Get information of type in Scala3 macro

Question

I'm struggling to get information of type in Scala3 macro implementation. I'll explain problem through code.

Here is application logic:

object BlockServiceImpl extends BlockService:
  def authenticateUser0() = new ServiceCall[AuthUser,AuthUserResponse]:
     def invoke(request: AuthUser): Future[AuthUserResponse] = 
       println("BlockServiceImpl authenticateUser0 called")
       Future.successful(AuthUserResponse("test"))   

Now, for the logic I want to make endpoints with help of macro.

defineRoute("POST","/v1/block",BlockServiceImpl.authenticateUser0)

This is inline method:

inline def defineRoute[Q: RootJsonFormat ,R: RootJsonFormat](method: String, uri: String,inline call: () =>  ServiceCall[Q,R]): AkkaHttpCall = ${ methodImpl[Q,R]('uri, 'call)}

And this is implementation of macro:

def methodImpl[Q: Type,R: Type](uri: Expr[String],expr: Expr[Function0[ServiceCall[Q,R]]])(using ctx: Quotes): Expr[AkkaHttpCall] = ...

How can I get information that Q is AuthUser type during macro expansion in a compile time?

Outhouse

Q: Type, R: Type you have no bounds on your methodImpl, can you bind them accordingly? (I'm not super familiar with the new macros, but how can you enforce a certain type if the type has no bounds?) – Grammar

How to bind them? – Outhouse

I think using upper bound : Q <: AuthUser : Type. – Edwardedwardian

There will be different defineRoute calls with different Q type. – Outhouse

@Outhouse Type bounds can be extremely useful if the type hierarchy is well defined. For the same or sub-classes, we can use an "upper bound" or "[T <: R]". For the same or super-classes, we can use a "lower bound" or "[T >: R]". We can bind both ends simultaneously using "[T >: R <: W]". I've used this as a reference in the past: blog.knoldus.com/scala-type-bounds – Grammar

Considering that several of your base types are not provided, specifically addressing your actual implementation issues are not possible without guessing. "Black-Box Types" AKA "Types that are givens for you but are not necessarily known to me": BlockService, ServiceCall, AuthUser, AuthUserResponse, RootJsonFormat , AkkaHttpCall – Grammar

@RandomnessSlayer, yes I understand the idea, but it can be any case class/object, they are request/response actually. I just wanted to know if user used special cases for request/response (NotUsed, Done, etc). So, that information I put in serializers. Lagom framework implemented it in that way. – Outhouse

Answer 1

I've solved it by putting information if Q and R were special cases (NotUsed, Done,..) in serializers for Q and R. The idea is took from the Lagom framework.

Outhouse

Your own answer contains zero technical information to help another person solve similar problems. You've basically accepted an answer saying "I've fixed it myself using a framework called Lagom" which, without context, is not helpful to fix a similar but not exactly the same value. Which is the purpose of StackOverflow questions and answers, to be able to understand the why or mechanisms to the question/problem at hand. – Grammar

Answer 2

A possible solution could be to use pattern matching on quoted expressions.

So for example, you can define a method that is used to retrieve the compile-time type:

def tag[A <: AnyKind] = throw new IllegalStateException("use it only to pattern match types")

And then, in the macro expansion, you can perform pattern match as:

'{ tag[Q] } match {
      case '{ tag[AuthUser] } => // here I am sure that Q is AuthUser, since Q is matched with AuthUser
}

It is quite a trick (and is not very extensible as you have to add each type) so take everything I say with a grain of salt... I think that exists a clearer solution depends on your particular application logic :)

Edwardedwardian

This could work. I don't need to figure out information for every type but just for one special case. I will try it later. – Outhouse

I've solved it differently. I've added information for special type in serializers. So, in a serializer I know if it were NotUsed,Done. – Outhouse

Good, why you don't answer the question yourself so, in the future, other people have a possible solution for this problem? :) – Edwardedwardian

I forget it :). – Outhouse

Answer 3

Bounding (See: Scala 3 Book: Context Bounds) the type of a parameter in a function can be achieved in several ways.

---

THIS IS WRONG (TY Dmytro!): When using generic parameters like so: [T : Type] we are aliasing a type

CORRECTION: When using generic parameters like so: [T : R] we are using syntactic sugar which represents an implicit parameter of type R[T]

---

For many applications, including yours, it can be beneficial to restrict the type of our generic parameter.

---

There are two main bounds, an "upper" and a "lower" bound.

The "upper" bound e.g. [T <: U] specifies that T must be of type U, or a subclass of U

The "lower" bound e.g. [T >: U] specifies that T must be of type U, or a super-class of U

It is possible to restrict both bounds, by first specifying the lower bound then the upper bound, e.g. [T >: Cat <: Animal]

Grammar

"When using generic parameters like so: [T : Type] we are aliasing a type" Context bounds do not alias a type. – Kershner