zio-gen-magnolia.scala

import magnolia.{ CaseClass, Magnolia, SealedTrait }
import mercator.Monadic
import zio.random.Random
import zio.test.{ Gen, Sized }

object Generators {

  implicit val genUnit: Typeclass[Unit]     = Gen.unit
  implicit val genBool: Typeclass[Boolean]  = Gen.boolean
  implicit val genString: Typeclass[String] = Gen.small(Gen.stringN(_)(Gen.alphaNumericChar))
  implicit val genLong: Typeclass[Long]     = Gen.anyLong
  implicit val genInt: Typeclass[Int]       = Gen.anyInt

  implicit def genOption[A](implicit ev: Typeclass[A]): Typeclass[Option[A]]                           = Gen.option(ev)
  implicit def genEither[A, B](implicit ev1: Typeclass[A], ev2: Typeclass[B]): Typeclass[Either[A, B]] = Gen.either(ev1, ev2)
  implicit def genList[A](implicit ev: Typeclass[A]): Typeclass[List[A]]                               = Gen.small(Gen.listOfN(_)(ev))
  implicit def genSet[A](implicit ev: Typeclass[A]): Typeclass[Set[A]]                                 = genList(ev).map(_.toSet)
  implicit def genTuple[A, B](implicit ev1: Typeclass[A], ev2: Typeclass[B]): Typeclass[(A, B)]        = ev1.flatMap(a => ev2.map((a, _)))
  implicit def genMap[A, B](implicit ev1: Typeclass[A], ev2: Typeclass[B]): Typeclass[Map[A, B]]       = genList(genTuple(ev1, ev2)).map(_.toMap)
  
  object Derivation {
    type Typeclass[T] = Gen[Random with Sized, T]

    implicit val monadic: Monadic[Typeclass] = new Monadic[Typeclass] {
      override def point[A](value: A): Typeclass[A]                                       = Gen.const(value)
      override def flatMap[A, B](from: Typeclass[A])(fn: A => Typeclass[B]): Typeclass[B] = from.flatMap(fn)
      override def map[A, B](from: Typeclass[A])(fn: A => B): Typeclass[B]                = from.map(fn)
    }

    def combine[T](caseClass: CaseClass[Typeclass, T]): Typeclass[T]      = caseClass.constructMonadic(_.typeclass)
    def dispatch[T](sealedTrait: SealedTrait[Typeclass, T]): Typeclass[T] = Gen.oneOf(sealedTrait.subtypes.map(_.typeclass): _*)
    implicit def gen[T]: Typeclass[T] = macro Magnolia.gen[T]
  }
}