feat: adding on-demand default evaluation

core/src/main/scala/magnolia1/impl.scala

@@ -102,7 +102,7 @@ object CaseClassDerivation:
         paramFromMaps[Typeclass, A, p](
           label,
           CallByNeed(summonInline[Typeclass[p]]),
-          CallByNeed(defaults.get(label).flatten.flatMap(d => unsafeCast(d.apply))),
+          CallByNeed.withValueEvaluator(defaults.get(label).flatten.flatMap(d => unsafeCast(d.apply))),
           repeated,
           annotations,
           inheritedAnnotations,

core/src/main/scala/magnolia1/interface.scala

@@ -1,5 +1,7 @@
 package magnolia1
 
+import magnolia1.CaseClass.getDefaultEvaluatorFromDefaultVal
+
 import scala.annotation.tailrec
 import scala.reflect.*
 
@@ -40,6 +42,9 @@ object CaseClass:
       *   default argument value, if any
       */
     def default: Option[PType]
+
+    /** provides a function to evaluate the default value for this parameter, as defined in the case class constructor */
+    def evaluateDefault: Option[() => PType] = None
     def inheritedAnnotations: IArray[Any] = IArray.empty[Any]
     override def toString: String = s"Param($label)"
 
@@ -63,6 +68,7 @@ object CaseClass:
       ):
         type PType = P
         def default: Option[PType] = defaultVal.value
+        override def evaluateDefault: Option[() => PType] = CaseClass.getDefaultEvaluatorFromDefaultVal(defaultVal)
         def typeclass = cbn.value
         override def inheritedAnnotations = inheritedAnns
         def deref(value: T): P =
@@ -87,10 +93,18 @@ object CaseClass:
       ):
         type PType = P
         def default: Option[PType] = defaultVal.value
+        override def evaluateDefault: Option[() => PType] = getDefaultEvaluatorFromDefaultVal(defaultVal)
         def typeclass = cbn.value
         def deref(value: T): P =
           value.asInstanceOf[Product].productElement(idx).asInstanceOf[P]
   end Param
+
+  private def getDefaultEvaluatorFromDefaultVal[P](defaultVal: CallByNeed[Option[P]]): Option[() => P] =
+    defaultVal.valueEvaluator.flatMap { evaluator =>
+      evaluator().fold[Option[() => P]](None) { _ =>
+        Some(() => evaluator().get)
+      }
+    }
 end CaseClass
 
 /** In the terminology of Algebraic Data Types (ADTs), case classes are known as 'product types'.
@@ -162,6 +176,7 @@ abstract class CaseClass[Typeclass[_], Type](
     ):
       type PType = P
       def default: Option[PType] = defaultVal.value
+      override def evaluateDefault: Option[() => PType] = getDefaultEvaluatorFromDefaultVal(defaultVal)
       def typeclass = cbn.value
       override def inheritedAnnotations = inheritedAnns
       def deref(value: Type): P =
@@ -345,10 +360,44 @@ object SealedTrait:
 end SealedTrait
 
 object CallByNeed:
-  def apply[A](a: => A): CallByNeed[A] = new CallByNeed(() => a)
+  /** Initializes a class that allows for suspending evaluation of a value until it is needed. Evaluation of a value via `.value` can only
+    * happen once.
+    */
+  def apply[A](a: => A): CallByNeed[A] = new CallByNeed(() => a, () => false)
 
-final class CallByNeed[+A](private[this] var eval: () => A) extends Serializable:
-  lazy val value: A =
-    val result = eval()
-    eval = null
-    result
+  /** Initializes a class that allows for suspending evaluation of a value until it is needed. Evaluation of a value via `.value` can only
+    * happen once. Evaluation of a value via `.valueEvaluator.map(evaluator => evaluator())` will happen every time the evaluator is called
+    */
+  def withValueEvaluator[A](a: => A): CallByNeed[A] = new CallByNeed(() => a, () => true)
+end CallByNeed
+
+// Both params are later nullified to reduce overhead and increase performance.
+// The supportDynamicValueEvaluation is passed as a function so that it can be nullified. Otherwise, there is no need for the function value.
+final class CallByNeed[+A] private (private[this] var eval: () => A, private var supportDynamicValueEvaluation: () => Boolean)
+    extends Serializable {
+
+  // This second constructor is necessary to support backwards compatibility for v1.3.6 and earlier
+  def this(eval: () => A) = this(eval, () => false)
+
+  val valueEvaluator: Option[() => A] = {
+    val finalRes = if (supportDynamicValueEvaluation()) {
+      val res = Some(eval)
+      eval = null
+      res
+    } else {
+      None
+    }
+    supportDynamicValueEvaluation = null
+    finalRes
+  }
+
+  lazy val value: A = {
+    if (eval == null) {
+      valueEvaluator.get.apply()
+    } else {
+      val result = eval()
+      eval = null
+      result
+    }
+  }
+}

