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Appendix M: Chapter 14 Exercise Solutions Written by Massimo Carli

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Exercise 14.1

ResultAp<E, T> is very similar to Either<E, T>. Can you implement flatMap for it as well?

Exercise 14.1 solution

A possible flatMap implementation for ResultAp<E, T> is the following:

fun <E : Throwable, B, D> ResultAp<E, B>.flatMap(
  fn: (B) -> ResultAp<E, D>
): ResultAp<E, D> = // 1
  when (this) {
    is Error<E> -> ResultAp.error(error) // 2
    is Success<B> -> { // 3
      val result = fn(value)
      when (result) {
        is Error<E> -> ResultAp.error(result.error) // 4
        is Success<D> -> ResultAp.success(result.value) // 4
      }
    }
  }

Exercise 14.2

In the previous paragraphs, you implemented a simple system to fetch and parse data using both Optional<T> and Either<E, T>. Can you do the same using ResultAp<E, T>?

Exercise 14.2 solution

In this case, you have to follow the same process you did in the chapter. Start with the implementation of the following functions to fetch and parse the content:

fun fetchTvShowResultAp(
  query: String
): ResultAp<IOException, String> = try {
  ResultAp.success(TvShowFetcher.fetch(query))
} catch (ioe: IOException) {
  ResultAp.error(ioe)
}

/** Invokes the parser returning a ResultAp<E, T> */
fun parseTvShowResultAp(
  json: String
): ResultAp<SerializationException, List<ScoredShow>> = try {
  ResultAp.success(TvShowParser.parse(json))
} catch (e: SerializationException) {
  ResultAp.error(e)
}
fun fetchAndParseTvShowResultAp(query: String) =
  fetchTvShowResultAp(query)
    .flatMap(::parseTvShowResultAp)
fun main() {
  fetchAndParseTvShowResultAp("Big Bang Theory")
    .errorMap {
      println("Error: $it")
      it
    }
    .successMap {
      println("Result: $it")
    }
}

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