Hi people have you ever encountered this error ``...

# arrow

[itbhp]

Hi people have you ever encountered this error

kotlin.NoWhenBranchMatchedException: null
	at arrow.core.computations.EitherEffect$DefaultImpls.bind(either.kt:20) ~[arrow-core-jvm-1.0.1.jar:?]

I am using the either monad comprehension block and the bind extension function but maybe I am missing something.

I am using arrow version 1.0.1

Basically I have few interface with suspended methods, and one interface exposing a method not suspended (and using jooq in the implementation to handle transactions)

interface Repository {
  suspend fun getBy(userId: UserId): Either<Error, List<DomainObject>>
  suspend fun update(obj: DomainObject) : Either<ErrorResult, Unit>
}

class TransactionMgmt(private val dslContext: DSLContext) {

    fun <T> transaction(
        block: () -> Either<Error, T>
    ): Either<ErrorResult, T> ...

interface Observer {

    suspend fun onExpired(obj: DomainObject): Either<ErrorResult, Unit>
}

class ExpireUseCase(
    private val repository: Repository,
    private val transactionMgmt: TransactionMgmt,
    private val observer: Observer
) : UseCase<Request, Response> {
    override suspend fun execute(request: Request): Either<Error, Response> =
        repository.getBy(request.userId)
            .flatMap { obj ->
                updateAndNotify(obj)
            }

    private suspend fun updateAndNotify(objs: List<DomainObject>): Either<Error, Response> {
        return objs.fold(initial) { acc, obj ->
            transactionMgmt.transaction {
                runBlocking {
                    expire(obj, acc)
                }
            }
        }
    }

    private suspend fun expire(
        obj: DomainObject,
        acc: Either<Error, Response>
    ): Either<Error, Response> = either {
        val expiredObj = obj.copy(status = EXPIRED)
        repository.update(expiredObj).bind()
        observer.onExpired(expiredObj).bind()
        Response(acc.bind().count + 1)
    }

    private val initial: Either<Error, Response> = Response(0).right()
}

Basically I am getting that error in the method

private suspend fun expire(
        obj: DomainObject,
        acc: Either<Error, Response>
    ): Either<Error, Response> = either {
        val expiredObj = obj.copy(status = EXPIRED)
        repository.update(expiredObj).bind()
        observer.onExpired(expiredObj).bind()
        Response(acc.bind().count + 1)
    }

Sorry for the very long post, and maybe I am missing something really stupid but I cannot see it.

(this is a similar version of the actual code I am using, I cannot post the original one)

[itbhp]

Sorry for this, it was indeed a test problem, I was mocking not in the right way the observer using mockito

[itbhp]

what happened was that mockito was not stubbing the mock method because it didn’t got the argument equality, and in that case returned null

[itbhp]

sorry for the silly question

[raulraja]

Hi @itbhp,

Looks by your exception that you are passing a null value where it would otherwise expect a typed value non nullable of Left or Right.
bind is currently defined in the Either.bind effect as an exaustive match over the Either data structure.

public suspend fun <B> Either<E, B>.bind(): B =
    when (this) {
      is Either.Right -> value
      is Left -> control().shift(this@bind)
    }

[raulraja]

if in that case this is null and incorrectly typed it would not be able to match either of the two cases.

[raulraja]

Can’t tell if there is anything causing that to be null but if you can come up with a small repro example on a gist or repo I’ll be happy to look at it.

[itbhp]

Thanks for your answer, it was mockito returning null instead of Unit.right(), I made a mistake in the test arrangement