Create extension methods for onSubtype/on.../eachItem/each... in DSL

benchmarks/src/main/scala/io/scalaland/chimney/benchmarks/fixtures.scala

@@ -104,10 +104,10 @@ object fixtures {
       }
 
       object nonFunhappy {
-        def validateA(a: Int): Int = throw new Exception("a not nice")
+        def validateA(a: Int): Int = sys.error("a not nice")
         def validateB(b: Double): Double = b
-        def validateC(c: String): String = throw new Exception("c not pretty")
-        def validateD(d: Option[String]): Option[String] = throw new Exception("I don't like this d")
+        def validateC(c: String): String = sys.error("c not pretty")
+        def validateD(d: Option[String]): Option[String] = sys.error("I don't like this d")
       }
 
       object happy {

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

@@ -34,6 +34,9 @@ package object dsl {
   implicit def TryPartialTransformerOps[A](`try`: Try[A]): syntax.TryPartialTransformerOps[A] =
     syntax.TryPartialTransformerOps(`try`)
 
+  implicit def TransformationPathOps[A](a: A): syntax.TransformationPathOps[A] =
+    syntax.TransformationPathOps(a)
+
   // Extension methods in dsl.* summon TypeClass.AutoDerived while extension methods in syntax.* summon TypeClass.
   // This help us preserve legacy behavior in dsl code while keeping stricter separation in auto/syntax imports.
 

chimney/src/main/scala-2/io/scalaland/chimney/syntax/package.scala

@@ -1,5 +1,8 @@
 package io.scalaland.chimney
 
+import io.scalaland.chimney.internal.runtime.{IsCollection, IsEither, IsMap, IsOption}
+
+import scala.annotation.unused
 import scala.util.Try
 
 /** Imports only extension methods for summoning and using Transformer, PartialTransformer or Patcher
@@ -177,4 +180,29 @@ package object syntax {
     def toPartialResult: partial.Result[A] =
       partial.Result.fromTry(`try`)
   }
+
+  // TODO: docs
+  implicit final class TransformationPathOps[A](@unused private val a: A) extends AnyVal {
+
+    def onSubtype[B <: A]: B =
+      sys.error(".onSubtype should be only called within Chimney DSL")
+
+    def onSome[B](implicit @unused ev: IsOption.Of[A, B]): B =
+      sys.error(".onSome should be only called within Chimney DSL")
+
+    def onLeft[L, R](implicit @unused ev: IsEither.Of[A, L, R]): L =
+      sys.error(".onLeft should be only called within Chimney DSL")
+
+    def onRight[L, R](implicit @unused ev: IsEither.Of[A, L, R]): R =
+      sys.error(".onRight should be only called within Chimney DSL")
+
+    def eachItem[B](implicit @unused ev: IsCollection.Of[A, B]): B =
+      sys.error(".eachItem should be only called within Chimney DSL")
+
+    def eachMapKey[K, V](implicit @unused ev: IsMap.Of[K, V, A]): K =
+      sys.error(".eachMapKey should be only called within Chimney DSL")
+
+    def eachMapValue[K, V](implicit @unused ev: IsMap.Of[K, V, A]): V =
+      sys.error(".eachMapValue should be only called within Chimney DSL")
+  }
 }

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

@@ -5,7 +5,17 @@ import io.scalaland.chimney.internal.compiletime.derivation.transformer.Transfor
 import io.scalaland.chimney.internal.compiletime.derivation.patcher.PatcherMacros
 
 export io.scalaland.chimney.inlined.{into, intoPartial, using}
-export io.scalaland.chimney.syntax.{toPartialResult, toPartialResultOrString}
+export io.scalaland.chimney.syntax.{
+  eachItem,
+  eachMapKey,
+  eachMapValue,
+  onLeft,
+  onRight,
+  onSome,
+  onSubtype,
+  toPartialResult,
+  toPartialResultOrString
+}
 
 // Extension methods in dsl.* summon TypeClass.AutoDerived while extension methods in syntax.* summon TypeClass.
 // This help us preserve legacy behavior in dsl code while keeping stricter separation in auto/syntax imports.

chimney/src/main/scala-3/io/scalaland/chimney/syntax/syntax.scala

@@ -1,7 +1,9 @@
 package io.scalaland.chimney.syntax
 
 import io.scalaland.chimney.{partial, PartialTransformer, Patcher, Transformer}
+import io.scalaland.chimney.internal.runtime.{IsCollection, IsEither, IsMap, IsOption}
 
