Error context in error handling in Scala

Question

Suppose I need to call a remote JSON/HTTP service. I make a JSON request, send it by HTTP to the server, and receive and parse the JSON response.

Suppose I have data type MyError for errors and all my functions return Either[MyError, R]

type Result[A] = Either[MyError, A]    

def makeJsonRequest(requestData: RequestData): Result[String] = ...

def invoke(url: URL, jsonRequest: String): Result[String] = ...

def parseJsonResponse(jsonResponse: String): Result[ResponseData] = ...

I can combine them to write a new function:

def invokeService(url: URL, requestData: RequestData) Result[ResponseData] = for {
   jsonRequest <- makeJsonRequest(requestData).right
   jsonResponse <- invoke(url, req).right 
   responseData <- parseJsonResponse(jsonResponse).right
} yield responseData

Now what if parseJsonResponse fails ?

I get the error but I need also the whole context . That is I need url , requestData , and jsonRequest . How would you suggest me do it ?

Answer 1

If this is a specific case I would make MyError into a trait (ADT), and allow one of the possible values to be JsonParsingFailed(jsonRequest, ...) .

If it's something more generic I might use Writer (or rather type MyWriter[A] = Writer[Vector[MyLogEntry], A] and then use EitherT[MyWriter, MyError, A] ) to "log the event" at every stage.