From dc05f7a8ac738fe7da672427b67da862e8926b7e Mon Sep 17 00:00:00 2001
From: Ben Plommer <ben.plommer@itv.com>
Date: Thu, 28 Jan 2021 14:46:10 +0000
Subject: [PATCH 1/8] narrow type ef `encode` from `Any` to `AnyRef`

---
 modules/core/src/main/scala/vulcan/Codec.scala | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/modules/core/src/main/scala/vulcan/Codec.scala b/modules/core/src/main/scala/vulcan/Codec.scala
index e1654054..e9491378 100644
--- a/modules/core/src/main/scala/vulcan/Codec.scala
+++ b/modules/core/src/main/scala/vulcan/Codec.scala
@@ -41,7 +41,7 @@ sealed abstract class Codec[A] {
   def schema: Either[AvroError, Schema]
 
   /** Attempts to encode the specified value using the provided schema. */
-  def encode(a: A): Either[AvroError, Any]
+  def encode(a: A): Either[AvroError, AnyRef]
 
   /** Attempts to decode the specified value using the provided schema. */
   def decode(value: Any, schema: Schema): Either[AvroError, A]
@@ -565,7 +565,7 @@ object Codec extends CodecCompanionCompat {
     */
   final def instance[A](
     schema: Either[AvroError, Schema],
-    encode: A => Either[AvroError, Any],
+    encode: A => Either[AvroError, AnyRef],
     decode: (Any, Schema) => Either[AvroError, A]
   ): Codec[A] = {
     val _schema = schema
@@ -576,7 +576,7 @@ object Codec extends CodecCompanionCompat {
       override final val schema: Either[AvroError, Schema] =
         _schema
 
-      override final def encode(a: A): Either[AvroError, Any] =
+      override final def encode(a: A): Either[AvroError, AnyRef] =
         _encode(a)
 
       override final def decode(value: Any, schema: Schema): Either[AvroError, A] =
@@ -594,7 +594,7 @@ object Codec extends CodecCompanionCompat {
     expectedValueType: String,
     decodingTypeName: String,
     schema: Either[AvroError, Schema],
-    encode: A => Either[AvroError, Any],
+    encode: A => Either[AvroError, AnyRef],
     decode: PartialFunction[(Any, Schema), Either[AvroError, A]]
   ): Codec[A] = {
     instance(

From 1bef917ba5375505c5e47f2ce7b2ae99e04a34f1 Mon Sep 17 00:00:00 2001
From: Ben Plommer <ben.plommer@itv.com>
Date: Thu, 28 Jan 2021 14:51:59 +0000
Subject: [PATCH 2/8] Add Repr type to codec

---
 .../core/src/main/scala/vulcan/Codec.scala    | 45 ++++++++++++-------
 .../main/scala/vulcan/generic/package.scala   |  4 +-
 .../test/scala/vulcan/generic/CodecSpec.scala |  2 +-
 3 files changed, 32 insertions(+), 19 deletions(-)

diff --git a/modules/core/src/main/scala/vulcan/Codec.scala b/modules/core/src/main/scala/vulcan/Codec.scala
index e9491378..38a58d59 100644
--- a/modules/core/src/main/scala/vulcan/Codec.scala
+++ b/modules/core/src/main/scala/vulcan/Codec.scala
@@ -36,12 +36,13 @@ import scala.util.Try
   "could not find implicit Codec[${A}]; ensure no imports are missing or manually define an instance"
 )
 sealed abstract class Codec[A] {
+  type Repr <: AnyRef
 
   /** The schema or an error if the schema could not be generated. */
   def schema: Either[AvroError, Schema]
 
   /** Attempts to encode the specified value using the provided schema. */
-  def encode(a: A): Either[AvroError, AnyRef]
+  def encode(a: A): Either[AvroError, Repr]
 
