EitherNet Save

A pluggable sealed API result type for modeling Retrofit responses.

Project README


A pluggable sealed API result type for modeling Retrofit responses.


By default, Retrofit uses exceptions to propagate any errors. This library leverages Kotlin sealed types to better model these responses with a type-safe single point of return and no exception handling needed!

The core type for this is ApiResult<out T, out E>, where T is the success type and E is a possible error type.

ApiResult has two sealed subtypes: Success and Failure. Success is typed to T with no error type and Failure is typed to E with no success type. Failure in turn is represented by four sealed subtypes of its own: Failure.NetworkFailure, Failure.ApiFailure, Failure.HttpFailure, and Failure.UnknownFailure. This allows for simple handling of results through a consistent, non-exceptional flow via sealed when branches.

when (val result = myApi.someEndpoint()) {
  is Success -> doSomethingWith(result.response)
  is Failure -> when (result) {
    is NetworkFailure -> showError(result.error)
    is HttpFailure -> showError(result.code)
    is ApiFailure -> showError(result.error)
    is UnknownFailure -> showError(result.error)

Usually, user code for this could just simply show a generic error message for a Failure case, but the sealed subtypes also allow for more specific error messaging or pluggability of error types.

Simply change your endpoint return type to the typed ApiResult and include our call adapter and delegating converter factory.

interface TestApi {
  suspend fun getData(): ApiResult<SuccessResponse, ErrorResponse>

val api = Retrofit.Builder()

If you don't have custom error return types, simply use Unit for the error type.

Decoding Error Bodies

If you want to decode error types in HttpFailures, annotate your endpoint with @DecodeErrorBody:

interface TestApi {
  suspend fun getData(): ApiResult<SuccessResponse, ErrorResponse>

Now a 4xx or 5xx response will try to decode its error body (if any) as ErrorResponse. If you want to contextually decode the error body based on the status code, you can retrieve a @StatusCode annotation from annotations in a custom Retrofit Converter.

// In your own converter factory.
override fun responseBodyConverter(
  type: Type,
  annotations: Array<out Annotation>,
  retrofit: Retrofit
): Converter<ResponseBody, *>? {
  val (statusCode, nextAnnotations) = annotations.statusCode()
    ?: return null
  val errorType = when (statusCode.value) {
    401 -> Unauthorized::class.java
    404 -> NotFound::class.java
    // ...
  val errorDelegate = retrofit.nextResponseBodyConverter<Any>(this, errorType.toType(), nextAnnotations)
  return MyCustomBodyConverter(errorDelegate)

Note that error bodies with a content length of 0 will be skipped.


A common pattern for some APIs is to return a polymorphic 200 response where the data needs to be dynamically parsed. Consider this example:

  "ok": true,
  "data": {

The same API may return this structure in an error event

  "ok": false,
  "error_message": "Please try again."

This is hard to model with a single concrete type, but easy to handle with ApiResult. Simply throw an ApiException with the decoded error type in a custom Retrofit Converter and it will be automatically surfaced as a Failure.ApiFailure type with that error instance.

suspend fun getData(): ApiResult<SuccessResponse, ErrorResponse>

// In your own converter factory.
class ErrorConverterFactory : Converter.Factory() {
  override fun responseBodyConverter(
    type: Type,
    annotations: Array<out Annotation>,
    retrofit: Retrofit
  ): Converter<ResponseBody, *>? {
    // This returns a `@ResultType` instance that can be used to get the error type via toType()
    val (errorType, nextAnnotations) = annotations.errorType() ?: return null
    return ResponseBodyConverter(errorType.toType())

  class ResponseBodyConverter(
    private val errorType: Type
  ) : Converter<ResponseBody, *> {
    override fun convert(value: ResponseBody): String {
      if (value.isErrorType()) {
        val errorResponse = ...
        throw ApiException(errorResponse)
      } else {
        return SuccessResponse(...)


EitherNet ships with a Test Fixtures artifact containing a EitherNetController API to allow for easy testing with EitherNet APIs. This is similar to OkHttp’s MockWebServer, where results can be enqueued for specific endpoints.

Simply create a new controller instance in your test using one of the newEitherNetController() functions.

val controller = newEitherNetController<PandaApi>() // reified type

Then you can access the underlying faked api property from it and pass that on to whatever’s being tested.

// Take the api instance from the controller and pass it to whatever's being tested
val provider = PandaDataProvider(controller.api)

Finally, enqueue results for endpoints as needed.

// Later in a test you can enqueue results for specific endpoints
controller.enqueue(PandaApi::getPandas, ApiResult.success("Po"))

You can also optionally pass in full suspend functions if you need dynamic behavior

controller.enqueue(PandaApi::getPandas) {
  // This is a suspend function!

In instrumentation tests with DI, you can provide the controller and its underlying API in a test module and replace the standard one. This works particularly well with Anvil.

  scope = UserScope::class,
  replaces = [PandaApiModule::class] // Replace the standard module
object TestPandaApiModule {
  fun providePandaApiController(): EitherNetController<PandaApi> = newEitherNetController()

  fun providePandaApi(
    controller: EitherNetController<PandaApi>
  ): PandaApi = controller.api

Then you can inject the controller in your test while users of PandaApi will get your test instance.

Java Interop

For Java interop, there is a limited API available at JavaEitherNetControllers.enqueueFromJava.


EitherNetController will run some small validation on API endpoints under the hood. If you want to add your own validations on top of this, you can provide implementations of ApiValidator via ServiceLoader. See ApiValidator's docs for more information.


Maven Central

dependencies {

  // Test fixtures

Snapshots of the development version are available in Sonatype's snapshots repository.


Copyright 2020 Slack Technologies, LLC

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at


Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
See the License for the specific language governing permissions and
limitations under the License.
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