Replace placeholders using Applicatives (Scala)

    // Usign Applicatives to replace ':value' placeholders in a string
    // All errors are accumulated into a ValidationNEL
sealed trait ParamsReplacer {
  val str: String
  private val r = ":([a-zA-Z0-9]+)".r
  private def f(p:Params) = (m:Match) => {
    val s = m.toString.drop(1)
    (p get s).toSuccess(s"Param '$s' is required").toValidationNEL
  }

  def replace(params: Params) = {
    val reduce = (a: String, b: (Match, String)) => b._1.matched.r.replaceFirstIn(a, b._2)
    val zipAndReduce = (a:List[Match], b:List[String]) => (a zip b).foldLeft(str)(reduce)
    val matchList = r.findAllMatchIn(str).toList

    (matchList.successNel[String] |@|
      (matchList <*> f(params).pure[List]).sequenceU)(zipAndReduce)
  }
}

object ParamsReplacer {
  def apply(s: String):ParamsReplacer = new ParamsReplacer {
    val str = s
  }
}

// Use:
scala> val rep = ParamsReplacer("retweets_of_me.:format.:test")
rep: com.logikujo.Spore.ParamsReplacer = com.logikujo.Spore.ParamsReplacer$$anon$1@7ce9a760

// Scary type :) The cool part is the Failure(NonEmptyList(Error1, Error2))
scala> rep.replace(Map.empty)
res1: scalaz.Unapply[scalaz.Apply,scalaz.ValidationNEL[String,List[scala.util.matching.Regex.Match]]]{type M[X] = scalaz.ValidationNEL[String,X]; type A = List[scala.util.matching.Regex.Match]}#M[String] = Failure(NonEmptyList(Param 'format' is required, Param 'test' is required)

scala> rep.replace(Map("format" -> "json"))
res3: scalaz.Unapply[scalaz.Apply,scalaz.ValidationNEL[String,List[scala.util.matching.Regex.Match]]]{type M[X] = scalaz.ValidationNEL[String,X]; type A = List[scala.util.matching.Regex.Match]}#M[String] = Failure(NonEmptyList(Param 'test' is required))

// A Success! :)
scala> rep.replace(Map("format" -> "json", "test" -> "JustATest"))
res4: scalaz.Unapply[scalaz.Apply,scalaz.ValidationNEL[String,List[scala.util.matching.Regex.Match]]]{type M[X] = scalaz.ValidationNEL[String,X]; type A = List[scala.util.matching.Regex.Match]}#M[String] = Success(retweets_of_me.json.JustATest)