How to access method valueOf of an unknown enum in Scala 3

I'm trying to create inline def to generate Json codec for any enums in Scala 3. For this I need to have access to valueOf method of the parent of the enum. Something like this:

inline def gen[T](using JsonCodec[String], T <:< reflect.Enum): JsonCodec[T] = ???

How this can be achieved?

After reading comments now I changed my code to:

    val decoder: JsonDecoder[T] = JsonDecoder[String].mapOrFail(v => Try(${getEnum[T]}(v)).fold(e => Left(e.getMessage), v => Right(v)))
    val encoder: JsonEncoder[T] = JsonEncoder[String].contramap(_.toString)
    JsonCodec.apply(encoder, decoder)

  def getEnum[T: Type](using Quotes): Expr[String => T] =
    import quotes.reflect.*
    val companion = Ref(TypeTree.of[T].symbol.companionModule)
    Select.unique(companion, "valueOf").asExprOf[String => T]

And compiler complains with:

Malformed macro.

Expected the splice ${...} to be at the top of the RHS:
  inline def foo(inline x: X, ..., y: Y): Int = ${ impl('x, ... 'y) }

 * The contents of the splice must call a static method
 * All arguments must be quoted

As a json library you seem to use https://zio.github.io/zio-json

Malformed macro.

Expected the splice ${...} to be at the top of the RHS

I guess the error is understandable. You're not supposed to use macro implementations directly like that.

Either you have a macro (inline method) getEnum and its implementation getEnumImpl (returning an Expr) and you use getEnum (not getEnumImpl)

import zio.json.{JsonDecoder, JsonEncoder, JsonCodec}
import scala.quoted.*

inline def gen[T]: JsonCodec[T] =
  val decoder: JsonDecoder[T] = JsonDecoder[String].mapOrFail(v => util.Try(getEnum[T](v)).fold(e => Left(e.getMessage), v => Right(v)))
  val encoder: JsonEncoder[T] = JsonEncoder[String].contramap(_.toString)
  JsonCodec.apply(encoder, decoder)

inline def getEnum[T]: String => T = ${ getEnumImpl[T] }

def getEnumImpl[T: Type](using Quotes): Expr[String => T] =
  import quotes.reflect.*

  val companion = Ref(TypeTree.of[T].symbol.companionModule)
  Select.unique(companion, "valueOf").asExprOf[String => T]

or getEnum is a macro implementation itself and you use it in another macro implementation

import zio.json.{JsonDecoder, JsonEncoder, JsonCodec}
import scala.quoted.*

inline def gen[T]: JsonCodec[T] = ${genImpl[T]}

def genImpl[T: Type](using Quotes): Expr[JsonCodec[T]] =
  '{
    val decoder: JsonDecoder[T] = JsonDecoder[String].mapOrFail(v => util.Try(${ getEnum[T] }(v)).fold(e => Left(e.getMessage), v => Right(v)) )
    val encoder: JsonEncoder[T] = JsonEncoder[String].contramap(_.toString)
    JsonCodec.apply(encoder, decoder)
  }

def getEnum[T: Type](using Quotes): Expr[String => T] =
  import quotes.reflect.*

  val companion = Ref(TypeTree.of[T].symbol.companionModule)
  Select.unique(companion, "valueOf").asExprOf[String => T]

---

Now the error is

enum Color:
  case Red, Green, Blue

gen[Color].encodeJson(Color.Red, None)

// Exception occurred while executing macro expansion.
// java.lang.Exception: Expected an expression.
// This is a partially applied Term. Try eta-expanding the term first.

