How to find class parameter datatype at runtime in scala

Question

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

def getType[T: TypeTag](obj: T) = typeOf[T]

case class Thing(
  val id: Int,
  var name: String
)
val thing = Thing(1, "Apple")

val dataType = getType(thing).decl(TermName("id")).asTerm.typeSignature

dataType match {
 case t if t =:= typeOf[Int] => println("I am Int")
 case t if t =:= typeOf[String] => println("String, Do some stuff")
 case _ => println("Absurd")
}

Not able to digest why result is Absurd instead of I am int?

My aim is to know data-type of class parameter at runtime and match it to predefined types.

What's the goal? Looks like an x/y question where you think that runtime reflection is the way to complete a goal you don't explain (while runtime reflection is rarely the only way to go, and even less the good one). — cchantep
– cchantep, Commented Nov 29, 2020 at 16:24
@cchantep Actually, because the question is about Types similar question can be asked about compile-time reflection. — Dmytro Mitin
– Dmytro Mitin, Commented Nov 29, 2020 at 17:32

Dmytro Mitin · Accepted Answer · 2020-12-02 04:15:16Z

Both current dataType and typeOf[Int] are printed as Int but if you do showRaw you'll see why they don't match

showRaw(dataType) // NullaryMethodType(TypeRef(ThisType(scala), scala.Int, List()))
showRaw(typeOf[Int]) // TypeRef(ThisType(scala), scala.Int, List())

The thing is that just the type Int and the type of nullary method returning Int are different types.

Try to add .resultType

val dataType = getType(thing).decl(TermName("id")).asTerm.typeSignature.resultType

dataType match {
  case t if t =:= typeOf[Int] => println("I am Int")
  case t if t =:= typeOf[String] => println("String, Do some stuff")
  case _ => println("Absurd")
} // I am Int

It's also worth mentioning that .decl(TermName("id")) returns getter symbol, it's .decl(TermName("id ")) (with a blank space) that returns field symbol. So alternatively you can do with a blank space in the symbol name and without .resultType

val dataType = getType(thing).decl(TermName("id ")).asTerm.typeSignature

I'll add to @TomerShetah's answer that if the goal is "pattern matching" all fields of a case class then this can be done also at compile time (mostly) with Shapeless:

import shapeless.Poly1
import shapeless.syntax.std.product._

object printTypes extends Poly1 {
  implicit val int: Case.Aux[Int, Unit] = at(t => println(s"I am Int: $t"))
  implicit val string: Case.Aux[String, Unit] = at(t => println(s"String, Do some stuff: $t"))
  implicit def default[V]: Case.Aux[V, Unit] = at(t => println(s"Absurd: $t"))
}
  
thing.toHList.map(printTypes)
// I am Int: 1
// String, Do some stuff: Apple

https://scastie.scala-lang.org/DmytroMitin/N4Idk4KcRumQJZE2CHC0yQ

Tomer Shetah · Accepted Answer · 2020-11-30 11:28:28Z

1

@Dmytrio answer is a great explanation why the reflection didn't work as you expected.

I can understand from your question, that what you are trying to do, is actually pattern match all variables you have in a case class. Please consider doing it in the following way:

case class Thing(id: Int, name: String)
val thing = Thing(1, "Apple")

thing.productIterator.foreach {
  case t: Int => println(s"I am Int: $t")
  case t: String => println(s"String, Do some stuff: $t")
  case t => println(s"Absurd: $t")
}

Code run at Scastie.

answered Nov 30, 2020 at 11:28

Tomer Shetah

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