Add withFallback DSL method, DSL macros and TransformerOverride type

chimney/src/main/scala-2/io/scalaland/chimney/dsl/PartialTransformerDefinition.scala

@@ -357,6 +357,27 @@ final class PartialTransformerDefinition[From, To, Overrides <: TransformerOverr
     macro PartialTransformerDefinitionMacros
       .withSealedSubtypeRenamedImpl[From, To, Overrides, Flags, FromSubtype, ToSubtype]
 
+  /** To use `fallback` when the source of type `From` is missing fields.
+    *
+    * Fallbacks can be stacked - then they will be tried in the order in which they were added.
+    *
+    * @see
+    *   TODO
+    *
+    * @tparam FromFallback
+    *   type of the fallback value which would be checked for fields when the `From` value would be missing
+    * @param fallback
+    *   fallback value which would be checked for fields when the `From` value would be missing
+    * @return
+    *   [[io.scalaland.chimney.dsl.PartialTransformerDefinition]]
+    *
+    * @since TODO
+    */
+  def withFallback[FromFallback](
+      fallback: FromFallback
+  ): PartialTransformerDefinition[From, To, ? <: TransformerOverrides, Flags] =
+    macro PartialTransformerDefinitionMacros.withFallbackImpl[From, To, Overrides, Flags, FromFallback]
+
   /** Use `f` instead of the primary constructor to construct the `To` value.
     *
     * Macro will read the names of Eta-expanded method's/lambda's parameters and try to match them with `From` getters.

chimney/src/main/scala-2/io/scalaland/chimney/dsl/PartialTransformerInto.scala

@@ -359,6 +359,27 @@ final class PartialTransformerInto[From, To, Overrides <: TransformerOverrides,
   def withEnumCaseRenamed[FromSubtype, ToSubtype]: PartialTransformerInto[From, To, ? <: TransformerOverrides, Flags] =
     macro PartialTransformerIntoMacros.withSealedSubtypeRenamedImpl[From, To, Overrides, Flags, FromSubtype, ToSubtype]
 
+  /** To use `fallback` when the source of type `From` is missing fields.
+    *
+    * Fallbacks can be stacked - then they will be tried in the order in which they were added.
+    *
+    * @see
+    *   TODO
+    *
+    * @tparam FromFallback
+    *   type of the fallback value which would be checked for fields when the `From` value would be missing
+    * @param fallback
+    *   fallback value which would be checked for fields when the `From` value would be missing
+    * @return
+    *   [[io.scalaland.chimney.dsl.TransformerOverrides]]
+    *
+    * @since TODO
+    */
+  def withFallback[FromFallback](
+      fallback: FromFallback
+  ): PartialTransformerInto[From, To, ? <: TransformerOverrides, Flags] =
+    macro PartialTransformerIntoMacros.withFallbackImpl[From, To, Overrides, Flags, FromFallback]
+
   /** Use `f` instead of the primary constructor to construct the `To` value.
     *
     * Macro will read the names of Eta-expanded method's/lambda's parameters and try to match them with `From` getters.

chimney/src/main/scala-2/io/scalaland/chimney/dsl/TransformerDefinition.scala

@@ -232,6 +232,27 @@ final class TransformerDefinition[From, To, Overrides <: TransformerOverrides, F
   def withEnumCaseRenamed[FromSubtype, ToSubtype]: TransformerDefinition[From, To, ? <: TransformerOverrides, Flags] =
     macro TransformerDefinitionMacros.withSealedSubtypeRenamedImpl[From, To, Overrides, Flags, FromSubtype, ToSubtype]
 
