Confusion about Result.zip extensions

[frko]

I have been looking at

arrow/arrow-libs/core/arrow-core/src/commonMain/kotlin/arrow/core/Result.kt

Line 181 in 5d0bc9e

)?.let { success(it) } ?: composeErrors(

for a while and I am slightly confused about its implementation.

I wonder if it is intentional that the implementation will throw an NPE when a Result contains null as success value, or, at least I think it does looking at the source.

Regards, Frank

[nomisRev]

Hey Frank,

Okay, I see what you mean, and this is definitely a bug.

val r = Result.success<Int?>(null)
r.zip(r) { x, y -> (x?.plus(y ?: 0)) ?: -1 } // shouldBe -1 but is java.lang.NullPointerException

The proper implementation would check for isSuccess and not rely on null for success, similar to how Result works internally.

inline fun <A, B, C> Result<A>.zip(b: Result<B>, transform: (A, B) -> C): Result<C> =
  if (isSuccess && b.isSuccess) Result.success(transform(getOrNull() as A, b.getOrNull() as B))
  else Result.failure((exceptionOrNull()?.apply { addSuppressed(b.exceptionOrNull()) } ?: b.exceptionOrNull())!!)

[frko]

Great you looked into this that fast .

Was wondering about the usage of suppressed for exception collection here as it feels off to me semantically. Is this to prevent 'yet another exception' to be created with it's own stacktrace while in fact the situation it is handling is not exceptional?

[nomisRev]

Hey @frko,

It's to prevent a stack-trace from getting lost. Let's take following example:

val runtime = Result.failure<Int>(RuntimeException("BOOM!"))
val state = Result.failure<Int>(IllegalStateException("Wrong Builder State!"))

fun main() {
  val res = runtime.zip(state) { x, y -> x + y }.getOrThrow()
  println(res)
}

Without addSuppressed of state into runtime we will get:

Exception in thread "main" java.lang.RuntimeException: BOOM!
	at arrow.core.continuations.MainKt.<clinit>(Main.kt:8)

If we add the suppression:

Exception in thread "main" java.lang.RuntimeException: BOOM!
	at arrow.core.continuations.MainKt.<clinit>(Main.kt:8)
	Suppressed: java.lang.IllegalStateException: Wrong Builder State!
		at arrow.core.continuations.MainKt.<clinit>(Main.kt:9)

So through addSuppressed we make sure that no information is lost when combining different Result.

[nomisRev]

I opened a PR with a fix #2804