shapeless-show.scala

// https://xebia.com/blog/how-to-derive-type-class-instances-with-shapeless-3/
//> using dep org.typelevel::shapeless3-deriving::3.5.0

package deriving

import shapeless3.deriving.*

trait Show[A]:
    def show(a: A): String

object Show:
    given Show[Int] = _.toString
    given Show[Boolean] = _.toString
    given Show[String] = identity(_)

    def deriveShowProduct[
        A: {Labelling as labelling, ([x] =>> K0.ProductInstances[Show, x]) as pInst},
    ]: Show[A] =
        (a: A) =>
            labelling.elemLabels.zipWithIndex
                .map { (label, index) =>
                    val value = pInst.project(a)(index)([t] => (st: Show[t], pt: t) => st.show(pt))
                    s"$label = $value"
                }
                .mkString(s"${labelling.label}(", ", ", ")")

    def deriveShowSum[A: ([x] =>> K0.CoproductInstances[Show, x]) as cInst]: Show[A] =
        (a: A) => cInst.fold(a)([a] => (st: Show[a], a: a) => st.show(a))

    inline given derived[A: K0.Generic as gen]: Show[A] =
        gen.derive(deriveShowProduct, deriveShowSum)

extension [A: Show as s](a: A) inline def show: String = s.show(a)

case class Pos(x: Int, y: Int) derives Show

enum Expr derives Show:
    case Var(name: String)
    case Add(left: Expr, right: Expr)
    case Mul(left: Expr, right: Expr)

@main def main() =
    println(Pos(3, 4).show)
    println(Expr.Add(Expr.Var("x"), Expr.Mul(Expr.Var("y"), Expr.Var("z"))).show)