+  /** To use `fallback` when the source of type `From` is missing fields.
+    *
+    * Fallbacks can be stacked - then they will be tried in the order in which they were added.
+    *
+    * @see
+    *   TODO
+    *
+    * @tparam FromFallback
+    *   type of the fallback value which would be checked for fields when the `From` value would be missing
+    * @param fallback
+    *   fallback value which would be checked for fields when the `From` value would be missing
+    * @return
+    *   [[io.scalaland.chimney.dsl.TransformerDefinition]]
+    *
+    * @since TODO
+    */
+  def withFallback[FromFallback](
+      fallback: FromFallback
+  ): TransformerDefinition[From, To, ? <: TransformerOverrides, Flags] =
+    macro TransformerDefinitionMacros.withFallbackImpl[From, To, Overrides, Flags, FromFallback]
+
   /** Use `f` instead of the primary constructor to construct the `To` value.
     *
     * Macro will read the names of Eta-expanded method's/lambda's parameters and try to match them with `From` getters.

chimney/src/main/scala-2/io/scalaland/chimney/dsl/TransformerInto.scala

@@ -219,6 +219,25 @@ final class TransformerInto[From, To, Overrides <: TransformerOverrides, Flags <
   def withEnumCaseRenamed[FromSubtype, ToSubtype]: TransformerInto[From, To, ? <: TransformerOverrides, Flags] =
     macro TransformerIntoMacros.withSealedSubtypeRenamedImpl[From, To, Overrides, Flags, FromSubtype, ToSubtype]
 
+  /** To use `fallback` when the source of type `From` is missing fields.
+    *
+    * Fallbacks can be stacked - then they will be tried in the order in which they were added.
+    *
+    * @see
+    *   TODO
+    *
+    * @tparam FromFallback
+    *   type of the fallback value which would be checked for fields when the `From` value would be missing
+    * @param fallback
+    *   fallback value which would be checked for fields when the `From` value would be missing
+    * @return
+    *   [[io.scalaland.chimney.dsl.TransformerInto]]
+    *
+    * @since TODO
+    */
+  def withFallback[FromFallback](fallback: FromFallback): TransformerInto[From, To, ? <: TransformerOverrides, Flags] =
+    macro TransformerIntoMacros.withFallbackImpl[From, To, Overrides, Flags, FromFallback]
+
   /** Use `f` instead of the primary constructor to construct the `To` value.
     *
     * Macro will read the names of Eta-expanded method's/lambda's parameters and try to match them with `From` getters.

chimney/src/main/scala-2/io/scalaland/chimney/internal/compiletime/dsl/PartialTransformerDefinitionMacros.scala

@@ -159,6 +159,17 @@ class PartialTransformerDefinitionMacros(val c: whitebox.Context) extends utils.
       ]]
     )
 
+  def withFallbackImpl[
+      From: WeakTypeTag,
+      To: WeakTypeTag,
+      Overrides <: TransformerOverrides: WeakTypeTag,
+      Flags <: TransformerFlags: WeakTypeTag,
+      FromFallback: WeakTypeTag
+  ](fallback: Tree): Tree =
+    c.prefix.tree
+      .addOverride(fallback)
+      .asInstanceOfExpr[PartialTransformerDefinition[From, To, Fallback[FromFallback, Path.Root, Overrides], Flags]]
+
   def withConstructorImpl[
       From: WeakTypeTag,
       To: WeakTypeTag,

chimney/src/main/scala-2/io/scalaland/chimney/internal/compiletime/dsl/PartialTransformerIntoMacros.scala

@@ -170,6 +170,17 @@ class PartialTransformerIntoMacros(val c: whitebox.Context) extends utils.DslMac
       .asInstanceOfExpr[PartialTransformerInto[From, To, Constructor[Args, Path.Root, Overrides], Flags]]
   }.applyFromBody(f)
 
+  def withFallbackImpl[
+      From: WeakTypeTag,
+      To: WeakTypeTag,
+      Overrides <: TransformerOverrides: WeakTypeTag,
+      Flags <: TransformerFlags: WeakTypeTag,
+      FromFallback: WeakTypeTag
+  ](fallback: Tree): Tree =
+    c.prefix.tree
+      .addOverride(fallback)
+      .asInstanceOfExpr[PartialTransformerInto[From, To, Fallback[FromFallback, Path.Root, Overrides], Flags]]
+
   def withConstructorToImpl[
       From: WeakTypeTag,
       To: WeakTypeTag,

chimney/src/main/scala-2/io/scalaland/chimney/internal/compiletime/dsl/TransformerDefinitionMacros.scala

