Define a generic value class trait using:
trait ValueType extends Any { this: AnyVal =>
type T
def value: T
}And a concrete implementation of that class using:
case class AccountId(value: UUID) extends AnyVal with ValueType { type T = UUID }I'm trying to write a macro to generate an implicit lift function from the underlying type to the value class type (eg: UUID => AccountId):
implicit def liftValueType1[T0, M0 <: ValueType { type T = T0 }](value: T0): M0 = macro Macros.liftValueType1[T0, M0]Macro def looks like this:
class Macros(val c: blackbox.Context) {
import c.universe._
/** Attempt 1:
*
* def liftValueType1(value: T0): M0
*/
def liftValueType1[T0: c.WeakTypeTag, M0: c.WeakTypeTag](value: c.Expr[T0]): c.Expr[M0] = {
val from = weakTypeOf[T0]
val to = weakTypeOf[M0]
c.echo(c.enclosingPosition, s"T0: $from, M0: $to")
c.Expr[M0](q"${to.typeSymbol.companion}.apply($value)")
}
...
}
I get this compile error (can't infer types?):
Information:(31, 24) ValueType.liftValueType1 is not a valid implicit value for java.util.UUID => AccountId because:
hasMatchingSymbol reported error: type mismatch;
found : (value: Any)Nothing
required: java.util.UUID => AccountId
val conv = implicitly[UUID => AccountId]
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