Add macro-based implementation of the typeclass derivation example

Co-authored-by: Fengyun Liu <[email protected]>

Author: biboudis

tests/run-macros/i8007.check

@@ -1,3 +1,9 @@
 List("name", "age")
 
 Test 23 
+()
+
+true
+
+false
+

tests/run-macros/i8007/Macro_1.scala

@@ -12,10 +12,9 @@ object Macro1 {
       case '[Unit] => Nil
     }
 
-  // Macro method 1
-  //   Demonstrates the use of quoted pattern matching
-  //   over a refined type, extracting the tuple type
-  //   for e.g., MirroredElemLabels
+  // Demonstrates the use of quoted pattern matching
+  // over a refined type extracting the tuple type
+  // for e.g., MirroredElemLabels
   inline def test1[T](value: =>T): List[String] =
     ${ test1Impl('value) }
 

tests/run-macros/i8007/Macro_3.scala

@@ -0,0 +1,68 @@
+import scala.deriving._
+import scala.quoted._
+import scala.quoted.matching._
+import scala.compiletime.{erasedValue, summonFrom, constValue}
+
+object Macro3 {
+
+  trait Eq[T] {
+    def eqv(x: T, y: T): Boolean
+  }
+
+  object Eq {
+    given Eq[String] {
+      def eqv(x: String, y: String) = x == y
+    }
+
+    given Eq[Int] {
+      def eqv(x: Int, y: Int) = x == y
+    }
+
+    def eqProduct[T](body: (T, T) => Boolean): Eq[T] =
+      new Eq[T] {
+        def eqv(x: T, y: T): Boolean = body(x, y)
+      }
+
+    def summonAll[T](t: Type[T])(given qctx: QuoteContext): List[Expr[Eq[_]]] = t match {
+      case '[$tpe *: $tpes] => summonExpr(given '[Eq[$tpe]]).get :: summonAll(tpes)
+      case '[Unit] => Nil
+    }
+
+    def derived[T: Type](ev: Expr[Mirror.Of[T]])(given qctx: QuoteContext): Expr[Eq[T]] = {
+      import qctx.tasty.{_, given}
+
+      val elementTypes = ev match {
+        case '{ $m: Mirror.ProductOf[T] { type MirroredElemTypes = $elem } } => elem
+      }
+
+      val elemInstances = summonAll(elementTypes)
+
+      val eqProductBody: (Expr[T], Expr[T]) => Expr[Boolean] = (x, y) => {
+        elemInstances.zipWithIndex.foldLeft(Expr(true: Boolean)) {
+          case (acc, (elem, index)) =>
+            val e1 = '{$x.asInstanceOf[Product].productElement(${Expr(index)})}
+            val e2 = '{$y.asInstanceOf[Product].productElement(${Expr(index)})}
+            '{ $acc && $elem.asInstanceOf[Eq[Any]].eqv($e1, $e2) }
+        }
+      }
+
+      '{
+        eqProduct((x: T, y: T) => ${eqProductBody('x, 'y)})
+      }
+    }
+  }
+
+  inline def test3[T](value: =>T, value2: =>T): Boolean = ${ test3Impl('value, 'value2) }
+
+  def test3Impl[T: Type](value: Expr[T], value2: Expr[T])(given qctx: QuoteContext): Expr[Boolean] = {
+    import qctx.tasty.{_, given}
+
+    val mirrorTpe = '[Mirror.Of[T]]
+    val mirrorExpr = summonExpr(given mirrorTpe).get
+    val derivedInstance = Eq.derived(mirrorExpr)
+
+    '{
+      $derivedInstance.eqv($value, $value2)
+    }
+  }
+}

tests/run-macros/i8007/Test_3.scala

@@ -0,0 +1,21 @@
+import Macro1._
+import Macro2._
+import Macro3._
+
+@main def Test() = {
+  val t1 = test1(Person("Test", 23))
+  println(t1)
+  println
+
+  val t2 = test2(Person("Test", 23))
+  println(t2)
+  println
+
+  val t3 = test3(Person("Test", 23), Person("Test", 23))
+  println(t3) // true
+  println
+
+  val t4 = test3(Person("Test", 23), Person("Test", 24))
+  println(t4) // false
+  println
+}