OptionalExercises.kt

package ft.typesafety

import arrow.core.Option
import arrow.core.getOrElse

/**
 * Use pattern matching and recursion.  No vars, no loops, no overriding.
 *
 * `Option` is an implementation of optional functionality.
 *
 * We've made these exercises to give you greater insight into how an optional pattern
 * might work in a functional language.
 *
 * When you see `Option` think: "It may exist, or it may not"
 *
 * There are two ways to construct an `Option`:
 *
 *   `Some()` represents something that exists
 *
 *   `None` represents something that doesn't exist
 *
 * We use `Option` in situations where there isn't certainty that a meaningful
 * value will be returned to us.
 *
 *
 * TODO - Rewrite these instructions for Kotlin
 *
 *   The `get()` method on the key to value store `Map` is a great example of this.
 *
 *     We expect `get()` to take a key and give us a value in return.
 *
 *     But what happens when our Map doesn't know about the key we gave it?
 *
 *   A Map here is the same as in any other language,
 *   we just need to tell it about the types we're working with.
 *
 *     This is the type of the key
 *                      |
 *                      |     This is the type of the value
 *                      |      |
 *                      |      |
 *     val myMap = mapOf<Int, String>( 1 to "one", 2 to "two", ...)
 *
 *
 *   When we call `get()` on Map we will always get back an `Option` type
 *
 *     myMap.get(1) = Some("one")  //The value exists and it's the string "one"
 *
 *     myMap.get(0) = None      //The value doesn't exist so we get None
 *
 *   `Some("one")` and `None` are both of the type Option
 *
 *   Since `Some` and `None` are the same type we can pattern match on them!
 *
 *   We can have one set of logic when we get Some back and a different set
 *   of logic when we get `None` back!
 *
 *   val mightBeSomething: Option<String> = myMap.get(3)
 *
 *   val result: String = mightBeSomething match {
 *     case Some(string) => "I got a String back!"
 *     case None => "I got None back"
 *   }
 *
 * Good luck!
 *
 */

object OptionalExercises1 {

  val config = mapOf("host" to "squareup.com", "port" to "8080")

  fun getFromConfig(key: String): Option<String> = Option.fromNullable(config[key])

  fun lengthOfHost(): Option<Int> = getFromConfig("host").map { it.length }

  fun portPlus1000(): Option<Int> = getFromConfig("port").map { it.toInt() + 1000 }
}

object OptionalExercises2 {

  val hosts = mapOf("host1" to "squareup.com", "host2" to "test.squareup.com", "host3" to "netflix.com")
  val envs = mapOf("squareup.com" to "prod", "test.squareup.com" to "test", "amazon.com" to "stage")

  // Should return the env string if successful or "couldn't resolve" if unsuccessful
  fun getEnvForHost(host: String): String = Option.fromNullable(hosts[host])
      .flatMap { Option.fromNullable(envs[it]) }
      .getOrElse { "couldn't resolve" }

  // See how many ways you can implement this.
  // Will either return "Connected to <squareup host>" or "not connected"
  fun connectToSquareupHostsOnly(host: String): String = Option.fromNullable(hosts[host])
      .filter { it.endsWith("squareup.com") }
      .map { createConnection(it) }
      .getOrElse { "not connected" }

  private fun createConnection(domain: String): String = "connected to $domain"
}

/**
 * Here we make the trait `Maybe`, which is our version of `Option`
 *
 * `Just` has the same behavior as `Some`
 * `Nothing` has the same behavior as `None`
 *
 * We use this exercise to illustrate that we can create our own optional behavior
 * with just a few functions.
 *
 */

object OptionalExercises3 {

  interface Maybe<out A>

  data class Just<A>(val get: A) : Maybe<A>

  object Nothing : Maybe<kotlin.Nothing>

  fun <A, B> flatMap(m: Maybe<A>, f: (A) -> Maybe<B>): Maybe<B> = when(m) {
    is Just -> f(m.get)
    else -> Nothing
  }

  fun <A, B> map(m: Maybe<A>, f: (A) -> B): Maybe<B> = when(m) {
    is Just -> Just(f(m.get))
    else -> Nothing
  }

  fun <A, B> fold(m: Maybe<A>, default: () -> B, f: (A) -> B): B = when(m) {
    is Just -> f(m.get)
    else -> default()
  }

  fun <A> orElse(m: Maybe<A>, otherwise: () -> Maybe<A>): Maybe<A> = when(m) {
    is Just -> m
    else -> otherwise()
  }

  fun <A> orSome(m: Maybe<A>, default: () -> A): A = when(m) {
    is Just -> m.get
    else -> default()
  }

  fun <A, B, C> map2(m1: Maybe<A>, m2: Maybe<B>, f: (A, B) -> C): Maybe<C> =
    flatMap(m1) { a -> map(m2) { b -> f(a, b) } }

  fun <A> sequence(l: List<Maybe<A>>): Maybe<List<A>> = l.fold(Just(emptyList())) { acc, next ->
    map2(acc, next) { xs, x -> xs.plus(x) }
  }

  fun <A, B> ap(m1: Maybe<A>, m2: Maybe<(A) -> B>): Maybe<B> = flatMap(m1) { a ->
    map(m2) { f -> f(a) }
  }
}