Failure handling using Either & # 8594; Where is the stack located?

Question

I heard from some people that in Scala we tend (like other functional languages) not to interrupt the control flow ... Instead, we return an error in Either Left .

But how do we get stracktrace from this exception? At the moment, I am returning to the left a simple Error class with code, message and reason ( Error too). But if I have an error, I cannot get stacktrace. If my application gets complicated, it can be difficult to find the code block that returned this Error value ... The main reason is important.

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So what do we do in practice?

Should I return a java Exception or Throwable in my Left instead of a custom Error ? What is the best practice for handling Scala exceptions without losing important information like stacktrace and reason?

Answer 1

I would suggest using Either[java.lang.Throwable, A] (where Throwable still gives you access to the stack trace) and (in general) so that your custom error types are extended by java.lang.Exception .

This is the practice used by Dispatch 0.9 , for example, where Either[Throwable, A] used to represent computations that might fail, and custom error types look like this:

 case class StatusCode(code: Int) extends Exception("Unexpected response status: %d".format(code)) 

Scalaz 7 Validation.fromTryCatch(a: => T) also returns a Validation[Throwable, T] , where Validation roughly equivalent to Either .