从TypeTag和方法获取精确的返回类型

Question

Say I have

trait Foo[T] { def bar: Bar[T] }

and want to obtain return type of bar when called on a Foo[Int] (ie Bar[Int] in this case) from the type tag of Foo[Int] and the name "bar" (we can assume there are no overloads or we can tell them apart). Can this be done done with scala-reflect?

Answer 1

Is this close to what you want (using asSeenFrom )?

val tt = typeTag[Foo[Int]]

scala> tt.tpe
    .members
    .find(_.name.toString == "bar").get
    .asMethod
    .returnType
    .asSeenFrom(tt.tpe, tt.tpe.typeSymbol)

res68: reflect.runtime.universe.Type = Bar[Int]

I've of course thrown type-safety out the window. Slightly better:

scala> tt.tpe
     .members
     .find(_.name.toString == "bar")
     .filter(_.isMethod)
     .map(_.asMethod.returnType.asSeenFrom(tt.tpe, tt.tpe.typeSymbol))

res74: Option[reflect.runtime.universe.Type] = Some(Bar[Int])

Answer 2

In case someone runs into this in the future: with this variation

trait Foo[T] { def bar: Option[T] }
trait Bar[T] extends Foo[T]

val tt = typeTag[Bar[Int]]

I had to slightly change @mz's answer:

val m = tt.tpe
  .member(newTermName("bar"))
  .asMethod

m.returnType
  .asSeenFrom(tt.tpe, m.owner)