Home Android & Kotlin Books Functional Programming in Kotlin by Tutorials

L
Appendix L: Chapter 13 Exercise Solutions Written by Massimo Carli

Heads up... You're reading this book for free, with parts of this chapter shown beyond this point as scrambled text.

You can unlock the rest of this book, and our entire catalogue of books and videos, with a raywenderlich.com Professional subscription.

Exercise 13.1

How would you make the Optional<T> data type you created in Chapter 9, “Data Types”, a monad?

Exercise 13.1 solution

In Chapter 9, “Data Types” you implemented the Optional<T> type like this:

sealed class Optional<out T> {
  companion object {
    @JvmStatic
    fun <T> lift(value: T): Optional<T> = Some(value)

    @JvmStatic
    fun <T> empty(): Optional<T> = None
  }
}

object None : Optional<Nothing>()
data class Some<T>(val value: T) : Optional<T>()
fun <A, B> Optional<A>.map(fn: Fun<A, B>): Optional<B> =
  when (this) {
    is Some<A> -> Some(fn(this.value))
    is None -> None
  }
fun <T> Optional<Optional<T>>.optionalFlatten(): Optional<T> = when (this) {
  is Some<Optional<T>> -> when (this.value) {
    is Some<T> -> Optional.lift<T>(this.value.value)
    is None -> Optional.empty()
  }
  is None -> Optional.empty()
}
infix fun <B, C> Optional<B>.optionalBind(
  g: Fun<B, Optional<C>>
): Optional<C> =
  map(g).optionalFlatten()
infix fun <A, B, C> Fun<A, Optional<B>>.optionalFish(
  g: Fun<B, Optional<C>>
): Fun<A, Optional<C>> = { a: A ->
  this(a).optionalBind(g)
}

Exercise 13.2

What’s the relation between the fish operator, >=>, and flatMap? Can you express the latter in terms of the former for Optional<T>?

Exercise 13.2 solution

The >=> operator for Optional<T> has type:

((A) -> Optional<B>, (B) -> Optional<C>) -> (A) -> Optional<C>
fun <A> Optional<A>.lift(value: A): Optional<A> =
  Optional.lift(value) // 1

fun <A, B> Optional<A>.flatMap(fn: Fun<A, Optional<B>>): Optional<B> =
  map(::lift optionalFish fn).optionalFlatten() // 2

Have a technical question? Want to report a bug? You can ask questions and report bugs to the book authors in our official book forum here.

© 2022 Razeware LLC

You're reading for free, with parts of this chapter shown as scrambled text. Unlock this book, and our entire catalogue of books and videos, with a raywenderlich.com Professional subscription.

Unlock Now

To highlight or take notes, you’ll need to own this book in a subscription or purchased by itself.