Get type class from generic in scala

Question

I want to get actual type class from Generic type.

When I try this

I get class scala.runtime.Nothing only.

import scala.reflect._

class Car {

}

object Car extends Model[Car] {

}

trait Model[T] {

  def something[T:ClassTag]():Unit = {
    println(classTag[T].runtimeClass)
  }

}

Car.something() // <- scala.runtime.Nothing$

The two `T`s are different. Try: `Car.something[String]()` for example. — Gábor Bakos, Dec 26 '14 at 21:14

score 3 · Answer 1 · edited May 23 '17 at 11:45

3

You might want something like the following: The T=:=U would unify the two types and the :ClassTag would just create a ClassTag for it.

Edit: the correct, but still too complicated form:

trait Model[T] {
  def something[U](implicit ev: T =:= U, tag: ClassTag[U]): Unit = {
    println(tag.runtimeClass)
  }

In general, I would also use https://stackoverflow.com/a/27666845/1502148 (when I am not sleepy :) ).

edited May 23 '17 at 11:45

Community

1
1

answered Dec 26 '14 at 21:19

Gábor Bakos

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This is 1) not a legal syntax; 2) more complicated than required if you fix the syntax to `def something[U](implicit ev: T =:= U, tag: ClassTag[U])`. – Alexey Romanov Dec 27 '14 at 11:40

score 2 · Answer 2 · answered Dec 26 '14 at 21:19

2

class Model[T:ClassTag] {

  def something():Unit = {
    println(classTag[T].runtimeClass)
  }

}

I solved by modifying Model trait to class and provide ClassTag to Generic.

answered Dec 26 '14 at 21:19

Scalalang2

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score 1 · Answer 3 · answered Dec 27 '14 at 11:37

If you want to allow creating a Model without ClassTag and only require ClassTag for some methods, you can do

trait Model[T] {
  def something(implicit tag: ClassTag[T]):Unit = ...
}

Obviously this applies to e.g. Numeric, Ordering, etc. constraints as well.

Get type class from generic in scala

3 Answers3