Question

So I’m getting into Scala and I have a question if what I want to do is possible, or if there’s a better way.

I want to be able to take as a parameter a Map whose keys are either strings, or 0-args function returning a string. So for example

def main(args: List[String]){
    f = F(
      Map(
      "key" -> "value",
      "key2" ->(()=> {"valule2"})
    )
    )

    println(f("key"))

}
    case class F(arg: Map[String, ???]){
        def apply(s: String): String = {arg(s)}
    }

This obviously doesn’t compile. Is there any way to do this?

Answer 1

In this case you can use scala.Either

scala> val map: Map[String, Either[String, () => String]] = Map.empty
map: Map[String,Either[String,() => String]] = Map()

scala> map + ("key" -> Left("value"))
res0: scala.collection.immutable.Map[String,Either[String,() => String]] = Map(key -> Left(value))

scala> res0("key")
res1: Either[String,() => String] = Left(value)

scala> map + ("key2" -> Right(() => "value2"))
res2: scala.collection.immutable.Map[String,Either[String,() => String]] = Map(key2 -> Right(<function0>))

scala> res2("key2")
res3: Either[String,() => String] = Right(<function0>)

Update

You can hide implementation from caller using something like this.

def toEither[T <: Any : Manifest](x: T): Either[String, () => String] =
  x match {
    case x: String => Left(x)
    case x: Function0[String] if manifest[T] <:< manifest[() => String] => Right(x)
    case _ => throw new IllegalArgumentException
  }

The actual type of Function0 is eliminated due type erasure, but it can be verified with Manifest

scala> map + ("key" -> toEither("value"))
res1: scala.collection.immutable.Map[String,Either[String,() => String]] = Map(key -> Left(value))

scala> map + ("key2" -> toEither(() => "value2"))
res2: scala.collection.immutable.Map[String,Either[String,() => String]] = Map(key2 -> Right(<function0>))

scala> res2("key2").right.get()
res3: String = value2

scala> map + ("key2" -> toEither(() => 5))
java.lang.IllegalArgumentException

scala> map + ("key2" -> toEither(false))
java.lang.IllegalArgumentException

Update2

As @Submonoid rightly corrected me in the comments below, there is much simpler way of dealing with Either.

type StringOrFun = Either[String, () => String]
implicit def either(x: String): StringOrFun = Left(x)
implicit def either(x: () => String): StringOrFun = Right(x)

val m: Map[String, StringOrFun] = Map("key" -> "value", "key2" -> (() => "value2"))