+import scala.annotation.unused
 import scala.util.Try
 
 // Extension methods in dsl.* summon TypeClass.AutoDerived while extension methods in syntax.* summon TypeClass.
@@ -172,3 +174,27 @@ extension [T](`try`: Try[T]) {
   transparent inline def toPartialResult: partial.Result[T] =
     partial.Result.fromTry(`try`)
 }
+
+extension [A](@unused a: A) {
+
+  def onSubtype[B <: A]: B =
+    sys.error(".onSubtype should be only called within Chimney DSL")
+
+  def onSome[B](implicit @unused ev: IsOption.Of[A, B]): B =
+    sys.error(".onSome should be only called within Chimney DSL")
+
+  def onLeft[L, R](implicit @unused ev: IsEither.Of[A, L, R]): L =
+    sys.error(".onLeft should be only called within Chimney DSL")
+
+  def onRight[L, R](implicit @unused ev: IsEither.Of[A, L, R]): R =
+    sys.error(".onRight should be only called within Chimney DSL")
+
+  def eachItem[B](implicit @unused ev: IsCollection.Of[A, B]): B =
+    sys.error(".eachItem should be only called within Chimney DSL")
+
+  def eachMapKey[K, V](implicit @unused ev: IsMap.Of[K, V, A]): K =
+    sys.error(".eachMapKey should be only called within Chimney DSL")
+
+  def eachMapValue[K, V](implicit @unused ev: IsMap.Of[K, V, A]): V =
+    sys.error(".eachMapValue should be only called within Chimney DSL")
+}

chimney/src/main/scala/io/scalaland/chimney/internal/compiletime/derivation/transformer/Configurations.scala

@@ -150,7 +150,7 @@ private[compiletime] trait Configurations { this: Derivation =>
           case _           => false
         }
 
-        override def toString: String = s".whenSubtype[${Type.prettyPrint(tpe.Underlying)}]"
+        override def toString: String = s".onSubtype[${Type.prettyPrint(tpe.Underlying)}]"
       }
       case object EachItem extends Segment {
         override def toString: String = ".eachItem"

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

@@ -0,0 +1,17 @@
+package io.scalaland.chimney.internal.runtime
+
+import scala.annotation.unused
+
+sealed trait IsCollection[C] {
+  type Item
+}
+object IsCollection {
+  type Of[C, A] = IsCollection[C] { type Item = A }
+
+  private object Impl extends IsCollection[Nothing]
+
+  // build-in Chimney support for collections assumes that they are BOTH Iterable and have a Factory
+  implicit def scalaCollectionIsCollection[A, C <: Iterable[A]](implicit
+      @unused ev: scala.collection.compat.Factory[A, C]
+  ): IsCollection.Of[C, A] = Impl.asInstanceOf[IsCollection.Of[C, A]]
+}

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

@@ -0,0 +1,14 @@
+package io.scalaland.chimney.internal.runtime
+
+sealed trait IsEither[E] {
+  type Left
+  type Right
+}
+object IsEither {
+
+  type Of[E, L, R] = IsEither[E] { type Left = L; type Right = R }
+
+  private object Impl extends IsEither[Nothing]
+
+  implicit def eitherOsEither[L, R, E <: Left[L, R]]: IsEither.Of[E, L, R] = Impl.asInstanceOf[IsEither.Of[E, L, R]]
+}

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

@@ -0,0 +1,17 @@
+package io.scalaland.chimney.internal.runtime
+import scala.annotation.unused
+
+sealed trait IsMap[M] {
+  type Key
+  type Value
+}
+object IsMap {
+  type Of[M, K, V] = IsMap[M] { type Key = K; type Value = V }
+
+  private object Impl extends IsMap[Nothing]
+
+  // build-in Chimney support for maps assumes that they are BOTH Map and have a Factory
+  implicit def scalaMapIsMap[K, V, M <: Map[K, V]](implicit
+      @unused ev: scala.collection.compat.Factory[(K, V), M]
+  ): IsMap.Of[K, V, M] = Impl.asInstanceOf[IsMap.Of[K, V, M]]
+}

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

@@ -0,0 +1,13 @@
+package io.scalaland.chimney.internal.runtime
+
+sealed trait IsOption[O] {
+  type Value
+}
+object IsOption {
+
+  type Of[O, A] = IsOption[O] { type Value = A }
+
+  private object Impl extends IsOption[Nothing]
+
+  implicit def optionIsOption[A, O <: Option[A]]: IsOption.Of[O, A] = Impl.asInstanceOf[IsOption.Of[O, A]]
+}

chimney/src/test/scala/io/scalaland/chimney/IssuesSpec.scala

@@ -756,3 +756,4 @@ class IssuesSpec extends ChimneySpec {
       .transform ==> To(uuid1, uuid2, "test")
   }
 }
+∑

chimney/src/test/scala/io/scalaland/chimney/PartialTransformerProductSpec.scala

@@ -373,7 +373,7 @@ class PartialTransformerProductSpec extends ChimneySpec {
 
       val result7 = Person("John", 10, 140)
         .intoPartial[User]
-        .withFieldComputed(_.age, _ => throw new Exception("error happened"))
+        .withFieldComputed(_.age, _ => sys.error("error happened"))
         .transform
       result7.asOption ==> None
       result7.asEither.isLeft ==> true