   /** Attempts to decode the specified value using the provided schema. */
   def decode(value: Any, schema: Schema): Either[AvroError, A]
@@ -51,7 +52,7 @@ sealed abstract class Codec[A] {
     * for encoding and decoding, mapping back-and-forth
     * between types `A` and `B`.
     */
-  final def imap[B](f: A => B)(g: B => A): Codec[B] =
+  final def imap[B](f: A => B)(g: B => A): Codec.Aux[B, Repr] =
     Codec.instance(
       schema,
       b => encode(g(b)),
@@ -66,7 +67,7 @@ sealed abstract class Codec[A] {
     * Similar to [[Codec#imap]], except the mapping from
     * `A` to `B` might be unsuccessful.
     */
-  final def imapError[B](f: A => Either[AvroError, B])(g: B => A): Codec[B] =
+  final def imapError[B](f: A => Either[AvroError, B])(g: B => A): Codec.Aux[B, Repr] =
     Codec.instance(
       schema,
       b => encode(g(b)),
@@ -81,10 +82,10 @@ sealed abstract class Codec[A] {
     * Similar to [[Codec#imap]], except the mapping from
     * `A` to `B` might be unsuccessful.
     */
-  final def imapTry[B](f: A => Try[B])(g: B => A): Codec[B] =
+  final def imapTry[B](f: A => Try[B])(g: B => A): Codec.Aux[B, Repr] =
     imapError(f(_).toEither.leftMap(AvroError.fromThrowable))(g)
 
-  private[vulcan] def withDecodingTypeName(decodingTypeName: String): Codec[A] =
+  private[vulcan] def withDecodingTypeName(decodingTypeName: String): Codec.Aux[A, Repr] =
     Codec.instance(
       schema,
       encode,
@@ -131,6 +132,10 @@ sealed abstract class Codec[A] {
   */
 object Codec extends CodecCompanionCompat {
 
+  type Aux[A, Repr0 <: AnyRef] = Codec[A] {
+    type Repr = Repr0
+  }
+
   /**
     * Returns the [[Codec]] for the specified type.
     *
@@ -209,7 +214,7 @@ object Codec extends CodecCompanionCompat {
       "Collection",
       "Chain",
       codec.schema.map(Schema.createArray),
-      _.toList.traverse(codec.encode).map(_.asJava), {
+      _.toList.traverse(codec.encode(_)).map(_.asJava), {
         case (collection: java.util.Collection[_], schema) =>
           collection.asScala.toList
             .traverse(codec.decode(_, schema.getElementType()))
@@ -563,20 +568,22 @@ object Codec extends CodecCompanionCompat {
     *
     * @group Create
     */
-  final def instance[A](
+  final def instance[A, Repr0 <: AnyRef](
     schema: Either[AvroError, Schema],
-    encode: A => Either[AvroError, AnyRef],
+    encode: A => Either[AvroError, Repr0],
     decode: (Any, Schema) => Either[AvroError, A]
-  ): Codec[A] = {
+  ): Codec.Aux[A, Repr0] = {
     val _schema = schema
     val _encode = encode
     val _decode = decode
 
     new Codec[A] {
+      type Repr = Repr0
+
       override final val schema: Either[AvroError, Schema] =
         _schema
 
-      override final def encode(a: A): Either[AvroError, AnyRef] =
+      override final def encode(a: A): Either[AvroError, Repr] =
         _encode(a)
 
       override final def decode(value: Any, schema: Schema): Either[AvroError, A] =
@@ -590,13 +597,13 @@ object Codec extends CodecCompanionCompat {
     }
   }
 
-  private[vulcan] final def instanceForTypes[A](
+  private[vulcan] final def instanceForTypes[A, Repr <: AnyRef](
     expectedValueType: String,
     decodingTypeName: String,
     schema: Either[AvroError, Schema],
-    encode: A => Either[AvroError, AnyRef],
+    encode: A => Either[AvroError, Repr],
     decode: PartialFunction[(Any, Schema), Either[AvroError, A]]
-  ): Codec[A] = {
+  ): Codec.Aux[A, Repr] = {
     instance(
       schema,
       encode,
@@ -667,7 +674,7 @@ object Codec extends CodecCompanionCompat {
       "Collection",
       "List",
       codec.schema.map(Schema.createArray),
-      _.traverse(codec.encode).map(_.asJava), {
+      _.traverse(codec.encode(_)).map(_.asJava), {
         case (collection: java.util.Collection[_], schema) =>
           collection.asScala.toList.traverse(codec.decode(_, schema.getElementType()))
       }
@@ -1080,7 +1087,7 @@ object Codec extends CodecCompanionCompat {
       a =>
         alts
           .foldMapK { alt =>
-            alt.prism.getOption(a).map(alt.codec.encode)
+            alt.prism.getOption(a).map(alt.codec.encode(_))
           }
           .getOrElse {
             Left(AvroError.encodeExhaustedAlternatives(a, None))
@@ -1178,7 +1185,7 @@ object Codec extends CodecCompanionCompat {
       "Collection",
       "Vector",
       codec.schema.map(Schema.createArray),
-      _.traverse(codec.encode).map(_.asJava), {
+      _.traverse(codec.encode(_)).map(_.asJava), {
         case (collection: java.util.Collection[_], schema) =>
           collection.asScala.toVector.traverse(codec.decode(_, schema.getElementType()))
       }
@@ -1199,6 +1206,12 @@ object Codec extends CodecCompanionCompat {
   implicit final def codecShow[A]: Show[Codec[A]] =
     Show.fromToString
 