You can wrap Select.unique with Apply (and replace functions String => T with methods). So either

inline def gen[T <: reflect.Enum]: JsonCodec[T] =
  val decoder: JsonDecoder[T] = JsonDecoder[String].mapOrFail(v => util.Try(getEnum[T](v)).fold(e => Left(e.getMessage), v => Right(v)))
  val encoder: JsonEncoder[T] = JsonEncoder[String].contramap(_.toString)
  JsonCodec.apply(encoder, decoder)

inline def getEnum[T](v: String): T = ${ getEnumImpl[T]('v) }

def getEnumImpl[T: Type](v: Expr[String])(using Quotes): Expr[T] =
  import quotes.reflect.*

  val companion = Ref(TypeTree.of[T].symbol.companionModule)
  Apply(Select.unique(companion, "valueOf"), List(v.asTerm)).asExprOf[T]

or

inline def gen[T <: reflect.Enum]: JsonCodec[T] = ${genImpl[T]}

def genImpl[T: Type](using Quotes): Expr[JsonCodec[T]] =
  '{
    val decoder: JsonDecoder[T] = JsonDecoder[String].mapOrFail(v => util.Try(${ getEnum[T]('v) }).fold(e => Left(e.getMessage), v => Right(v)) )
    val encoder: JsonEncoder[T] = JsonEncoder[String].contramap(_.toString)
    JsonCodec.apply(encoder, decoder)
  }

def getEnum[T: Type](v: Expr[String])(using Quotes): Expr[T] =
  import quotes.reflect.*

  val companion = Ref(TypeTree.of[T].symbol.companionModule)
  Apply(Select.unique(companion, "valueOf"), List(v.asTerm)).asExprOf[T]

---

Also in order to derive JsonCodec for enums you can use Mirror

import scala.compiletime.{constValue, erasedValue, summonInline}
import scala.deriving.Mirror

inline given [T <: reflect.Enum with Singleton](using
  m: Mirror.ProductOf[T]
): JsonCodec[T] =
  val decoder: JsonDecoder[T] =
    val label = constValue[m.MirroredLabel]
    JsonDecoder[String].mapOrFail(v => Either.cond(label == v, m.fromProduct(EmptyTuple), s"$label != $v"))
  val encoder: JsonEncoder[T] = JsonEncoder[String].contramap(_.toString)
  JsonCodec.apply(encoder, decoder)

inline def mkSumDecoder[T, Tup <: Tuple]: JsonDecoder[T] = inline erasedValue[Tup] match
  case _: (h *: EmptyTuple) => summonInline[JsonDecoder[h & T]].widen[T]
  case _: (h *: t) => summonInline[JsonDecoder[h & T]].widen[T] <> mkSumDecoder[T, t]

inline given [T <: reflect.Enum](using
  m: Mirror.SumOf[T]
): JsonCodec[T] =
  val decoder: JsonDecoder[T] = mkSumDecoder[T, m.MirroredElemTypes]
  val encoder: JsonEncoder[T] = JsonEncoder[String].contramap(_.toString)
  JsonCodec.apply(encoder, decoder)

Testing:

import zio.json.given

(Color.Blue: Color.Blue.type).toJson // "Blue"
(Color.Blue: Color).toJson           // "Blue"
""" "Blue" """.fromJson[Color.Blue.type] // Right(Blue)
""" "Blue" """.fromJson[Color]           // Right(Blue)

---

There can be also cases depending on a parameter like in

enum Color:
  case Red(i: Int)
  case Green, Blue

We can also use https://github.com/typelevel/shapeless-3 for derivation or derive in Shapeless 2 style

sealed trait Coproduct extends Product with Serializable
sealed trait +:[+H, +T <: Coproduct] extends Coproduct
final case class Inl[+H, +T <: Coproduct](head: H) extends (H +: T)
final case class Inr[+H, +T <: Coproduct](tail: T) extends (H +: T)
sealed trait CNil extends Coproduct

object Coproduct:
  def unsafeToCoproduct(length: Int, value: Any): Coproduct =
    (0 until length).foldLeft[Coproduct](Inl(value))((c, _) => Inr(c))

