有没有办法用中缀表示法调用scala方法（具有类型参数）

Question

I have a piece of code in implicit class -

implicit class Path(bSONValue: BSONValue) {
      def |<[S, T <:{def value:S}] = {
        bSONValue.asInstanceOf[T].value
      }

} 

The problem is if I want to call |< method after BSONValue I need to call with . . eg

(doc/"_id").|<[String,BSONString]

The problem is without . scala raises error because it does not allow type parameter method with infix notation. So always I have to wrap doc/"_id" portion with () . Is their any way of using type parameter method without . eg

doc/"_id"|<[String,BSONString]

Answer 1

All types T that you want to get out of BSONValue s will probably have a companion object with the same name. You could use that companion object as an intuitive placeholder for the type that you actually want to get. Something along these lines:

trait Extract[A, BComp, B] {
  def extractValue(a: A): B
}

implicit class Extractable[A](a: A) {
  def |<[BC, B]
    (companion: BC)
    (implicit e: Extract[A, BC, B])
  : B = e.extractValue(a)
}

implicit def extractIntFromString
  : Extract[String, Int.type, Int] = _.toInt

implicit def extractDoubleFromString
  : Extract[String, Double.type, Double] = _.toDouble

val answer = "42" |< Int
val bnswer = "42.1" |< Double

This allows you to use infix syntax, because all those things are ordinary values.

---

Still, only because it's possible, it doesn't mean that you have to do it. For instance, I wouldn't know what to expect from a |< -operator. Many other people also wouldn't know what to do with it. They'd have to go and look it up. Then they would see this signature:

def |<[BC, B](companion: BC)(implicit e: Extract[A, BC, B]): B

I can imagine that the vast majority of people (myself in one week included) would not be immediately enlightened by this signature.

Maybe you could consider something more lightweight:

type BSONValue = String

trait Extract[B] {
  def extractValue(bsonValue: BSONValue): B
}


def extract[B](bson: BSONValue)(implicit efb: Extract[B])
  : B = efb.extractValue(bson)

implicit def extractIntFromString
  : Extract[Int] = _.toInt

implicit def extractDoubleFromString
  : Extract[Double] = _.toDouble

val answer = extract[Int]("42")
val bnswer = extract[Double]("42.1")

println(answer)
println(bnswer)

It seems to do roughly the same as the |< operator, but with much less magic going on.