+  /**
+    * @group Cats
+    */
+  implicit final def codecAuxShow[A, Repr <: AnyRef]: Show[Codec.Aux[A, Repr]] =
+    Show.fromToString
+
   /**
     * @group Create
     */
diff --git a/modules/generic/src/main/scala/vulcan/generic/package.scala b/modules/generic/src/main/scala/vulcan/generic/package.scala
index 00b90cf3..29e66e01 100644
--- a/modules/generic/src/main/scala/vulcan/generic/package.scala
+++ b/modules/generic/src/main/scala/vulcan/generic/package.scala
@@ -45,8 +45,8 @@ package object generic {
         }
       },
       _.eliminate(
-        headCodec.encode,
-        tailCodec.value.encode
+        headCodec.encode(_),
+        tailCodec.value.encode(_)
       ),
       (value, schema) => {
         val schemaTypes =
diff --git a/modules/generic/src/test/scala/vulcan/generic/CodecSpec.scala b/modules/generic/src/test/scala/vulcan/generic/CodecSpec.scala
index fb247537..b54ada79 100644
--- a/modules/generic/src/test/scala/vulcan/generic/CodecSpec.scala
+++ b/modules/generic/src/test/scala/vulcan/generic/CodecSpec.scala
@@ -61,7 +61,7 @@ final class CodecSpec extends AnyFunSpec with ScalaCheckPropertyChecks with Eith
             coproductCodec[Int, CNil](
               Codec.int,
               shapeless.Lazy {
-                Codec.instance[CNil](
+                Codec.instance[CNil, Null](
                   Right(SchemaBuilder.builder().nullType()),
                   _ => Left(AvroError("encode")),
                   (_, _) => Left(AvroError("decode"))

From 9b765e764051a43ab28aaffc4bbfa0cb84afe39f Mon Sep 17 00:00:00 2001
From: Ben Plommer <ben.plommer@itv.com>
Date: Thu, 28 Jan 2021 15:03:55 +0000
Subject: [PATCH 3/8] use Aux types for codecs

---
 .../core/src/main/scala/vulcan/Codec.scala    | 89 +++++++++++--------
 1 file changed, 54 insertions(+), 35 deletions(-)

