Hacking generalized type constraints.

GenTypeConstraints.scala

object GenTypeConstraints {
  abstract class =:=|[F,A,B] extends (F => Either[A,B])

  object =:=| {
    implicit def eqA[F,X]: =:=|[F,F,X] = new =:=|[F,F,X] {def apply(v:F) = Left(v)}
    implicit def eqB[F,X]: =:=|[F,X,F] = new =:=|[F,X,F] {def apply(v:F) = Right(v)}
  }

  def main(args: Array[String]) {
    println(f(1))
    println(f("FOO"))
    // compilation error println(f(1.0D))
  }

  def f[T](v: T)(implicit ev: =:=|[T,String,Int]): String = {
    ev(v) match {
      case Left(s) => s + s
      case Right(i) => (i + 1).toString
    }
  }
}

汎用っぽくしてみたけど型パラメータが3つになってしまったのでinfixできなくてカッコワルい…