I'm wondering about the `contract` specified for `...

# arrow

[Huib Donkers]

I'm wondering about the contract specified for Either::getOrNull . It seems incorrect in case B (right) is nullable.
For reference:

public fun getOrNull(): B? {
    contract {
      returns(null) implies (this@Either is Left<A>)
      returnsNotNull() implies (this@Either is Right<B>)
    }
    return getOrElse { null }
  }

But maybe I'm misunderstanding something about contracts (or arrow). I'm actually unable to get my examples to work, seems my setup is wacky..

fun foo(either: Either<String, Int?>) = if (either.isLeft()) "Left(${either.value})" else "Right(${either.value})" // IntelliJ highlights as of this compiles and either is smart cast to Left and Right, but when compiling it gives the error "Unresolved reference: value"

fun bar(either: Either<String, Int?>) = if (either.getOrNull() == null) "Left(${either.value})" else "Right(${either.value})" // IntelliJ highlighting and compiler agree: no smart casting (but what does the contract do then?)

But regardless of my potentially wacky setup, what should happen when I call bar(Either.Right(null))?

[Youssef Shoaib [MOD]]

Huh, I never thought about that. Not sure why your example isn't working btw. I'll try to replicate this issue!

[Youssef Shoaib [MOD]]

It seems like the contract doesn't even work yeah! It means we can easily change it though

[Youssef Shoaib [MOD]]

3 things:
• contracts still don't work if coming from a 3rd-party library
• Arrow's Kotlin version a little bit behind your and my IDE Kotlin version
• Thus, I have to redefine getOrNull and use a new Kotlin version to see this issue, but you're correct, this compiles and fails with a CCE at runtime:

@Test
fun getOrNullNullable() = runTest {
  val either: Either<String, Int?> = Right(null)
  if (either.gon() == null) {
    val left: Either.Left<*> = either
    left.value
  }
}
fun <A, B> Either<A, B>.gon(): B? {
  contract {
    returns(null) implies (this@gon is Left<A>)
    returnsNotNull() implies (this@gon is Right<B>)
  }
  return getOrElse { null }
}

[Huib Donkers]

Ah thanks for looking into this! Great that the trouble in ~re~producing the bug means a whole lot less trouble correcting it 🙂

[Youssef Shoaib [MOD]]

https://github.com/arrow-kt/arrow/pull/3703

[Daniel Pitts]

Wait, contracts in libraries aren't honored?

[Youssef Shoaib [MOD]]

callsInPlace is. implies seems to not be yet. I know that has been a long standing bug

[Daniel Pitts]

Ah, okay.