diff --git a/modules/core/src/main/scala/vulcan/Codec.scala b/modules/core/src/main/scala/vulcan/Codec.scala
index 38a58d59..3f5c0acd 100644
--- a/modules/core/src/main/scala/vulcan/Codec.scala
+++ b/modules/core/src/main/scala/vulcan/Codec.scala
@@ -147,7 +147,7 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group General
     */
-  implicit final val boolean: Codec[Boolean] =
+  implicit final val boolean: Codec.Aux[Boolean, java.lang.Boolean] =
     Codec.instanceForTypes(
       "Boolean",
       "Boolean",
@@ -161,7 +161,7 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group General
     */
-  implicit final lazy val byte: Codec[Byte] = {
+  implicit final lazy val byte: Codec.Aux[Byte, java.lang.Integer] = {
     val min: Int = Byte.MinValue.toInt
     val max: Int = Byte.MaxValue.toInt
     Codec.int
@@ -176,7 +176,7 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group General
     */
-  implicit final val bytes: Codec[Array[Byte]] =
+  implicit final val bytes: Codec.Aux[Array[Byte], ByteBuffer] =
     Codec.instance(
       Right(SchemaBuilder.builder().bytesType()),
       ByteBuffer.wrap(_).asRight,
@@ -209,7 +209,9 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group Cats
     */
-  implicit final def chain[A](implicit codec: Codec[A]): Codec[Chain[A]] =
+  implicit final def chain[A](
+    implicit codec: Codec[A]
+  ): Codec.Aux[Chain[A], java.util.Collection[codec.Repr]] =
     Codec.instanceForTypes(
       "Collection",
       "Chain",
@@ -225,7 +227,7 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group General
     */
-  implicit final val char: Codec[Char] =
+  implicit final val char: Codec.Aux[Char, Utf8] =
     Codec.instanceForTypes(
       "Utf8",
       "Char",
@@ -246,7 +248,7 @@ object Codec extends CodecCompanionCompat {
   final def decimal(
     precision: Int,
     scale: Int
-  ): Codec[BigDecimal] = {
+  ): Codec.Aux[BigDecimal, ByteBuffer] = {
     val conversion = new Conversions.DecimalConversion()
     val logicalType = LogicalTypes.decimal(precision, scale)
     val schema = AvroError.catchNonFatal {
@@ -307,7 +309,7 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group General
     */
-  implicit final val double: Codec[Double] =
+  implicit final val double: Codec.Aux[Double, java.lang.Double] =
     Codec.instance(
       Right(SchemaBuilder.builder().doubleType()),
       java.lang.Double.valueOf(_).asRight,
@@ -346,7 +348,7 @@ object Codec extends CodecCompanionCompat {
   implicit final def either[A, B](
     implicit codecA: Codec[A],
     codecB: Codec[B]
-  ): Codec[Either[A, B]] =
+  ): Codec.Aux[Either[A, B], AnyRef] =
     Codec.union(alt => alt[Left[A, B]] |+| alt[Right[A, B]])
 
   /**
@@ -372,7 +374,7 @@ object Codec extends CodecCompanionCompat {
     doc: Option[String] = None,
     aliases: Seq[String] = Seq.empty,
     props: Props = Props.empty
-  ): Codec[A] = {
+  ): Codec.Aux[A, AnyRef] = {
     val typeName = if (namespace.isEmpty) name else s"$namespace.$name"
     val schema = AvroError.catchNonFatal {
       props.toChain.map { props =>
@@ -429,7 +431,8 @@ object Codec extends CodecCompanionCompat {
     doc: Option[String] = None,
     aliases: Seq[String] = Seq.empty,
     props: Props = Props.empty
-  ): Codec[A] = enumeration(name, namespace, symbols, encode, decode, default, doc, aliases, props)
+  ): Codec.Aux[A, AnyRef] =
+    enumeration(name, namespace, symbols, encode, decode, default, doc, aliases, props)
 
   /**
     * Returns a new fixed [[Codec]] for type `A`.
@@ -450,7 +453,7 @@ object Codec extends CodecCompanionCompat {
     doc: Option[String] = None,
     aliases: Seq[String] = Seq.empty,
     props: Props = Props.empty
-  ): Codec[A] = {
+  ): Codec.Aux[A, GenericFixed] = {
     val typeName = if (namespace.isEmpty) name else s"$namespace.$name"
     val schema = AvroError.catchNonFatal {
       props.toChain.map { props =>
@@ -474,7 +477,7 @@ object Codec extends CodecCompanionCompat {
           val bytes = encode(a)
           if (bytes.length <= size) {
             val buffer = ByteBuffer.allocate(size).put(bytes)
-            schema.map(GenericData.get().createFixed(null, buffer.array(), _))
+            schema.map(new GenericData.Fixed(_, buffer.array()))
           } else {
             Left(AvroError.encodeExceedsFixedSize(bytes.length, size, typeName))
           }
@@ -499,7 +502,7 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group General
     */
-  implicit final val float: Codec[Float] =
+  implicit final val float: Codec.Aux[Float, java.lang.Float] =
     Codec.instance(
       Right(SchemaBuilder.builder().floatType()),
       java.lang.Float.valueOf(_).asRight,
@@ -632,7 +635,7 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group JavaTime
     */
-  implicit final val instant: Codec[Instant] =
+  implicit final val instant: Codec.Aux[Instant, java.lang.Long] =
     Codec.instanceForTypes(
       "Long",
       "Instant",
@@ -649,7 +652,7 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group General
     */
-  implicit final val int: Codec[Int] =
+  implicit final val int: Codec.Aux[Int, java.lang.Integer] =
     Codec.instanceForTypes(
       "Int",
       "Int",
@@ -663,13 +666,15 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group General
     */
-  implicit final def left[A, B](implicit codec: Codec[A]): Codec[Left[A, B]] =
+  implicit final def left[A, B](implicit codec: Codec[A]): Codec.Aux[Left[A, B], codec.Repr] =
     codec.imap(Left[A, B](_))(_.value)
 