@@ -100,6 +100,17 @@ class TransformerDefinitionMacros(val c: whitebox.Context) extends utils.DslMacr
       ]]
     )
 
+  def withFallbackImpl[
+      From: WeakTypeTag,
+      To: WeakTypeTag,
+      Overrides <: TransformerOverrides: WeakTypeTag,
+      Flags <: TransformerFlags: WeakTypeTag,
+      FromFallback: WeakTypeTag
+  ](fallback: Tree): Tree =
+    c.prefix.tree
+      .addOverride(fallback)
+      .asInstanceOfExpr[TransformerDefinition[From, To, Fallback[FromFallback, Path.Root, Overrides], Flags]]
+
   def withConstructorImpl[
       From: WeakTypeTag,
       To: WeakTypeTag,

chimney/src/main/scala-2/io/scalaland/chimney/internal/compiletime/dsl/TransformerIntoMacros.scala

@@ -100,6 +100,17 @@ class TransformerIntoMacros(val c: whitebox.Context) extends utils.DslMacroUtils
       ]]
     )
 
+  def withFallbackImpl[
+      From: WeakTypeTag,
+      To: WeakTypeTag,
+      Overrides <: TransformerOverrides: WeakTypeTag,
+      Flags <: TransformerFlags: WeakTypeTag,
+      FromFallback: WeakTypeTag
+  ](fallback: Tree): Tree =
+    c.prefix.tree
+      .addOverride(fallback)
+      .asInstanceOfExpr[TransformerInto[From, To, Fallback[FromFallback, Path.Root, Overrides], Flags]]
+
   def withConstructorImpl[
       From: WeakTypeTag,
       To: WeakTypeTag,

chimney/src/main/scala-3/io/scalaland/chimney/dsl/PartialTransformerDefinition.scala

@@ -364,6 +364,27 @@ final class PartialTransformerDefinition[From, To, Overrides <: TransformerOverr
         .withSealedSubtypeRenamedImpl[From, To, Overrides, Flags, FromSubtype, ToSubtype]('this)
     }
 
+  /** To use `fallback` when the source of type `From` is missing fields.
+    *
+    * Fallbacks can be stacked - then they will be tried in the order in which they were added.
+    *
+    * @see
+    *   TODO
+    *
+    * @tparam FromFallback
+    *   type of the fallback value which would be checked for fields when the `From` value would be missing
+    * @param fallback
+    *   fallback value which would be checked for fields when the `From` value would be missing
+    * @return
+    *   [[io.scalaland.chimney.dsl.PartialTransformerDefinition]]
+    *
+    * @since TODO
+    */
+  transparent inline def withFallback[FromFallback](
+      inline fallback: FromFallback
+  ): PartialTransformerDefinition[From, To, ? <: TransformerOverrides, Flags] =
+    ${ PartialTransformerDefinitionMacros.withFallbackImpl('this, 'fallback) }
+
   /** Use `f` instead of the primary constructor to construct the `To` value.
     *
     * Macro will read the names of Eta-expanded method's/lambda's parameters and try to match them with `From` getters.

chimney/src/main/scala-3/io/scalaland/chimney/dsl/PartialTransformerInto.scala

@@ -370,6 +370,27 @@ final class PartialTransformerInto[From, To, Overrides <: TransformerOverrides,
       )
     }
 