core/src/main/scala/magnolia1/magnolia.scala

@@ -1,10 +1,6 @@
 package magnolia1
 
-import scala.compiletime.*
 import scala.deriving.Mirror
-import scala.reflect.*
-
-import Macro.*
 
 trait CommonDerivation[TypeClass[_]]:
   type Typeclass[T] = TypeClass[T]

examples/src/main/scala/magnolia1/examples/default.scala

@@ -5,6 +5,7 @@ import magnolia1._
 /** typeclass for providing a default value for a particular type */
 trait HasDefault[T]:
   def defaultValue: Either[String, T]
+  def getDynamicDefaultValueForParam(paramLabel: String): Option[Any] = None
 
 /** companion object and derivation object for [[HasDefault]] */
 object HasDefault extends AutoDerivation[HasDefault]:
@@ -19,6 +20,16 @@ object HasDefault extends AutoDerivation[HasDefault]:
           case None      => param.typeclass.defaultValue
         }
       }
+
+      override def getDynamicDefaultValueForParam(paramLabel: String): Option[Any] =
+        IArray
+          .genericWrapArray {
+            ctx.params
+              .filter(_.label == paramLabel)
+          }
+          .toArray
+          .headOption
+          .flatMap(_.evaluateDefault.map(res => res()))
     }
 
   /** chooses which subtype to delegate to */
@@ -28,6 +39,12 @@ object HasDefault extends AutoDerivation[HasDefault]:
         case Some(sub) => sub.typeclass.defaultValue
         case None      => Left("no subtypes")
 
+      override def getDynamicDefaultValueForParam(paramLabel: String): Option[Any] =
+        ctx.subtypes.headOption match {
+          case Some(sub) => sub.typeclass.getDynamicDefaultValueForParam(paramLabel)
+          case _         => None
+        }
+
   /** default value for a string; the empty string */
   given string: HasDefault[String] with
     def defaultValue = Right("")
@@ -39,6 +56,9 @@ object HasDefault extends AutoDerivation[HasDefault]:
   given boolean: HasDefault[Boolean] with
     def defaultValue = Left("truth is a lie")
 
+  given double: HasDefault[Double] with
+    def defaultValue = Right(0)
+
   /** default value for sequences; the empty sequence */
   given seq[A]: HasDefault[Seq[A]] with
     def defaultValue = Right(Seq.empty)

test/src/test/scala/magnolia1/tests/DefaultValuesTests.scala

@@ -37,6 +37,19 @@ class DefaultValuesTests extends munit.FunSuite:
     assertEquals(res, Right(Item("", 1, 0)))
   }
 
+  test("access dynamic default constructor values") {
+    val res1 = summon[HasDefault[ParamsWithDynamicDefault]].getDynamicDefaultValueForParam("a")
+    val res2 = summon[HasDefault[ParamsWithDynamicDefault]].getDynamicDefaultValueForParam("a")
+
+    assertEquals(res1.isDefined, true)
+    assertEquals(res2.isDefined, true)
+
+    for {
+      default1 <- res1
+      default2 <- res2
+    } yield assertNotEquals(default1, default2)
+  }
+
   test("construct a HasDefault instance for a generic product with default values") {
     val res = HasDefault.derived[ParamsWithDefaultGeneric[String, Int]].defaultValue
     assertEquals(res, Right(ParamsWithDefaultGeneric("A", 0)))
@@ -52,6 +65,8 @@ object DefaultValuesTests:
 
   case class ParamsWithDefault(a: Int = 3, b: Int = 4)
 
+  case class ParamsWithDynamicDefault(a: Double = scala.math.random())
+
   case class ParamsWithDefaultGeneric[A, B](a: A = "A", b: B = "B")
 
   case class ParamsWithDefaultDeepGeneric[A, B](a: Option[A] = Some("A"), b: Option[B] = Some("B"))