How to use Shapeless in a Quasiquote?

Question

I'm trying to call a Shapeless macro from inside a quasiquote with Scala and I'm not getting what I would like to get.

My macro doesn't return any errors but it doesn't expand Witness(fieldName) into Witness.Lt[String]

val implicits = schema.fields.map { field =>
  val fieldName:String = field.name
  val fieldType = TypeName(field.valueType.fullName)
  val in = TermName("implicitField"+fieldName)
  val tn = TermName(fieldName)
  val cc = TermName("cc")
  q"""implicit val $in = Field.apply[$className,$fieldType](Witness($fieldName), ($cc:   $className) => $cc.$tn)"""
}

Here is my Field definition:

sealed abstract class Field[CC, FieldName] {
  val  fieldName: String
  type fieldType

  // How to extract this field
  def  get(cc : CC) : fieldType
}

object Field {
  // fieldType is existencial in Field but parametric in Fied.Aux
  // used to explict constraints on fieldType
  type Aux[CC, FieldName, fieldType_] = Field[CC, FieldName] {
    type fieldType = fieldType_
  }

  def apply[CC, fieldType_](fieldWitness : Witness.Lt[String], ext : CC => fieldType_) : Field.Aux[CC, fieldWitness.T, fieldType_] =
    new Field[CC, fieldWitness.T] {
      val fieldName  : String = fieldWitness.value
      type fieldType = fieldType_
      def get(cc : CC) : fieldType = ext(cc)
    }
}

In this case the implicit I generate looks like:

implicit val implicitFieldname : Field[MyCaseClass, fieldWitness.`type`#T]{
  override type fieldType = java.lang.String
}

If it had been defined outside a quasiquote it would generate something like:

implicit val implicitFieldname : Field.Aux[MyCaseClass, Witness.Lt[String]#T, String] = ...

Is there something that can be done?

Are you using this in a macro annotation? Have you tried providing a type annotation for $in (which I think will require using ConstantType)? — Travis Brown
@TravisBrown yes I'm building this using a macro annotation (Macro Paradise). I have tried to provide a type like this : q"""implicit val $in : Field.Aux[$className, Witness.Lt[String]#T, String] = Field.apply[$className,$fieldType](Witness($fieldName), ($cc: $className) => $cc.$tn)""" — Roch
You'll need the specific field name in the type annotation, though (see e.g. my old pre-Shapeless 2.0 blog post here for an example of using ConstantType). Do you happen to have a complete working example around? — Travis Brown

Federico Pellegatta Federico Pellegatta · Accepted Answer · 2017-08-18T15:46:29

This is my working solution using old-style macro annotations.

import scala.language.experimental.macros
import scala.reflect.macros.blackbox.Context
import scala.annotation.StaticAnnotation

class fieldable extends StaticAnnotation {
  def macroTransform(annottees: Any*): Any = macro fieldableMacro.impl
}

object fieldableMacro {
  def impl(c: Context)(annottees: c.Expr[Any]*): c.Tree = {
    import c.universe._
    annottees.map(_.tree) match {
      case (param @ q"case class $className(..$fields)") :: Nil => {
        val implicits = fields.collect {
          case field @ q"$mods val $tname: $tpt" => q"""
            implicit val $tname = Field.apply[$className,$tpt](
              Witness(${tname.decodedName.toString}), _.$tname
            )"""
        }; q"$param; object ${className.toTermName} {..$implicits}"
      }
    }
  }
}

It can be, for sure, improved using better quasiquotes, but my goal was to show something as cleaner as possible.

It can be used as:

@fieldable
case class MyCaseClass(foo: String, bar: Int)

This produces a MyCaseClass companion object having required Fields implicits:

implicit val foo = Field.apply[MyCaseClass, String](Witness("foo"), ((x$1) => x$1.foo));
implicit val bar = Field.apply[MyCaseClass, Int](Witness("bar"), ((x$2) => x$2.bar));

As it was already pointed out, without a complete working example, it's quite difficult to write an exhaustive answer.

How to use Shapeless in a Quasiquote?

1 Answers