+  /** To use `fallback` when the source of type `From` is missing fields.
+    *
+    * Fallbacks can be stacked - then they will be tried in the order in which they were added.
+    *
+    * @see
+    *   TODO
+    *
+    * @tparam FromFallback
+    *   type of the fallback value which would be checked for fields when the `From` value would be missing
+    * @param fallback
+    *   fallback value which would be checked for fields when the `From` value would be missing
+    * @return
+    *   [[io.scalaland.chimney.dsl.PartialTransformerInto]]
+    *
+    * @since TODO
+    */
+  transparent inline def withFallback[FromFallback](
+      inline fallback: FromFallback
+  ): PartialTransformerInto[From, To, ? <: TransformerOverrides, Flags] =
+    ${ PartialTransformerIntoMacros.withFallbackImpl('this, 'fallback) }
+
   /** Use `f` instead of the primary constructor to construct the `To` value.
     *
     * Macro will read the names of Eta-expanded method's/lambda's parameters and try to match them with `From` getters.

chimney/src/main/scala-3/io/scalaland/chimney/dsl/TransformerDefinition.scala

@@ -244,6 +244,27 @@ final class TransformerDefinition[From, To, Overrides <: TransformerOverrides, F
       )
     }
 
+  /** To use `fallback` when the source of type `From` is missing fields.
+    *
+    * Fallbacks can be stacked - then they will be tried in the order in which they were added.
+    *
+    * @see
+    *   TODO
+    *
+    * @tparam FromFallback
+    *   type of the fallback value which would be checked for fields when the `From` value would be missing
+    * @param fallback
+    *   fallback value which would be checked for fields when the `From` value would be missing
+    * @return
+    *   [[io.scalaland.chimney.dsl.TransformerDefinition]]
+    *
+    * @since TODO
+    */
+  transparent inline def withFallback[FromFallback](
+      inline fallback: FromFallback
+  ): TransformerDefinition[From, To, ? <: TransformerOverrides, Flags] =
+    ${ TransformerDefinitionMacros.withFallbackImpl('this, 'fallback) }
+
   /** Use `f` instead of the primary constructor to construct the `To` value.
     *
     * Macro will read the names of Eta-expanded method's/lambda's parameters and try to match them with `From` getters.

chimney/src/main/scala-3/io/scalaland/chimney/dsl/TransformerInto.scala

@@ -225,6 +225,27 @@ final class TransformerInto[From, To, Overrides <: TransformerOverrides, Flags <
       : TransformerInto[From, To, ? <: TransformerOverrides, Flags] =
     ${ TransformerIntoMacros.withSealedSubtypeRenamedImpl[From, To, Overrides, Flags, FromSubtype, ToSubtype]('this) }
 
+  /** To use `fallback` when the source of type `From` is missing fields.
+    *
+    * Fallbacks can be stacked - then they will be tried in the order in which they were added.
+    *
+    * @see
+    *   TODO
+    *
+    * @tparam FromFallback
+    *   type of the fallback value which would be checked for fields when the `From` value would be missing
+    * @param fallback
+    *   fallback value which would be checked for fields when the `From` value would be missing
+    * @return
+    *   [[io.scalaland.chimney.dsl.TransformerInto]]
+    *
+    * @since TODO
+    */
+  transparent inline def withFallback[FromFallback](
+      inline fallback: FromFallback
+  ): TransformerInto[From, To, ? <: TransformerOverrides, Flags] =
+    ${ TransformerIntoMacros.withFallbackImpl('this, 'fallback) }
+
   /** Use `f` instead of the primary constructor to construct the `To` value.
     *
     * Macro will read the names of Eta-expanded method's/lambda's parameters and try to match them with `From` getters.

chimney/src/main/scala-3/io/scalaland/chimney/internal/compiletime/dsl/PartialTransformerDefinitionMacros.scala

@@ -257,6 +257,22 @@ object PartialTransformerDefinitionMacros {
         ]]
     }
 
