Add `CaseClass.rawConstruct` and new `Patcher` example

2 files changed, 31 insertions, 13 deletions

diff --git a/core/shared/src/main/scala/interface.scala b/core/shared/src/main/scala/interface.scala
index 193a6f9..86ef5b1 100644
--- a/core/shared/src/main/scala/interface.scala
+++ b/core/shared/src/main/scala/interface.scala
@@ -122,7 +122,22 @@ abstract class CaseClass[Typeclass[_], Type] private[magnolia] (
     *  @param makeParam  lambda for converting a generic [[Param]] into the value to be used for
     *                    this parameter in the construction of a new instance of the case class
     *  @return  a new instance of the case class */
-  def construct[Return](makeParam: Param[Typeclass, Type] => Return): Type
+  final def construct[Return](makeParam: Param[Typeclass, Type] => Return): Type =
+    rawConstruct(parameters map makeParam)
+
+  /** constructs a new instance of the case class type
+    *
+    *  Like [[construct]] this method is implemented by Magnolia and let's you construct case class
+    *  instances generically in user code, without knowing their type concretely.
+    *
+    *  `rawConstruct`, however, is more low-level in that it expects you to provide a [[Seq]]
+    *  containing all the field values for the case class type, in order and with the correct types.
+    *
+    * @param fieldValues contains the field values for the case class instance to be constructed,
+    *                    in order and with the correct types.
+    *  @return  a new instance of the case class
+    *  @throws  IllegalArgumentException if the size of `paramValues` differs from the size of [[parameters]] */
+  def rawConstruct(fieldValues: Seq[Any]): Type
 
   /** a sequence of [[Param]] objects representing all of the parameters in the case class
     *
diff --git a/core/shared/src/main/scala/magnolia.scala b/core/shared/src/main/scala/magnolia.scala
index f55dfa7..bc16a62 100644
--- a/core/shared/src/main/scala/magnolia.scala
+++ b/core/shared/src/main/scala/magnolia.scala
@@ -333,7 +333,7 @@ object Magnolia {
         val caseParams = caseParamsReversed.reverse
 
         val paramsVal: TermName = TermName(c.freshName("parameters"))
-        val fnVal: TermName = TermName(c.freshName("fn"))
+        val fieldValues: TermName = TermName(c.freshName("fieldValues"))
 
         val preAssignments = caseParams.map(_.typeclass)
 
@@ -381,15 +381,18 @@ object Magnolia {
               false,
               $isValueClass,
               $paramsVal,
-              ($fnVal: $magnoliaPkg.Param[$typeConstructor, $genericType] => Any) =>
-                new $genericType(..${caseParams.zipWithIndex.map {
-              case (typeclass, idx) =>
-                val arg = q"$fnVal($paramsVal($idx)).asInstanceOf[${typeclass.paramType}]"
-                if (typeclass.repeated) q"$arg: _*" else arg
-            }})
-            ))
-          }"""
-          )
+              ($fieldValues: $scalaPkg.Seq[Any]) => {
+                if ($fieldValues.lengthCompare($paramsVal.length) != 0) {
+                  val msg = "`" + $className + "` has " + $paramsVal.length + " fields, not " + $fieldValues.size
+                  throw new java.lang.IllegalArgumentException(msg)
+                }
+                new $genericType(..${
+                  caseParams.zipWithIndex.map { case (typeclass, idx) =>
+                    val arg = q"$fieldValues($idx).asInstanceOf[${typeclass.paramType}]"
+                    if (typeclass.repeated) q"$arg: _*" else arg
+                  }
+            })}))
+          }""")
         )
       } else if (isSealedTrait) {
         val genericSubtypes = classType.get.knownDirectSubclasses.to[List]
@@ -541,9 +544,9 @@ object Magnolia {
                           obj: Boolean,
                           valClass: Boolean,
                           params: Array[Param[Tc, T]],
-                          constructor: (Param[Tc, T] => Any) => T) =
+                          constructor: Seq[Any] => T): CaseClass[Tc, T] =
     new CaseClass[Tc, T](name, obj, valClass, params) {
-      def construct[R](param: Param[Tc, T] => R): T = constructor(param)
+      def rawConstruct(fieldValues: Seq[Any]): T = constructor(fieldValues)
     }
 }