Demo of how the Nothing type in Kotlin can allow a Swift-style guard function

GuardTest.kt

import com.google.common.truth.Truth.assertThat
import org.junit.Test

inline fun <T> guard(receiver: T?, block: () -> Nothing): T {
  if (receiver == null) {
    block()
  }
  return receiver
}

class GuardTest {

  fun example() {
    val myTaco: String? = "taco"
    val myGuardedTaco = guard(myTaco) { return }
    
    // The following bit won't compile because the passed function doesn't return or throw an exception!
//    val myUnguardedTaco = guard(myTaco) {
//      // Not legal because I haven't returned or thrown an exception!
//    }
  }

  fun returnTypeExample(): String {
    val myTaco: String? = "taco"
    return guard(myTaco) { return "No taco :(" }
  }

  @Test
  fun test() {
    val foo = "foo"
    val bar = guard(foo) {
      throw AssertionError("Should never reach here because foo is not null")
    }
    assertThat(bar).isEqualTo("foo")
  }

  @Test
  fun testNull() {
    val foo: String? = null
    val bar = guard(foo) { return }
    throw AssertionError("Should never reach here because foo is null")
  }

}