I have the following code. It returns a flow where...

# arrow

[jean]

I have the following code. It returns a flow where I emit a payment status while it’s being processed.
• I start by requesting a qrCode, customers use it to open a payment app preconfigured with payment recipient and sum.
• I then need to poll an api that will answer “not approved” until the customer has pay.
• Once the customer has pay, the process is done
I need to constraint the polling to an arbitrary timeout value, is there a better/cleaner way to do this?

fun processPayment(price: Int) = flow {
    emit(PaymentStatus.Working)
    either {
        val qrCode = getQrCode(price).bind()
        emit(PaymentStatus.ShowQrCode(qrCode.url))
        try {
            withTimeout(twoMinutes()) {
                var paymentIsApproved = false
                while (!paymentIsApproved) {
                    val paymentStatus = getPaymentFromApi(qrCode.reference).bind()
                    paymentIsApproved = paymentStatus.aggregate.authorizedAmount.value == price.toLong()
                    delay(2000)
                }
                emit(PaymentStatus.Success)
            }
        } catch (e: Exception) {
            emit(paymentError(PaymentError.CouldNotVerifyPaymentBeforeTimeout))
        }
    }.mapLeft(::paymentError)
}

[Alejandro Serrano Mena]

my recommendation would be to look at Schedule.retryOrElse https://arrow-kt.io/docs/apidocs/arrow-fx-coroutines/arrow.fx.coroutines/retry-or-else.html

[Alejandro Serrano Mena]

fun processPayment(price: Int) = flow {
    emit(PaymentStatus.Working)
    either {
        val qrCode = getQrCode(price).bind()
        emit(PaymentStatus.ShowQrCode(qrCode.url))
        Schedule.spaced(2.seconds).whileOutput { it < 2.minutes }.retryOrElse {
          fa = {
             val paymentStatus = getPaymentFromApi(qrCode.reference).bind()
             val paymentIsApproved = paymentStatus.aggregate.authorizedAmount.value == price.toLong()
             if (paymentIsApproved) emit(PaymentStatus.Success)
          },
          orElse = {
             emit(paymentError(PaymentError.CouldNotVerifyPaymentBeforeTimeout))
          }
        }
    }.mapLeft(::paymentError)
}

[Alejandro Serrano Mena]

the code above does not compile, it’s just a sketch; but the idea is that using Schedule you can retry something following a policy (like “every 2 seconds, up to 2 minutes”)

[Francis Reynders]

a bit offtopic, but the flow lambda argument type returns Unit so I don't think the Either value is returned. Makes me wonder what is the best way to capture the getQrCode logic failure. Maybe define processPayment with Raise context or extension?

[jean]

@simon.vergauwen do you have an opinion on this?

[simon.vergauwen]

I think the code using Schedule is indeed nicer instead of manually implementing the Schedule, but Francis is also correct in that the `Eitherem`` value is never returned or rather emitted.

[jean]

Shouldn’t I use a else branch after if(paymentApproved) to throw a Throwable ? Otherwise it would not retry, right?

[jean]

And the return type after retryOrElse is Any not Either<PaymentError, PaymentStatus>

[simon.vergauwen]

I am a bit confused by the snippet. do PaymentError and PaymentStatus share a common parent?

[jean]

I guess my naming here wasn't optimal hehe. PaymentStatus is a sealed class with Working, Success and Error as sub-elements. Error takes a PaymentError as parameter. It can be Timeout or whatever comes from the bind()

[simon.vergauwen]

So the return type is Flow<PaymentStatus> ?

[jean]

Well I only care about the final result. So success or whatever was the reason for it to fail. And I would emit that value int flow { ... } That contains the schedule logic

[simon.vergauwen]

So you only care about the final value? 🤔 So why use Flow?

[jean]

I use the flow to emit the status of the payment :

flow {
    emit(PaymentStatus.Working) // the process start, I emit "Working", the ui can show a spinner for example
    val result = Schedule... // Here I want to poll my endpoint until I get the expected response, then I emit that final "Success" or "Error" if the IO requests return 4xx, 5xx or if I get a timeout
    emit(result)
}

[jean]

This does what I want I think:
1. emit “Working”
2. emit “ShowQrCode”
3. emit “Success” when the payment is authorized OR “Error” if anything went wrong or timeout happened

Untitled.kt

[simon.vergauwen]

I think you can use something like this to not rely on Throwable there. So basically repeat polling until you reach Either.Right with the desired value.

Schedule
  .spaced<Either<VippsError, PaymentStatus>>(twoSeconds)
  .whileOutput { it < twoMinutes }
  .whileInput {
    it.map { it.aggregate.authorizedAmount.value != price.toLong() }
      .getOrElse { true }
  }
  .zipRight(Schedule.identity())
  .repeatOrElseEither(
    fa = { getPayment(qrCode)) },
    orElse = { VippsError.from(throwable).left() }
  ).fold(
    { emit(PaymentStatus.Success) },
    { emit(it) }
  )

[jean]

Nice, thanks for the help! 👏🏻