CODEWITHSUNDEEP
scalagenericsreflection
Mehdi
Mehdi

Reputation: 140

Creating typeTag from String and pass it to a polymorphic Method

I have a polymorphic method that takes a custom case class as a type parameter. Now, in order to support several case classes (defined in a config file as a String), I need to convert a String to the tagType of the case class.

For that, I used runtimeMirror method to get the class from the String, and then I used manifestToTypeTag to get the tagType (Getting TypeTag from a classname string)

import scala.reflect.runtime.universe._
import scala.reflect.runtime.universe
import scala.reflect.ManifestFactory

// My polymorphic method
def printMe[T](l: List[T])(implicit typeTag: TypeTag[T]): Unit = println(l)

// This works:
printMe(List("fdfg"))(typeTag[java.lang.String])

// Now, I want to build the typeTag dynamically from a String
val className = "java.lang.String" // a Custom case class 
val mirror = universe.runtimeMirror(getClass.getClassLoader)
val cls = Class.forName(className)
// Getting the typeTag from the class name
val t = internal.manifestToTypeTag(mirror,ManifestFactory.classType(cls))

// Call of the method with the generated typeTag
printMe(List("fdfg"))(t)

// Compilation error
Error:(12, 31) type mismatch;
found   : scala.reflect.api.Universe#TypeTag[Nothing]
required: reflect.runtime.universe.TypeTag[String]
Note: Nothing <: String, but trait TypeTag is invariant in type T.
You may wish to investigate a wildcard type such as `_ <: String`. (SLS 3.2.10)
printMe(List("fdfg"))(t)

However, when I pass the typeTag to my polymorphic method I get a "type match compilation error" shown above. Indeed, my polymorphic method requires a TypeTag[MyClassToto], and the TypeTag I generate is TypeTag[Nothing].

I wonder if is possible to cast the TypeTag I have got, or may be I have to change the signature of my polymorphic method ?

Upvotes: 3

Views: 522

Answers (1)

Mario Galic
Mario Galic

Reputation: 48420

Try suggestion from https://stackoverflow.com/a/23792152/5205022:

def printMe[T](l: List[T])(implicit typeTag: TypeTag[T]): Unit = println(l)

def stringToTypeTag[A](name: String): TypeTag[A] = {
  val c = Class.forName(name)
  val mirror = runtimeMirror(c.getClassLoader)
  val sym = mirror.staticClass(name)
  val tpe = sym.selfType
  TypeTag(mirror, new api.TypeCreator {
    def apply[U <: api.Universe with Singleton](m: api.Mirror[U]) =
      if (m eq mirror) tpe.asInstanceOf[U # Type]
      else throw new IllegalArgumentException(s"Type tag defined in $mirror cannot be migrated to other mirrors.")
  })
}

printMe(List("fdfg"))(stringToTypeTag("java.lang.String"))

Upvotes: 3

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