irevive/union-derivation

A micro-library to derive a typeclass for Scala 3 Union types.

union-derivation

A micro-library to derive a typeclass for Scala 3 Union types.

Getting started

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 trait

Versions 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+	+	+	+

Usage example

Typeclass definition

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)
}

1. The derivation mechanism. Checkout Scala 3 docs for more details.
2. derivedUnion has IsUnion constraint, therefore the method can be applied only to Union types.

Usage

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$$Lambda$16196/0x00000070037ccd80@4475cd21
val userShow: Show[User] = summon[Show[User]]
// userShow: Show[User] = repl.MdocSession$$anon$9@5e969cb4

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))

Generated code

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")
}

scala-cli

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

union-derivation's Issues

Add scala-cli example

Cross-publish for Scala.JS and Scala Native

Please consider upstreaming to Scala3

Hi, thanks for the awesome library! I'm using heaps of unions so this is a lifesaver. Would you consider upstreaming this to Scala3? I think this is a missing feature that really belongs in the standard lib.

Add more examples

• The current example is complex. The scala-cli example is much better for the showcase
• Introduce two examples: minimal and full

Support concrete members

This code works as expected:

//> using scala "3.2.0"
//> using lib "io.github.irevive::union-derivation-core:0.0.3"

import io.github.irevive.union.derivation.{IsUnion, UnionDerivation}

trait Show[A]:

  def showValue(value: A): String

  //def show(value: A): String = showValue(value)

given Show[String] with

  override def showValue(value: String): String = s"str: $value"

given Show[Int] with

  override def showValue(value: Int): String = s"int: $value"

inline given derivedUnion[A](using IsUnion[A]): Show[A] = UnionDerivation.derive[Show, A]

println(summon[Show[String | Int]].showValue(1)) //int: 1
println(summon[Show[String | Int]].showValue("1")) //str: 1

but when uncommenting the concrete method, the compiler throws the following error:

[error] ./union.sc:23:35: More than one abstract method detected in trait Show: showValue, show. Automatic derivation is impossible
[error] println(summon[Show[String | Int]].showValue("1"))
[error]                                   ^
[error] ./union.sc:22:35: More than one abstract method detected in trait Show: showValue, show. Automatic derivation is impossible
[error] println(summon[Show[String | Int]].showValue(1))
[error]                                   ^

I expect the code to still compile since the show member is concrete and shouldn't be redefined. Maybe an annotation would help to disambiguate and choose the method to reimplement?

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