How to implement that generic function with TypeTa

问题:

Suppose I need to write a function convert[T]: String => Option[T], which works as follows:

 import scala.util.Try

 def toInt(s: String): Option[Int] = Try(s.toInt).toOption
 def toDouble(s: String): Option[Double] = Try(s.toDouble).toOption
 def toBoolean(s: String): Option[Boolean] = Try(s.toBoolean).toOption

 // if T is either Int, Double, or Boolean return 
 // toInt(s), toDouble(s), toBoolean(s) respectively

 def convert[T](s: String): Option[T] = ???

Should I use TypeTag to implement it ?

回答1:

No, you should use the typeclass pattern. That way the types are resolved at compile time rather than runtime, which is much safer.

trait ConverterFor[T] {
  def convert(s: String): Option[T]
}
object ConverterFor {
  implicit def forInt = new ConverterFor[Int] {
    def convert(s: String) = Try(s.toInt).toOption }
  implicit def forDouble = ...
}

def convert[T](s: String)(implicit converter: ConverterFor[T]): Option[T] =
  converter.convert(s)

The correct ConvertorFor is resolved implicitly at compile time. If you try to call convert with a type for which there is no implicit ConverterFor available, it will fail to compile.