Try<T> computation for RxJava

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Try<T> is a simple sum type representing 2 cases: success and failure.

val success = Success(1)
success() // This will return 1.

val error = IllegalStateException()
val failure = Failure(error)
failure() // This will return the `error`.

val result = getResult() // Returns `Try<String>`
when (result) {
  is Success -> print(result())
  is Failure -> logError(result())
}

fun getTop10NearbyRideShares(val position: Coordinates): Single<List<Ride>> {
  return api.getRideShares(position = position, count = 10)
    .toTry()
}

Corresponding Java usage via compose():

Single<List<Ride>> getTop10NearbyRideShares(final Coordinates position) {
  return api.getRideShares(position, 10)
    .compose(toTrySingle())
}

If you use Kotlin, this library provides 2 extension functions toTry() so that you can compose with Single or Observable.

Whenever the user moves to a new location, you will ask PokémonService to retrieve top 10 nearby pokémons. Under the hood, PokémonService will shoot a network request to a RESTful backend service.

An initial impl can be like:

fun getTop10Pokemons(): Observable<List<Pokemon>> {
  // Assume that you have `getUserLocationStream(): Observable<Location>`.
  return getUserLocationStream()
    .switchMap(userLocation -> pokemonService.retrieveNearbyPokemons(count = 10))
}

But there's problem. If somehow retrieveNearbyPokemons() runs into network errors like, due to no Internet connection, or temporarily unreachable backend service, the stream returned by getTop10Pokemons() will be terminated. For example,

• When the user moves to (1, 2), we got pokemons A, and B from pokemonService.
• When the user moves to (2, 3), pokemonService fails due to IOException. The stream Observable<List<Pokemon>> triggers its onError(), thus terminates.
• Next, when the user moves to (4, 5), we receive no event from getTop10Pokemons() anymore because it already terminated before.

So, using toTry() can solve the problem here:

fun getTop10Pokemons(): Observable<Try<List<Pokemon>>> {
  // Assume that you have `getUserLocationStream(): Observable<Location>`.
  return getUserLocationStream()
    .switchMap(userLocation -> {
      pokemonService.retrieveNearbyPokemons(count = 10)
        // Assume that `isKindOfNetworkError()` is an extension function
        // in form of `(Throwable) -> Boolean`.
        .toTry { it.isKindOfNetworkError() }
    })
}

How can it be used from the outside?

getTop10Pokemons()
  .subscribe {
    when (it) {
      is Success -> showPokemons(it())
      is Failure -> showError(it())
    }
  }

But how can toTry() actually be helpful? Let's go through the emission again:

• When the user moves to (1, 2), we got pokemons A, and B from pokemonService, and they are wrapped into a Success.
• When the user moves to (2, 3), pokemonService fails due to IOException. toTry() will wrap the error into a Failure. But actually getTop10Pokemons() won't terminate at all. The Failure will be emitted via onNext().
• Next, when the user moves to (4, 5), we got pokemons C, and D from pokemonService. getTop10Pokemons() will continue to emit them via a Success again.

Anticipated errors will be emitted via onNext() with Failure while onError() is the place dedicated for unexpected errors. Some example of the 2 kinds of error:

• Anticipated errors: network errors (e.g. no Internet connection, temporarily unreachable backend service), validation errors (e.g. wrong email format).
• Unexpected errors: Mostly due to programmer mistake for example.

// With `Try<T>`
getTop10Pokemons()
  .subscribe({
    when (it) {
      is Success -> showPokemons(it())
      // showError() will handle anticipated errors.
      is Failure -> showError(it())
    }
  }, {
    // Unexpected errors will be reported to Crashlytics
    // for further investigation.
    logErrorViaCrashlytics(it)
  })

vs.

// Without `Try<T>`. Everything goes into `onError()`.
getTop10Pokemons()
  .subscribe({
    showPokemons(it)
  }, { error ->
    when (error) {
      is NoInternetConnectionError -> showNoInternetConnectionError(error)
      is UnreachableServerError -> showUnreachableServerError(error)
      else -> logErrorViaCrashlytics(error)
    }
  })