ConfGet: add getNested method

metaconfig-core/shared/src/main/scala/metaconfig/Conf.scala

@@ -35,6 +35,9 @@ sealed abstract class Conf extends Product with Serializable {
       default: T
   )(implicit ev: ConfDecoder[T]): Configured[T] =
     ConfGet.getOrElse(this, default, path, extraNames: _*)
+
+  def getNested[T](keys: String*)(implicit ev: ConfDecoder[T]): Configured[T] =
+    ConfGet.getNested(this, keys: _*)
 }
 
 object Conf {

metaconfig-core/shared/src/main/scala/metaconfig/internal/ConfGet.scala

@@ -45,6 +45,24 @@ object ConfGet {
     }
   }
 
+  @tailrec
+  def getNested[T](conf: Conf, keys: String*)(
+      implicit ev: ConfDecoder[T]
+  ): Configured[T] =
+    keys.headOption match {
+      case None => ev.read(conf)
+      case Some(key) =>
+        conf match {
+          case obj: Conf.Obj =>
+            obj.values.collectFirst { case (`key`, v) => v } match {
+              case Some(v) => getNested(v, keys.tail: _*)
+              case _ => ConfError.missingField(obj, key).notOk
+            }
+          case _ =>
+            ConfError.typeMismatch(s"Conf.Obj with key '$key'", conf).notOk
+        }
+    }
+
   // Copy-pasted from scala.meta inputs because it's private.
   // TODO(olafur) expose utility in inputs to get offset from line
   private[metaconfig] def getOffsetByLine(chars: Array[Char]): Array[Int] = {

metaconfig-tests/shared/src/test/scala/metaconfig/ConfGetSuite.scala

@@ -0,0 +1,43 @@
+package metaconfig
+
+class ConfGetSuite extends munit.FunSuite {
+  import Conf._
+
+  test("getNested good") {
+    val confA = Str("a")
+    assertEquals(confA.getNested[String](), Configured.Ok("a"))
+
+    val confB = Obj("b" -> confA)
+    assertEquals(confB.getNested[String]("b"), Configured.Ok("a"))
+
+    val confC = Obj("c" -> confB)
+    assertEquals(confC.getNested[String]("c", "b"), Configured.Ok("a"))
+
+    val confD = Obj("d" -> confC)
+    assertEquals(confD.getNested[String]("d", "c", "b"), Configured.Ok("a"))
+  }
+
+  test("getNested fail") {
+    val confA = Str("a")
+    assertEquals(
+      confA.getNested[Boolean]().toEither.left.get.msg,
+      """|Type mismatch;
+         |  found    : String (value: "a")
+         |  expected : Bool""".stripMargin
+    )
+
+    assertEquals(
+      confA.getNested[Boolean]("c").toEither.left.get.msg,
+      """|Type mismatch;
+         |  found    : String (value: "a")
+         |  expected : Conf.Obj with key 'c'""".stripMargin
+    )
+
+    val confB = Obj("b" -> confA)
+    assertEquals(
+      confB.getNested[String]("c").toEither.left.get.msg,
+      """{"b": "a"} has no field 'c'."""
+    )
+  }
+
+}