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Asked: 2026-06-14T10:17:05+00:00

Can we get the type of val s: String using reflection from the outside

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Can we get the type of val s: String using reflection from the outside of the function f?

val f = (r: {val s: String}) => {
}
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  1. Editorial Team
    scala> import scala.reflect.runtime.{universe => ru}
import scala.reflect.runtime.{universe=>ru}

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

scala> def typeOf[T: ru.TypeTag](x: T) = ru.typeOf[T] // capture compile-time type info
typeOf: [T](x: T)(implicit evidence$1: reflect.runtime.universe.TypeTag[T])reflect.runtime.universe.Type

scala> val f = (r: {val s: String}) => {}
f: AnyRef{val s: String} => Unit = <function1>

scala> val tpe = typeOf(f)
tpe: reflect.runtime.universe.Type = scala.AnyRef{val s: String} => Unit

scala> ru.showRaw(tpe)
res0: String = TypeRef(ThisType(scala), scala.Function1, List(RefinedType(List(TypeRef(ThisType(scala), newTypeName("AnyRef"), List())), Scope(newTermName("s"))), TypeRef(ThisType(scala), scala.Unit, List())))

scala> val ru.TypeRef(_, _, refinement :: _) = tpe
refinement: reflect.runtime.universe.Type = scala.AnyRef{val s: String}

    With Scala reflection one can also generate mocks for structural types as follows: https://gist.github.com/4008389. The linked gist does this using toolboxes and runtime reflection, but this scenario looks implementable with macros as well.

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