Nested Monads Composition in Scala

Question

Here is a code example:

import cats.data.Reader

trait Configuration {  

  type FailFast[A] = Either[List[String], A]

  def getValue(name: String)(map: Map[String, String]): FailFast[String] =
    map.get(name)
      .toRight(List(s"$name field not specified"))

  type PropReader[A] = Reader[Map[String, String], A]
  def propReader(name:String): PropReader[FailFast[String]] =
    Reader(map => validation.getValue(name)(map))

  type OptionalValue[A] = PropReader[FailFast[Option[A]]]
  //how to use propReader(Configuration.NEW_EVENT)
  //inside of 'event' to return 'OptionalValue':?
  def event:OptionalValue[String] = ???
}      

object Configuration extends Configuration {
  final val NEW_EVENT = "event.unique"
}

Cannot get it how to implement event with a composition of: propReader(Configuration.NEW_EVENT)

If there are more than 1 options, it would be great to consider all of them.

UPDATE thanks to @Travis Brown, I would implement it this way. Here is an updated implementation:

  import cats.instances.list._ //for monoid
  import cats.instances.either._

  type FailFast[A] = Either[List[String], A]
  type PropReaderT[A] = ReaderT[FailFast, Map[String, String], A]
  type OptionalReaderT[A] = ReaderT[FailFast, Map[String, String], Option[A]]

  def getValue(name: String)(map: Map[String, String]): FailFast[String] =
    map.get(name).toRight(List(s"$name field not specified"))

  def propReader(name: String): PropReaderT[String] =
    ReaderT(getValue(name))

  def value2Option(value:String):Option[String] =
    if (value == null || value.isEmpty) Option.empty
    else Some(value)

  def event: OptionalReaderT[String] =
    propReader(Configuration.KEY1)
      .map(result => value2Option(result))

The difference between this and Travis Brown's implementation: I need to see a difference between not having a key in the map (which is an error, and I need a clear error description of it) and a case when a key exists, but its value either null or empty string. So it does not work in the same way as Maps.get, which returns Option. So I cannot get rid of FailFast

Hope for someone, it will be useful.

Answer 1

The simplest approach would be to map into the result, promoting failures into a successful None:

import cats.data.Reader

trait Configuration {
  type FailFast[A] = Either[List[String], A]
  type PropReader[A] = Reader[Map[String, String], A]
  type OptionalValue[A] = PropReader[FailFast[Option[A]]]

  def getValue(name: String)(map: Map[String, String]): FailFast[String] =
    map.get(name).toRight(List(s"$name field not specified"))

  def propReader(name:String): PropReader[FailFast[String]] =
    Reader(getValue(name))

  def event: OptionalValue[String] = propReader(Configuration.NEW_EVENT).map(
    result => Right(result.right.toOption)
  )
}      

object Configuration extends Configuration {
  final val NEW_EVENT = "event.unique"
}

I think it's worth reconsidering the model a bit, though. Any time you have a function that looks like A => F[B] (like a lookup in a map), you can represent it as a ReaderT[F, A, B], which gives you nicer kinds of composition—instead of mapping through two layers, you only have one, for example.

The ReaderT approach also makes it a little nicer to change out the F (via mapK). For example, suppose as in your example you generally want to work with readers that return their values in a FailFast context, but you need to switch to an Option context occasionally. That would look like this:

import cats.~>
import cats.arrow.FunctionK
import cats.data.ReaderT

trait Configuration {
  type FailFast[A] = Either[List[String], A]
  type PropReader[A] = ReaderT[FailFast, Map[String, String], A]
  type OptionalReader[A] = ReaderT[Option, Map[String, String], A]

  private def eitherToOption[A](either: FailFast[A]): Option[A] =
    either.right.toOption

  def getValue(name: String)(map: Map[String, String]): FailFast[String] =
    map.get(name).toRight(List(s"$name field not specified"))

  def propReader(name: String): PropReader[String] =
    ReaderT(getValue(name))

  def event: OptionalReader[String] =
    propReader(Configuration.NEW_EVENT).mapK(FunctionK.lift(eitherToOption))
}      

object Configuration extends Configuration {
  final val NEW_EVENT = "event.unique"
}

The OptionalReader here isn't exactly the same as your OptionalValue, since it doesn't include the FailFast layer, but that layer is redundant in your code, since missing values are represented in the Option layer, so the OptionReader approach is likely to be a better fit.