Question

The following is a pretty common play framework 2 controller:

def save(ideaId : Long) = CORSAction { request =>
  Idea.findById(ideaId).map { idea =>
    request.body.asJson.map { json =>
      json.asOpt[Comment].map { comment =>
        comment.copy(idea = idea).save.fold(
          errors => JsonBadRequest(errors),
          comment => Ok(toJson(comment).toString)
        )
      }.getOrElse     (JsonBadRequest("Invalid Comment entity"))
    }.getOrElse       (JsonBadRequest("Expecting JSON data"))
  }.getOrElse         (JsonBadRequest("Could not find idea with id '%s'".format(ideaId)))
}

I find it a bit annoying all those nested .maps, and I also find a bit tedious that each error handling is at the bottom

How would you improve it to make more readable, and, at the same time remain as functional idiomatic scala code?

I was thinking perhaps something like this (it’s seudo code, still does not compile)

def save(ideaId : Long) = CORSAction { request =>

  val idea = Idea.findById(ideaId).getOrElse(
    return JsonBadRequest("Could not find idea with id '%s'".format(ideaId)))

  val json = request.body.asJson.getOrElse(
    return JsonBadRequest("Expecting JSON data"))

  val comment = json.asOpt[Comment].getOrElse(
    return JsonBadRequest("Invalid Comment entity"))

  comment.copy(idea = idea).save.fold(
    errors => JsonBadRequest(errors),
    comment => Ok(toJson(comment).toString)
  )

}

ps: I know It would be much better to avoid the return statement…

Answer 1

First to simplify. Suppose I have three methods that take a String and return an Option[String]:

def foo(s: String): Option[String] = if (s.size >= 4) Some(s + "1") else None
def bar(s: String): Option[String] = if (s(0) != 'A') Some(s + "2") else None
def baz(s: String): Option[String] = if (s toSet ' ') Some(s + "3") else None

I want a method that pipes a string through these and returns the appropriate error message if I get a None along the way. I could write this:

def all(s: String): Either[String, String] =
  foo(s).map { x =>
    bar(x).map { y =>
      baz(y).map { z =>
        Right(z)
      } getOrElse Left("Doesn't contain a space!")
    } getOrElse   Left("Starts with an A!")
  } getOrElse     Left("Too short!")

But right, this isn’t pretty. We can use a for-comprehension and the toRight method on Option to write a much clearer version:

def all(s: String): Either[String, String] = for {
  x <- (foo(s) toRight "Too short!"              ).right
  y <- (bar(x) toRight "Starts with an A!"       ).right
  z <- (baz(y) toRight "Doesn't contain a space!").right
} yield z

Calling toRight(msg) on an Option gives us a Left(msg) if it’s empty, and a Right(whatever) otherwise. We then have to take the right projection of the Either with .right, since Scala’s Either isn’t right-biased.

The equivalent in your case would be something like this:

def save(ideaId: Long) = CORSAction { request =>
  val saveResult = for {
    idea    <- (Idea.findById(ideaId) toRight "Could not find id"     ).right
    json    <- (request.body.asJson   toRight "Invalid Comment entity").right
    comment <- (json.asOpt[Comment]   toRight "Expecting JSON data"   ).right
    result  <- comment.copy(idea = idea).save().right
  } yield result

  saveResult.fold(
    error => JsonBadRequest(error),
    comment => Ok(toJson(comment).toString)
  )
}

Not quite as concise as your desired syntax, but the error messages appear in a more logical place, and we’ve gotten rid of the ugly nesting.