to GSON 
        val gsonConverter = gsonConverterFactory
            .responseBodyConverter(wrappedType, annotations, retrofit)

        return Converter<ResponseBody, Either<ApiError, Any>> { value ->
            val response =
                (gsonConverter as Converter<ResponseBody, JSONRpcResponseEnvelope<Any>>).convert(
                    value
                )
            val error = response?.error
            if (error == null) {
                Either.Right(response?.result as Any)
            } else {
                Either.Left(response.error)
            }
        }
    }
...
}

[raulraja]

nice!