Implementation of Generic in Dotty

In Scala2, shapeless provides Generic. Generic is a type class that expresses interconversion between algebraic data type and simple data type that is expressed with product and coproduct. Shapeless implements it with macros since Scala2 language does not provide any useful information about such interconversion.

Dotty supports Mirror type class for supporting type class derivation. The Mirror provides information about how to construct algebraic data type from product of its elements, so we can implement Generic without macros by utilizing Mirror.

sealed trait Foo
case class Bar (a: Int, b: String) extends Foo
case class Baz (c: String) extends Foo

import com.phenan.coproduct._
import com.phenan.generic._
import com.phenan.generic.given

val generic = summon[Generic[Bar, (Int, String)]]

val x: Bar = generic.from((10, "bar"))   // Bar(10, "bar")
val y: (Int, String) = generic.to(x)     // (10, "bar")

val z: Bar :+: Baz :+: CNil = generic.to(Baz("baz"))    // InR(InL(Baz("baz")))
val w: Foo = generic.from(z)                            // Baz("baz")

@phenan