  @scala.annotation.tailrec
  def unsafeFromCoproduct(c: Coproduct): Any = c match
    case Inl(h) => h
    case Inr(c) => unsafeFromCoproduct(c)
    case _: CNil => sys.error("impossible")

  type ToCoproduct[T <: Tuple] <: Coproduct = T match
    case EmptyTuple => CNil
    case h *: t => h +: ToCoproduct[t]

  type ToTuple[C <: Coproduct] <: Tuple = C match
    case CNil => EmptyTuple
    case h +: t => h *: ToTuple[t]

trait Generic[T]:
  type Repr
  def to(t: T): Repr
  def from(r: Repr): T

object Generic:
  type Aux[T, Repr0] = Generic[T] {type Repr = Repr0}
  def instance[T, Repr0](f: T => Repr0, g: Repr0 => T): Aux[T, Repr0] =
    new Generic[T]:
      override type Repr = Repr0
      override def to(t: T): Repr0 = f(t)
      override def from(r: Repr0): T = g(r)

  object ops:
    extension[A] (a: A)
      def toRepr(using g: Generic[A]): g.Repr = g.to(a)

    extension[Repr] (a: Repr)
      def to[A](using g: Generic.Aux[A, Repr]): A = g.from(a)

  given[T <: Product](using
    // ev: NotGiven[T <:< Tuple],
    // ev1: NotGiven[T <:< Coproduct],
    m: Mirror.ProductOf[T],
    m1: Mirror.ProductOf[m.MirroredElemTypes]
  ): Aux[T, m.MirroredElemTypes] = instance(
    m1.fromProduct,
    m.fromProduct
  )

  given[T, C <: Coproduct](using
    // ev: NotGiven[T <:< Tuple],
    // ev1: NotGiven[T <:< Coproduct],
    m: Mirror.SumOf[T],
    ev2: Coproduct.ToCoproduct[m.MirroredElemTypes] =:= C
  ): Generic.Aux[T, C/*Coproduct.ToCoproduct[m.MirroredElemTypes]*/] = instance(
    t => Coproduct.unsafeToCoproduct(m.ordinal(t), t).asInstanceOf[C],
    Coproduct.unsafeFromCoproduct(_).asInstanceOf[T]
  )

inline given singleton[T <: reflect.Enum with Singleton](using
  m: Mirror.ProductOf[T]
): JsonCodec[T] =
  val decoder: JsonDecoder[T] =
    val label = constValue[m.MirroredLabel]
    JsonDecoder[String].mapOrFail(v => Either.cond(label == v, m.fromProduct(EmptyTuple), s"$label != $v"))
  val encoder: JsonEncoder[T] = JsonEncoder[String].contramap(_.toString)
  JsonCodec(encoder, decoder)

inline given [T <: reflect.Enum](using
  m: Mirror.ProductOf[T]
): JsonCodec[T] = DeriveJsonCodec.gen[T]

given [T <: reflect.Enum](using
  gen: Generic.Aux[T, _ <: Coproduct],
  codec: JsonCodec[gen.Repr]
): JsonCodec[T] = codec.transform(gen.from, gen.to)

given [H, T <: Coproduct](using
  hCodec: JsonCodec[H],
  tCodec: JsonCodec[T]
): JsonCodec[H +: T] = (hCodec <+> tCodec).transform({
  case Left(h) => Inl(h)
  case Right(t) => Inr(t)
}, {
  case Inl(h) => Left(h)
  case Inr(t) => Right(t)
})

given JsonCodec[CNil] = JsonCodec[String].transform(_ => sys.error("impossible"), _ => sys.error("impossible"))

import zio.json.given

(Color.Blue: Color.Blue.type).toJson // "Blue"
(Color.Blue: Color).toJson           // "Blue"
(Color.Red(1): Color.Red).toJson // {"i":1}
(Color.Red(1): Color).toJson     // {"i":1}
""" "Blue" """.fromJson[Color.Blue.type] // Right(Blue)
""" "Blue" """.fromJson[Color]           // Right(Blue)
"""{"i":1}""".fromJson[Color.Red] // Right(Red(1))
"""{"i":1}""".fromJson[Color]     // Right(Red(1))