Scala Akka 流程 output 处理

Question

I am facing a problem using akka http and web socket. I have a function through my code that is returning me the following type:

Flow[String, Either[RuntimeException, T], Any]

And from that type I have to exit with the following type:

Either[Unit, Flow[String, T, Any]]

Can everyone tell me how to answer Unit in case the the out part of the flow is Left(RuntimeException) and T when it's right handed.

Thanks in advance for your help

Answer 1

The issue is probably that:

• Tapir expects that error to return to the user is Either[Unit, Flow[String, T, Ant]] (error algebra is not provided - Unit - so it expects that no error information will be returned to the user)
• OPs business logic provides Flow working with Either[RuntimeException, T]
• if there are some errors during websocket processing, their result can be stored in T (so T has to be internally something like Either or similar ADT)

So endpoint either never fails on connection, or on failure shows no information to the user (since Left T = Unit). Once connected it can just return errors as websocket responses in the form of T .

In such case whole flow could just be wrapped in Right(flow) and one obstacle is worked out.

Then RuntimeException can be converted to T and the second obstacle would be defeated.

def handleRequest(input: Input): Either[
  Unit, // Left(()) is failure but without any error information for users
  Flow[String, Either[RuntimeException, T], Any] // Right(flow) on success
] = ...

// implementation depends on T and is only possible if T can somehow store
// either success and failure result
val mapError: RuntimeException => T = ...

def tapirLogic(input: Input): Either[Unit, Flow[String, T, ]] = for {
  // if we should go Left on request handling this is the moment
  businessLogic <- handleRequest(input)
  // turns Either[RuntimeExeption, T] to T
  flow = businessLogic.map((_.fold(mapError, identity)))
} yield flow

What if T isn't able to store failures? What if it cannot be encoded eg as something like:

{ "success": "success response" }
{ "failure": "failure response" }

or similar?

In such case your API definition is not prepared for encoding error messages that you would have to sent to your users on RuntimeException result within your business logic definition. Maybe you don't want to send anything in such case - then you could use some Flow operator which eg flatMapConcat single value on Right and empty Flow on Left. But if you want to be able to return some message to the user, then Tapir has to know how to encode it, and so T has to somehow encode it. Maybe you need two different T s: encoding only success in original flow and encoding both T and RuntimeException messages in flow transformed for Tapir. Here types tell you what to do to make your implementation correct.