A micro-library to derive a typeclass for Scala 3 Union types.
To use union-derivation in an existing SBT project with Scala 3.3.1 or a later version, add the following configuration to your build.sbt:
libraryDependencies += "io.github.irevive" %% "union-derivation-core" % "0.1.0"
scalacOptions += "-Yretain-trees" // important for the detection of an abstract method in a traitVersions matrix:
| Scala | Library | JVM | Scala Native | Scala.js |
|---|---|---|---|---|
| 3.1.2 | 0.0.3 | + | - | - |
| 3.2.0+ | 0.0.4+ | + | + | - |
| 3.3.1+ | 0.1.0+ | + | + | + |
import io.github.irevive.union.derivation.{IsUnion, UnionDerivation}
import scala.compiletime.{erasedValue, summonInline}
import scala.deriving.*
trait Show[A] {
def show(a: A): String
}
object Show extends ShowLowPriority {
given Show[Int] = v => s"Int($v)"
given Show[Long] = v => s"Long($v)"
given Show[String] = v => s"String($v)"
inline given derived[A](using m: Mirror.Of[A]): Show[A] = { // (1)
val elemInstances = summonAll[m.MirroredElemTypes]
inline m match {
case s: Mirror.SumOf[A] => showSum(s, elemInstances)
case _: Mirror.ProductOf[A] => showProduct(elemInstances)
}
}
inline def summonAll[A <: Tuple]: List[Show[?]] =
inline erasedValue[A] match {
case _: EmptyTuple => Nil
case _: (t *: ts) => summonInline[Show[t]] :: summonAll[ts]
}
private def showA[A](a: A, show: Show[?]): String =
show.asInstanceOf[Show[A]].show(a)
private def showSum[A](s: Mirror.SumOf[A], elems: => List[Show[?]]): Show[A] =
new Show[A] {
def show(a: A): String = showA(a, elems(s.ordinal(a)))
}
private def showProduct[A](elems: => List[Show[?]]): Show[A] =
new Show[A] {
def show(a: A): String = {
val product = a.asInstanceOf[Product]
product.productIterator
.zip(product.productElementNames)
.zip(elems.iterator)
.map { case ((field, name), show) => s"$name = ${showA[Any](field, show)}" }
.mkString(product.productPrefix + "(", ", ", ")")
}
}
}
trait ShowLowPriority {
inline given derivedUnion[A](using IsUnion[A]): Show[A] = UnionDerivation.derive[Show, A] // (2)
}- The derivation mechanism. Checkout Scala 3 docs for more details.
derivedUnionhasIsUnionconstraint, therefore the method can be applied only to Union types.
type UnionType = Int | Long | String
final case class User(name: String, age: Long, flags: UnionType)
val unionShow: Show[UnionType] = summon[Show[UnionType]]
// unionShow: Show[UnionType] = repl.MdocSession$MdocApp$$Lambda$44884/0x0000007004e88d80@60f65413
val userShow: Show[User] = summon[Show[User]]
// userShow: Show[User] = repl.MdocSession$$anon$9@3ae6c4fd
println(unionShow.show(1))
// Int(1)
println(unionShow.show(2L))
// Long(2)
println(unionShow.show("3"))
// String(3)
println(userShow.show(User("Pablo", 22, 12L)))
// User(name = String(Pablo), age = Long(22), flags = Long(12))
println(userShow.show(User("Pablo", 33, 1)))
// User(name = String(Pablo), age = Long(33), flags = Int(1))The library creates a set of if-else statements for the known types of the union.
The simplified version of the generated code:
val instance: Show[Int | String | Long] = UnionDerivation.derive[Show, Int | String | Long]
// expands into
val instance: Show[Int | String | Long] = { (value: Int | String | Long) =>
if (value.isInstanceOf[Int]) summon[Show[Int]].show(value)
else if (value.isInstanceOf[String]) summon[Show[String]].show(value)
else if (value.isInstanceOf[Long]) summon[Show[Long]].show(value)
else sys.error("Impossible")
}The library works out of the box with scala-cli too.
//> using scala "3.3.1"
//> using lib "io.github.irevive::union-derivation-core:0.1.0"
//> using options "-Yretain-trees"
import io.github.irevive.union.derivation.{IsUnion, UnionDerivation}
trait Show[A] {
def show(value: A): String
}
given Show[String] = value => s"str: $value"
given Show[Int] = value => s"int: $value"
inline given derivedUnion[A](using IsUnion[A]): Show[A] = UnionDerivation.derive[Show, A]
println(summon[Show[String | Int]].show(1))
// int: 1
println(summon[Show[String | Int]].show("1"))
// str: 1
