Closed
Description
Compiler version
3.0.0-RC2
sbt 1.4.9 (GraalVM Community Java 1.8.0_282)
Minimized code
import scala.deriving.*
import scala.compiletime.{erasedValue, summonInline}
case class Simple(a: String, b: Boolean) derives Printable
case class SimpleT(a: (String, Boolean)) derives Printable
@main def hello: Unit = {
summon[Printable[Simple]].print // Prints STRING BOOLEAN as expected
summon[Printable[SimpleT]].print // java.lang.ClassCastException: SimpleT$$anon$1 cannot be cast to scala.deriving.Mirror$Product
}
trait Printable[T]:
def print: Unit
object Printable:
given Printable[String] with
def print: Unit = println("STRING")
given Printable[Boolean] with
def print: Unit = println("BOOLEAN")
def printProduct[T](p: Mirror.ProductOf[T], elems: => List[Printable[_]]): Printable[T] =
new Printable[T]:
def print: Unit =
elems.foreach(_.print)
inline given derived[T](using m: Mirror.Of[T]): Printable[T] =
val elemInstances = summonAllPrintable[m.MirroredElemTypes]
inline m match
case p: Mirror.ProductOf[T] => printProduct(p, elemInstances)
end Printable
inline def summonAllPrintable[T <: Tuple]: List[Printable[_]] =
inline erasedValue[T] match
case _: EmptyTuple => Nil
case _: (t *: ts) => summonInline[Printable[t]] :: summonAllPrintable[ts]Output
> run
[info] running hello
STRING
BOOLEAN
[error] (run-main-0) java.lang.ClassCastException: SimpleT$$anon$1 cannot be cast to scala.deriving.Mirror$Product
[error] java.lang.ClassCastException: SimpleT$$anon$1 cannot be cast to scala.deriving.Mirror$Product
[error] at SimpleT$.derived$Printable(Main.scala:5)
[error] at Main$package$.hello(Main.scala:11)
[error] at hello.main(Main.scala:7)
[error] at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
[error] at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
[error] at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
[error] at java.lang.reflect.Method.invoke(Method.java:498)
[error] stack trace is suppressed; run last Compile / bgRun for the full output
[error] Nonzero exit code: 1
[error] (Compile / run) Nonzero exit code: 1
[error] Total time: 3 s, completed Mar 31, 2021 9:16:29 PM
Expectation
I'd expect this program to produce compilation error about missing Printable[(String, Boolean)] or similar.
Trying to get familiar with type-classes-supporting-automatic-deriving I'm trying to implement typeclass which prints its own type.
Minimized project is available here.