   /**
     * @group Collection
     */
-  implicit final def list[A](implicit codec: Codec[A]): Codec[List[A]] =
+  implicit final def list[A](
+    implicit codec: Codec[A]
+  ): Codec.Aux[List[A], java.util.Collection[codec.Repr]] =
     Codec.instanceForTypes(
       "Collection",
       "List",
@@ -683,7 +688,7 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group JavaTime
     */
-  implicit final val localDate: Codec[LocalDate] =
+  implicit final val localDate: Codec.Aux[LocalDate, java.lang.Integer] =
     Codec.instanceForTypes(
       "Integer",
       "LocalDate",
@@ -700,7 +705,7 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group General
     */
-  implicit final val long: Codec[Long] =
+  implicit final val long: Codec.Aux[Long, java.lang.Long] =
     Codec.instance(
       Right(SchemaBuilder.builder().longType()),
       java.lang.Long.valueOf(_).asRight,
@@ -732,7 +737,9 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group Collection
     */
-  implicit final def map[A](implicit codec: Codec[A]): Codec[Map[String, A]] =
+  implicit final def map[A](
+    implicit codec: Codec[A]
+  ): Codec.Aux[Map[String, A], java.util.Map[Utf8, codec.Repr]] =
     Codec.instanceForTypes(
       "java.util.Map",
       "Map",
@@ -762,7 +769,7 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group General
     */
-  implicit final val none: Codec[None.type] =
+  implicit final val none: Codec.Aux[None.type, Null] =
     Codec.instanceForTypes(
       "null",
       "None",
@@ -774,7 +781,9 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group Cats
     */
-  implicit final def nonEmptyChain[A](implicit codec: Codec[A]): Codec[NonEmptyChain[A]] =
+  implicit final def nonEmptyChain[A](
+    implicit codec: Codec[A]
+  ): Codec.Aux[NonEmptyChain[A], java.util.Collection[codec.Repr]] =
     Codec
       .chain[A]
       .imapError(
@@ -785,7 +794,9 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group Cats
     */
-  implicit final def nonEmptyList[A](implicit codec: Codec[A]): Codec[NonEmptyList[A]] =
+  implicit final def nonEmptyList[A](
+    implicit codec: Codec[A]
+  ): Codec.Aux[NonEmptyList[A], java.util.Collection[codec.Repr]] =
     Codec
       .list[A]
       .imapError(
@@ -799,7 +810,7 @@ object Codec extends CodecCompanionCompat {
   implicit final def nonEmptySet[A](
     implicit codec: Codec[A],
     ordering: Ordering[A]
-  ): Codec[NonEmptySet[A]] =
+  ): Codec.Aux[NonEmptySet[A], java.util.Collection[codec.Repr]] =
     Codec
       .list[A]
       .imapError(
@@ -813,7 +824,9 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group Cats
     */
-  implicit final def nonEmptyVector[A](implicit codec: Codec[A]): Codec[NonEmptyVector[A]] =
+  implicit final def nonEmptyVector[A](
+    implicit codec: Codec[A]
+  ): Codec.Aux[NonEmptyVector[A], java.util.Collection[codec.Repr]] =
     Codec
       .vector[A]
       .imapError(
@@ -838,7 +851,7 @@ object Codec extends CodecCompanionCompat {
     doc: Option[String] = None,
     aliases: Seq[String] = Seq.empty,
     props: Props = Props.empty
-  )(f: FieldBuilder[A] => FreeApplicative[Field[A, *], A]): Codec[A] = {
+  )(f: FieldBuilder[A] => FreeApplicative[Field[A, *], A]): Codec.Aux[A, GenericRecord] = {
     val typeName = if (namespace.isEmpty) name else s"$namespace.$name"
     val free = f(FieldBuilder.instance)
     val schema = AvroError.catchNonFatal {
@@ -952,13 +965,15 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group General
     */
-  implicit final def right[A, B](implicit codec: Codec[B]): Codec[Right[A, B]] =
+  implicit final def right[A, B](implicit codec: Codec[B]): Codec.Aux[Right[A, B], codec.Repr] =
     codec.imap(Right[A, B](_))(_.value)
 
