How can I create an Option type at runtime (reflection)?

Question

Using reflection, I have determined the runtime type of a thing, t: Type. Now I want to create a new Type of Option[t]. How can I do that?

val t: Type = ...
val optT: Type = ???  // Option of whatever t is

Why I want this: I have a handler function that operates on a Type. At compile time I have something like this:

trait Thing { name: String }
case class BigThing(name: String) extends Thing

case class Stuff[T <: Thing]( id: Int, maybeThing: Option[T] ) // contrived

def handler( t: Type ): Output = {...}

I can reflect that if I have a class of type Stuff, it has a member maybeThing of type Object[T] or even Object[Thing]. At runtime let's say I can determine that a specific object has T = BigThing, so I want to pass Option[BigThing], not Option[T] or Option[Thing] to handler(). That's why I'm trying to create a runtime type of Option[BigThing].

I did try the following but Scala didn't like it:

val newType = staticClass(s"Option[${runtimeTypeTAsString}]")

So given `t: Type` representing a value of type `T`, you want a method `def makeOptionType(t: Type): Type` which returns the `Type` representing a value of type `Option[T]`? — Tim, Feb 17 '19 at 17:59

Dmytro Mitin · Accepted Answer · 2019-02-17T18:45:13.947

According to tutorial

there are three ways to instantiate a Type.

via method typeOf on scala.reflect.api.TypeTags, which is mixed into Universe (simplest and most common).

Standard Types, such as Int, Boolean, Any, or Unit are accessible through the available universe.

Manual instantiation using factory methods such as typeRef or polyType on scala.reflect.api.Types, (not recommended).

Using the third way,

import scala.reflect.runtime.universe._

class MyClass

val t: Type = typeOf[MyClass] //pckg.App.MyClass

val mirror = runtimeMirror(ClassLoader.getSystemClassLoader)

val optT: Type = mirror.universe.internal.typeRef(
  definitions.PredefModule.typeSignature, 
  definitions.OptionClass, 
  List(t)
) // Option[pckg.App.MyClass]

val optT1 : Type = typeOf[Option[MyClass]]

optT =:= optT1 // true

How can I create an Option type at runtime (reflection)?

1 Answers1