From e28748af5850f74a055aa15f3c594dcb9294cb2c Mon Sep 17 00:00:00 2001
From: Albert Meltzer <7529386+kitbellew@users.noreply.github.com>
Date: Fri, 21 May 2021 22:55:36 -0700
Subject: [PATCH] ConfDecoderEx: allow extending collections

---
 docs/reference.md                             | 25 +++++++++++++++++++
 .../scala/metaconfig/ConfDecoderExT.scala     | 20 +++++++++++++++
 .../DeriveConfDecoderExJVMSuite.scala         | 15 +++++++++--
 3 files changed, 58 insertions(+), 2 deletions(-)

diff --git a/docs/reference.md b/docs/reference.md
index 425ce6df..e489eeaa 100644
--- a/docs/reference.md
+++ b/docs/reference.md
@@ -202,6 +202,31 @@ for `ConfDecoderExT[A, A]`.
 If a decoder for type `T` is defined, the package defines implicits to derive
 decoders for `Option[T]`, `Seq[T]` and `Map[String, T]`.
 
+There's also a special for _extending_ collections rather than redefining them
+(works only for `ConfDecoderEx`, not the original `ConfDecoder`):
+```
+// sets list
+a = [ ... ]
+// sets map
+a = {
+  b { ... }
+  c { ... }
+}
+
+// extends list
+a = {
+  // must be the only key
+  "+" = [ ... ]
+}
+// extends map
+a = {
+  // must be the only key
+  "+" = {
+    d { ... }
+  }
+}
+```
+
 ## ConfEncoder
 
 To convert a class instance into `Conf` use `ConfEncoder[T]`. It's possible to
diff --git a/metaconfig-core/shared/src/main/scala/metaconfig/ConfDecoderExT.scala b/metaconfig-core/shared/src/main/scala/metaconfig/ConfDecoderExT.scala
index a3eb59f2..691b5214 100644
--- a/metaconfig-core/shared/src/main/scala/metaconfig/ConfDecoderExT.scala
+++ b/metaconfig-core/shared/src/main/scala/metaconfig/ConfDecoderExT.scala
@@ -110,6 +110,17 @@ object ConfDecoderExT {
     fromPartial(
       s"Map[String, ${classTag.runtimeClass.getName}]"
     ) {
+      case (stateOpt, Conf.Obj(List(("+", Conf.Obj(values))))) =>
+        val res =
+          buildFrom(none, values, ev, factory)(_._2, (x, y) => (x._1, y))
+        res.map { x =>
+          stateOpt.fold(x) { state =>
+            val builder = factory.newBuilder
+            builder ++= state
+            builder ++= x
+            builder.result()
+          }
+        }
       case (_, Conf.Obj(values)) =>
         buildFrom(none, values, ev, factory)(_._2, (x, y) => (x._1, y))
     }
@@ -136,6 +147,15 @@ object ConfDecoderExT {
     fromPartial(
       s"List[${classTag.runtimeClass.getName}]"
     ) {
+      case (stateOpt, Conf.Obj(List(("+", Conf.Lst(values))))) =>
+        buildFrom(none, values, ev, factory)(identity, (_, x) => x).map { x =>
+          stateOpt.fold(x) { state =>
+            val builder = factory.newBuilder
+            builder ++= state
+            builder ++= x
+            builder.result()
+          }
+        }
       case (_, Conf.Lst(values)) =>
         buildFrom(none, values, ev, factory)(identity, (_, x) => x)
     }
diff --git a/metaconfig-tests/jvm/src/test/scala/metaconfig/DeriveConfDecoderExJVMSuite.scala b/metaconfig-tests/jvm/src/test/scala/metaconfig/DeriveConfDecoderExJVMSuite.scala
index 2199c9c2..59343780 100644
--- a/metaconfig-tests/jvm/src/test/scala/metaconfig/DeriveConfDecoderExJVMSuite.scala
+++ b/metaconfig-tests/jvm/src/test/scala/metaconfig/DeriveConfDecoderExJVMSuite.scala
@@ -31,12 +31,18 @@ class DeriveConfDecoderExJVMSuite extends munit.FunSuite {
         |    k3 { param = 3 }
         |  }
         |}
+        |d {
+        |  "+" = [{
+        |    b { param = 40 }
+        |  }]
+        |}
         |""".stripMargin,
       Nested(
         a = 14,
         c = Nested2(a = "n2", b = OneParam(4), c = Map("k3" -> OneParam(3))),
         d = Seq(
-          Nested2("n1", OneParam(2), Map("k1" -> OneParam(1)))
+          Nested2("n1", OneParam(2), Map("k1" -> OneParam(1))),
+          Nested2(b = OneParam(40))
         )
       ),
       Nested(
@@ -54,6 +60,11 @@ class DeriveConfDecoderExJVMSuite extends munit.FunSuite {
         |  a = "xxx"
         |  b {
         |    b { param = 3 }
+        |    c {
+        |      "+" = {
+        |        k3 { param = 33 }
+        |      }
+        |    }
         |  }
         |}
         |""".stripMargin,
@@ -63,7 +74,7 @@ class DeriveConfDecoderExJVMSuite extends munit.FunSuite {
           b = Nested2(
             a = "zzz",
             b = OneParam(3),
-            c = Map("k2" -> OneParam(2))
+            c = Map("k2" -> OneParam(2), "k3" -> OneParam(33))
           )
         )
       ),