   /**
     * @group Collection
     */
-  implicit final def seq[A](implicit codec: Codec[A]): Codec[Seq[A]] =
+  implicit final def seq[A](
+    implicit codec: Codec[A]
+  ): Codec.Aux[Seq[A], java.util.Collection[codec.Repr]] =
     Codec
       .list[A]
       .imap[Seq[A]](_.toSeq)(_.toList)
@@ -967,7 +982,9 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group Collection
     */
-  implicit final def set[A](implicit codec: Codec[A]): Codec[Set[A]] =
+  implicit final def set[A](
+    implicit codec: Codec[A]
+  ): Codec.Aux[Set[A], java.util.Collection[codec.Repr]] =
     Codec
       .list[A]
       .imap(_.toSet)(_.toList)
@@ -976,7 +993,7 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group General
     */
-  implicit final val short: Codec[Short] = {
+  implicit final val short: Codec.Aux[Short, java.lang.Integer] = {
     val min: Int = Short.MinValue.toInt
     val max: Int = Short.MaxValue.toInt
     Codec.int
@@ -991,13 +1008,13 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group General
     */
-  implicit final def some[A](implicit codec: Codec[A]): Codec[Some[A]] =
+  implicit final def some[A](implicit codec: Codec[A]): Codec.Aux[Some[A], codec.Repr] =
     codec.imap(Some(_))(_.value)
 
   /**
     * @group General
     */
-  implicit final val string: Codec[String] =
+  implicit final val string: Codec.Aux[String, Utf8] =
     Codec.instance(
       Right(SchemaBuilder.builder().stringType()),
       new Utf8(_).asRight,
@@ -1074,7 +1091,7 @@ object Codec extends CodecCompanionCompat {
     *
     * @group Create
     */
-  final def union[A](f: AltBuilder[A] => Chain[Alt[A]]): Codec[A] = {
+  final def union[A](f: AltBuilder[A] => Chain[Alt[A]]): Codec.Aux[A, AnyRef] = {
     val alts = f(AltBuilder.instance)
     val schema = AvroError.catchNonFatal {
       alts.toList
@@ -1161,7 +1178,7 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group JavaUtil
     */
-  implicit final val uuid: Codec[UUID] =
+  implicit final val uuid: Codec.Aux[UUID, Utf8] =
     Codec.instanceForTypes(
       "Utf8",
       "UUID",
@@ -1180,7 +1197,9 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group Collection
     */
-  implicit final def vector[A](implicit codec: Codec[A]): Codec[Vector[A]] =
+  implicit final def vector[A](
+    implicit codec: Codec[A]
+  ): Codec.Aux[Vector[A], java.util.Collection[codec.Repr]] =
     Codec.instanceForTypes(
       "Collection",
       "Vector",

From 8f4a6d5a59cadae63bc6667fe06191a3838f3cbd Mon Sep 17 00:00:00 2001
From: Ben Plommer <ben.plommer@itv.com>
Date: Thu, 28 Jan 2021 15:26:41 +0000
Subject: [PATCH 4/8] narrow type we encode Array as