+  def withFallbackImpl[
+      From: Type,
+      To: Type,
+      Overrides <: TransformerOverrides: Type,
+      Flags <: TransformerFlags: Type,
+      FromFallback: Type
+  ](
+      td: Expr[PartialTransformerDefinition[From, To, Overrides, Flags]],
+      fallback: Expr[FromFallback]
+  )(using Quotes): Expr[PartialTransformerDefinition[From, To, ? <: TransformerOverrides, Flags]] =
+    '{
+      WithRuntimeDataStore
+        .update($td, $fallback)
+        .asInstanceOf[PartialTransformerDefinition[From, To, Fallback[FromFallback, Path.Root, Overrides], Flags]]
+    }
+
   def withConstructorImpl[
       From: Type,
       To: Type,

chimney/src/main/scala-3/io/scalaland/chimney/internal/compiletime/dsl/PartialTransformerIntoMacros.scala

@@ -242,6 +242,22 @@ object PartialTransformerIntoMacros {
         ]]
     }
 
+  def withFallbackImpl[
+      From: Type,
+      To: Type,
+      Overrides <: TransformerOverrides: Type,
+      Flags <: TransformerFlags: Type,
+      FromFallback: Type
+  ](
+      ti: Expr[PartialTransformerInto[From, To, Overrides, Flags]],
+      fallback: Expr[FromFallback]
+  )(using Quotes): Expr[PartialTransformerInto[From, To, ? <: TransformerOverrides, Flags]] =
+    '{
+      WithRuntimeDataStore
+        .update($ti, $fallback)
+        .asInstanceOf[PartialTransformerInto[From, To, Fallback[FromFallback, Path.Root, Overrides], Flags]]
+    }
+
   def withConstructorImpl[
       From: Type,
       To: Type,

chimney/src/main/scala-3/io/scalaland/chimney/internal/compiletime/dsl/TransformerDefinitionMacros.scala

@@ -151,6 +151,22 @@ object TransformerDefinitionMacros {
         ]]
     }
 
+  def withFallbackImpl[
+      From: Type,
+      To: Type,
+      Overrides <: TransformerOverrides: Type,
+      Flags <: TransformerFlags: Type,
+      FromFallback: Type
+  ](
+      ti: Expr[TransformerDefinition[From, To, Overrides, Flags]],
+      fallback: Expr[FromFallback]
+  )(using Quotes): Expr[TransformerDefinition[From, To, ? <: TransformerOverrides, Flags]] =
+    '{
+      WithRuntimeDataStore
+        .update($ti, $fallback)
+        .asInstanceOf[TransformerDefinition[From, To, Fallback[FromFallback, Path.Root, Overrides], Flags]]
+    }
+
   def withConstructorImpl[
       From: Type,
       To: Type,

chimney/src/main/scala-3/io/scalaland/chimney/internal/compiletime/dsl/TransformerIntoMacros.scala

@@ -169,6 +169,22 @@ object TransformerIntoMacros {
         }
     }(f)
 
+  def withFallbackImpl[
+      From: Type,
+      To: Type,
+      Overrides <: TransformerOverrides: Type,
+      Flags <: TransformerFlags: Type,
+      FromFallback: Type
+  ](
+      ti: Expr[TransformerInto[From, To, Overrides, Flags]],
+      fallback: Expr[FromFallback]
+  )(using Quotes): Expr[TransformerInto[From, To, ? <: TransformerOverrides, Flags]] =
+    '{
+      WithRuntimeDataStore
+        .update($ti, $fallback)
+        .asInstanceOf[TransformerInto[From, To, Fallback[FromFallback, Path.Root, Overrides], Flags]]
+    }
+
   def withConstructorToImpl[
       From: Type,
       To: Type,

chimney/src/main/scala/io/scalaland/chimney/internal/runtime/TransformerOverrides.scala

@@ -27,4 +27,6 @@ object TransformerOverrides {
   final class RenamedFrom[FromPath <: Path, ToPath <: Path, Tail <: Overrides] extends Overrides
   // Computes a value from matched subtype, targeting another subtype
   final class RenamedTo[FromPath <: Path, ToPath <: Path, Tail <: Overrides] extends Overrides
+  // Fallback value allowing merging several sources
+  final class Fallback[FromFallback, ToPath <: Path, Tail <: Overrides] extends Overrides
 }