Scala通用类型方法匹配

Question

I am building a Scala generic type method that takes tuple with different size and output a line of CSV string. The code I wrote is like this

object CSVFormat {
  protected def valueToCSV[T](value: Seq[T]): String =
    s""""${value.map(_.toString).mkString("{", ",", "}")}""""
  protected def valueToCSV[T](value: T): String =
    s""""${value.toString}""""

  def rowToCSV[A](row: (A)): String = valueToCSV(row)
  def rowToCSV[A, B](row: (A, B)): String =
    s"${rowToCSV((row._1))},${rowToCSV((row._2))}"
  def rowToCSV[A, B, C](row: (A, B, C)): String =
    s"${rowToCSV((row._1, row._2))},${rowToCSV((row._3))}"
  def rowToCSV[A, B, C, D](row: (A, B, C, D)): String =
    s"${rowToCSV((row._1, row._2, row._3))},${rowToCSV((row._4))}"
  def rowToCSV[A, B, C, D, E](row: (A, B, C, D, E)): String =
    s"${rowToCSV((row._1, row._2, row._3, row._4))},${rowToCSV((row._5))}"
}

What I am trying to get is something like

CSVFormat.rowToCSV(("hello", 1234, Seq("a", "b", "c")))

then the output should be

"hello","1234","{a,b,c}"

The major issue here is for Seq[T] type matching and just T type matching for valueToCSV method, it seems just T type always win, so

CSVFormat.rowToCSV((Seq("A")))

always results in

""[(List(A))]""

Any idea how to make Scala generic type matching, the Seq[T] has higher priority so that it got match first? Or should I determine type in runtime instead of compiling type instead? If so, how should I do it?

Answer 1

The flavor of valueToCSV method is resolved at compile time, when it is not known whether the type you pass in is going to be a sequence or not.

You need a dynamic type check instead. How about something like this:

 def rowToCSV[T <: Product](tuple: T) = tuple
  .productIterator
  .map { 
      case s: Seq[_] => s.mkString("{", ",", "}")
      case x => x.toString
   }.mkString("\"", "\",\"", "\"")

BTW, you don't need the parenthesis when calling a function, whose only argument is a tuple: rowToCSV("foo") is the same as rowToCSV(("foo")) and rowToCSV(1, "foo", Seq("bar", "baz")) is the same as rowToCSV((1, "foo", Seq("bar", "baz"))) .