---
 .../core/src/main/scala/vulcan/Codec.scala    | 20 +++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/modules/core/src/main/scala/vulcan/Codec.scala b/modules/core/src/main/scala/vulcan/Codec.scala
index 3f5c0acd..58832439 100644
--- a/modules/core/src/main/scala/vulcan/Codec.scala
+++ b/modules/core/src/main/scala/vulcan/Codec.scala
@@ -211,7 +211,7 @@ object Codec extends CodecCompanionCompat {
     */
   implicit final def chain[A](
     implicit codec: Codec[A]
-  ): Codec.Aux[Chain[A], java.util.Collection[codec.Repr]] =
+  ): Codec.Aux[Chain[A], java.util.List[codec.Repr]] =
     Codec.instanceForTypes(
       "Collection",
       "Chain",
@@ -674,7 +674,7 @@ object Codec extends CodecCompanionCompat {
     */
   implicit final def list[A](
     implicit codec: Codec[A]
-  ): Codec.Aux[List[A], java.util.Collection[codec.Repr]] =
+  ): Codec.Aux[List[A], java.util.List[codec.Repr]] =
     Codec.instanceForTypes(
       "Collection",
       "List",
@@ -783,7 +783,7 @@ object Codec extends CodecCompanionCompat {
     */
   implicit final def nonEmptyChain[A](
     implicit codec: Codec[A]
-  ): Codec.Aux[NonEmptyChain[A], java.util.Collection[codec.Repr]] =
+  ): Codec.Aux[NonEmptyChain[A], java.util.List[codec.Repr]] =
     Codec
       .chain[A]
       .imapError(
@@ -796,7 +796,7 @@ object Codec extends CodecCompanionCompat {
     */
   implicit final def nonEmptyList[A](
     implicit codec: Codec[A]
-  ): Codec.Aux[NonEmptyList[A], java.util.Collection[codec.Repr]] =
+  ): Codec.Aux[NonEmptyList[A], java.util.List[codec.Repr]] =
     Codec
       .list[A]
       .imapError(
@@ -810,7 +810,7 @@ object Codec extends CodecCompanionCompat {
   implicit final def nonEmptySet[A](
     implicit codec: Codec[A],
     ordering: Ordering[A]
-  ): Codec.Aux[NonEmptySet[A], java.util.Collection[codec.Repr]] =
+  ): Codec.Aux[NonEmptySet[A], java.util.List[codec.Repr]] =
     Codec
       .list[A]
       .imapError(
@@ -826,7 +826,7 @@ object Codec extends CodecCompanionCompat {
     */
   implicit final def nonEmptyVector[A](
     implicit codec: Codec[A]
-  ): Codec.Aux[NonEmptyVector[A], java.util.Collection[codec.Repr]] =
+  ): Codec.Aux[NonEmptyVector[A], java.util.List[codec.Repr]] =
     Codec
       .vector[A]
       .imapError(
@@ -973,7 +973,7 @@ object Codec extends CodecCompanionCompat {
     */
   implicit final def seq[A](
     implicit codec: Codec[A]
-  ): Codec.Aux[Seq[A], java.util.Collection[codec.Repr]] =
+  ): Codec.Aux[Seq[A], java.util.List[codec.Repr]] =
     Codec
       .list[A]
       .imap[Seq[A]](_.toSeq)(_.toList)
@@ -984,7 +984,7 @@ object Codec extends CodecCompanionCompat {
     */
   implicit final def set[A](
     implicit codec: Codec[A]
-  ): Codec.Aux[Set[A], java.util.Collection[codec.Repr]] =
+  ): Codec.Aux[Set[A], java.util.List[codec.Repr]] =
     Codec
       .list[A]
       .imap(_.toSet)(_.toList)
@@ -1166,7 +1166,7 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group General
     */
-  implicit final val unit: Codec[Unit] =
+  implicit final val unit: Codec.Aux[Unit, Null] =
     Codec.instanceForTypes(
       "null",
       "Unit",
@@ -1199,7 +1199,7 @@ object Codec extends CodecCompanionCompat {
     */
   implicit final def vector[A](
     implicit codec: Codec[A]
-  ): Codec.Aux[Vector[A], java.util.Collection[codec.Repr]] =
+  ): Codec.Aux[Vector[A], java.util.List[codec.Repr]] =
     Codec.instanceForTypes(
       "Collection",
       "Vector",

From f1cc6bbdab45d0f16ff8c6eeee32175fc8350bf0 Mon Sep 17 00:00:00 2001
From: Ben Plommer <ben.plommer@itv.com>
Date: Thu, 28 Jan 2021 15:30:41 +0000
Subject: [PATCH 5/8] reverse order of Aux parameters

