Lightweight Option

lwo.kt

inline fun <A, B> A?.fold(crossinline fn: () -> B, crossinline f: (A) -> B): B =
  if (this == null) fn() else f(this)

inline fun <A, B> A?.map(crossinline f: (A) -> B): B? = fold({ null }, f)

inline fun <A> A?.orElse(crossinline fn: () -> A): A = fold(fn, { it })

val a: Int? = null
val x = a.map { it + 1 }.orElse { 0 }
// 0

val a: Int? = 0
val x = a.map { it + 1 }.orElse { 0 }
// 1