Option.map: Pass None to new decoder when empty (zio#204)

PelamWolt · Mar 13, 2021 · 30c6b8a · 30c6b8a
1 parent 449499d
commit 30c6b8a
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Showing 2 changed files with 37 additions and 2 deletions.
diff --git a/zio-json/shared/src/main/scala/zio/json/decoder.scala b/zio-json/shared/src/main/scala/zio/json/decoder.scala
@@ -105,7 +105,7 @@ trait JsonDecoder[A] extends JsonDecoderPlatformSpecific[A] { self =>
   /**
    * Returns a new codec whose decoded values will be mapped by the specified function.
    */
-  final def map[B](f: A => B): JsonDecoder[B] =
+  def map[B](f: A => B): JsonDecoder[B] =
     new JsonDecoder[B] {
 
       def unsafeDecode(trace: List[JsonError], in: RetractReader): B =
@@ -260,7 +260,7 @@ object JsonDecoder extends GeneratedTupleDecoders with DecoderLowPriority0 {
   // If alternative behaviour is desired, e.g. pass null to the underlying, then
   // use a newtype wrapper.
   implicit def option[A](implicit A: JsonDecoder[A]): JsonDecoder[Option[A]] =
-    new JsonDecoder[Option[A]] {
+    new JsonDecoder[Option[A]] { self =>
       private[this] val ull: Array[Char] = "ull".toCharArray
 
       override def unsafeDecodeMissing(trace: List[JsonError]): Option[A] =
@@ -275,6 +275,17 @@ object JsonDecoder extends GeneratedTupleDecoders with DecoderLowPriority0 {
             in.retract()
             Some(A.unsafeDecode(trace, in))
         }
+
+      // overridden here to pass `None` to the new Decoder instead of throwing
+      // when called from a derived decoder
+      override def map[B](f: Option[A] => B): JsonDecoder[B] =
+        new JsonDecoder[B] {
+          override def unsafeDecodeMissing(trace: List[JsonError]): B =
+            f(None)
+
+          def unsafeDecode(trace: List[JsonError], in: RetractReader): B =
+            f(self.unsafeDecode(trace, in))
+        }
     }
 
   // supports multiple representations for compatibility with other libraries,

diff --git a/zio-json/shared/src/test/scala/zio/json/DecoderSpec.scala b/zio-json/shared/src/test/scala/zio/json/DecoderSpec.scala
@@ -72,6 +72,30 @@ object DecoderSpec extends DefaultRunnableSpec {
       test("unicode") {
         assert(""""€🐵🥰"""".fromJson[String])(isRight(equalTo("€🐵🥰")))
       },
+      test("Option: .map on derived JsonDecoder with missing value") {
+        // More information about use case here https://github.com/zio/zio-json/issues/198
+        // User wants to derive an alternative encoding of optionality
+        sealed trait Assumed[+A]
+
+        object Assumed {
+          case object MissingAssumed       extends Assumed[Nothing]
+          case class FoundAssumed[A](v: A) extends Assumed[A]
+
+          implicit def decoder[A](implicit decoding: JsonDecoder[A]): JsonDecoder[Assumed[A]] =
+            JsonDecoder.option[A].map {
+              case None    => Assumed.MissingAssumed
+              case Some(v) => Assumed.FoundAssumed[A](v)
+            }
+        }
+
+        case class Example(a: Assumed[Boolean])
+        implicit val exampleDecoder: JsonDecoder[Example] = DeriveJsonDecoder.gen[Example]
+
+        assert("""{ "a": null }""".fromJson[Example])(isRight(equalTo(Example(Assumed.MissingAssumed)))) &&
+        assert("""{ "a": true }""".fromJson[Example])(isRight(equalTo(Example(Assumed.FoundAssumed(true))))) &&
+        assert("""{ "a": false }""".fromJson[Example])(isRight(equalTo(Example(Assumed.FoundAssumed(false))))) &&
+        assert("""{ }""".fromJson[Example])(isRight(equalTo(Example(Assumed.MissingAssumed))))
+      },
       test("Seq") {
         val jsonStr  = """["5XL","2XL","XL"]"""
         val expected = Seq("5XL", "2XL", "XL")