a scalafix for viewbounds

viewboundfix.scala

package fix

import scalafix._
import scala.meta._

case object Viewbounds_v1_0 extends Rule("Viewbounds_v1_0") {

  def eliminateViewBound(defn: Defn.Def): Option[Defn.Def] = {

    def makeimplicit(i: Int, from: Name, to: Type): Term.Param = {
      val funname = s"viewbound_$i"
      val expr = s"implicit viewbound_$i: $from => $to"
      expr.parse[Term.Param].get
    }

    def isImplParameterList(params: List[Term.Param]): Boolean = params.exists {
      case Term.Param(mods, _, _, _) => mods.exists(_.is[Mod.Implicit])
    }
  
    defn match {
      case Defn.Def(x, name, typeparams, parameterlists, ascription, body) => {
        val (regular, impl) = parameterlists.span(pl => !isImplParameterList(pl))
        val (nobounds, names, bounds) = typeparams.map {
          case Type.Param(mods, name, tparams, typebounds, viewbounds, contextbounds) => (Type.Param(mods, name, tparams, typebounds, Nil, contextbounds), name, viewbounds)
        }.unzip3
      
        val b = names.zip(bounds).flatMap{
          case (name, targets) => targets.map(to => (name, to))
        }
      
        val convertedparams = b.zipWithIndex.map{ case ((from, to), i) => makeimplicit(i, from, to)}
        if (convertedparams.nonEmpty) {
          val totalimplicits = impl.flatten ::: convertedparams
          val paramlists = regular :+ totalimplicits
          Some(Defn.Def(x, name, nobounds, paramlists, ascription, body))
        } else None
      }
    }
  }

  override def description: String =
    "Rewrite that replaces viewbounds with an implicit conversion parameter"

  override def fix(ctx: RuleCtx): Patch = {
    ctx.tree.collect {
      case defn: Defn.Def => {
        val replacement = eliminateViewBound(defn)
        replacement.map(repl => ctx.replaceTree(defn, repl.toString)).getOrElse(Patch.empty)
      }
    }.asPatch
  }

}