Abstract reflection API in Scala 2.10

Question

Scala 2.10 comes with a great reflection API. There are two entry points to it, however: runtime universe and macro context universe.

When using runtime reflection, you should import scala.reflect.runtime.universe. When using reflection inside a macro implementation, you should import universe from the context.

Is it possible to write some code that works in both environments? How should one obtain the universe?

Consider this example:

class MyReflection(val u: scala.reflect.api.Universe) {
  import u._

  def foo[T: TypeTag] = implicitly[TypeTag[T]].tpe.members // returns MyReflection.u.MemberScope
}
val x = new MyReflection(scala.reflect.runtime.universe)
val members: scala.reflect.runtime.universe.MemberScope = x.foo[String] // BANG! Compiler error

This won't compile because of type mismatch. Same time, it is obvious that both scala.reflect.runtime.universe.MemberScope and MyReflection.u.MemberScope in this example share the same API. Is there a way to abstract over different universes?

Or am I possibly doing something philosophically wrong with trying to export reflection artifacts (MemberScope in this example)?

Community · Accepted Answer · 2017-05-23 11:43:17Z

2

You can just accept the universe as a parameter:

class MyReflection(val u: scala.reflect.api.Universe) {
  import u._

  def foo[T : TypeTag] = implicitly[TypeTag[T]].tpe.members
}


val x = new MyReflection(scala.reflect.runtime.universe)

Note that you'll have to refer to the universe via your instance of MyReflection to get the path-dependent types right.

val members: x.u.MemberScope = x.foo[String]

Have a look at this question for more examples and options.

edited May 23, 2017 at 11:43

CommunityBot

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answered Jun 7, 2013 at 19:21

gzm0

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3 Comments

Alexander Bulaev Over a year ago

Thank you for your anwser! While this hint is a solution, it has a noticeable downside for me. I've updated the question with the description of it.

gzm0 Over a year ago

Welcome to the downside of path-dependent types. You have no choice but to do everything you want to do under the scope of MyReflection, or refer to MyReflection's implementation of Universe. See updated answer.

Alexander Bulaev Over a year ago

Thanks! Looks like this (see section "Writing bigger macros") will help me.

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