---
 .../core/src/main/scala/vulcan/Codec.scala    | 88 +++++++++----------
 1 file changed, 44 insertions(+), 44 deletions(-)

diff --git a/modules/core/src/main/scala/vulcan/Codec.scala b/modules/core/src/main/scala/vulcan/Codec.scala
index 58832439..ecb49d8b 100644
--- a/modules/core/src/main/scala/vulcan/Codec.scala
+++ b/modules/core/src/main/scala/vulcan/Codec.scala
@@ -52,7 +52,7 @@ sealed abstract class Codec[A] {
     * for encoding and decoding, mapping back-and-forth
     * between types `A` and `B`.
     */
-  final def imap[B](f: A => B)(g: B => A): Codec.Aux[B, Repr] =
+  final def imap[B](f: A => B)(g: B => A): Codec.Aux[Repr, B] =
     Codec.instance(
       schema,
       b => encode(g(b)),
@@ -67,7 +67,7 @@ sealed abstract class Codec[A] {
     * Similar to [[Codec#imap]], except the mapping from
     * `A` to `B` might be unsuccessful.
     */
-  final def imapError[B](f: A => Either[AvroError, B])(g: B => A): Codec.Aux[B, Repr] =
+  final def imapError[B](f: A => Either[AvroError, B])(g: B => A): Codec.Aux[Repr, B] =
     Codec.instance(
       schema,
       b => encode(g(b)),
@@ -82,10 +82,10 @@ sealed abstract class Codec[A] {
     * Similar to [[Codec#imap]], except the mapping from
     * `A` to `B` might be unsuccessful.
     */
-  final def imapTry[B](f: A => Try[B])(g: B => A): Codec.Aux[B, Repr] =
+  final def imapTry[B](f: A => Try[B])(g: B => A): Codec.Aux[Repr, B] =
     imapError(f(_).toEither.leftMap(AvroError.fromThrowable))(g)
 
-  private[vulcan] def withDecodingTypeName(decodingTypeName: String): Codec.Aux[A, Repr] =
+  private[vulcan] def withDecodingTypeName(decodingTypeName: String): Codec.Aux[Repr, A] =
     Codec.instance(
       schema,
       encode,
@@ -132,7 +132,7 @@ sealed abstract class Codec[A] {
   */
 object Codec extends CodecCompanionCompat {
 
-  type Aux[A, Repr0 <: AnyRef] = Codec[A] {
+  type Aux[Repr0 <: AnyRef, A] = Codec[A] {
     type Repr = Repr0
   }
 
@@ -141,13 +141,13 @@ object Codec extends CodecCompanionCompat {
     *
     * @group Utilities
     */
-  final def apply[A](implicit codec: Codec[A]): Codec[A] =
+  final def apply[A](implicit codec: Codec[A]): codec.type =
     codec
 
   /**
     * @group General
     */
-  implicit final val boolean: Codec.Aux[Boolean, java.lang.Boolean] =
+  implicit final val boolean: Codec.Aux[java.lang.Boolean, Boolean] =
     Codec.instanceForTypes(
       "Boolean",
       "Boolean",
@@ -161,7 +161,7 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group General
     */
-  implicit final lazy val byte: Codec.Aux[Byte, java.lang.Integer] = {
+  implicit final lazy val byte: Codec.Aux[java.lang.Integer, Byte] = {
     val min: Int = Byte.MinValue.toInt
     val max: Int = Byte.MaxValue.toInt
     Codec.int
@@ -176,7 +176,7 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group General
     */
-  implicit final val bytes: Codec.Aux[Array[Byte], ByteBuffer] =
+  implicit final val bytes: Codec.Aux[ByteBuffer, Array[Byte]] =
     Codec.instance(
       Right(SchemaBuilder.builder().bytesType()),
       ByteBuffer.wrap(_).asRight,
@@ -211,7 +211,7 @@ object Codec extends CodecCompanionCompat {
     */
   implicit final def chain[A](
     implicit codec: Codec[A]
-  ): Codec.Aux[Chain[A], java.util.List[codec.Repr]] =
+  ): Codec.Aux[java.util.List[codec.Repr], Chain[A]] =
     Codec.instanceForTypes(
       "Collection",
       "Chain",
@@ -227,7 +227,7 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group General
     */
-  implicit final val char: Codec.Aux[Char, Utf8] =
+  implicit final val char: Codec.Aux[Utf8, Char] =
     Codec.instanceForTypes(
       "Utf8",
       "Char",
@@ -248,7 +248,7 @@ object Codec extends CodecCompanionCompat {
   final def decimal(
     precision: Int,
     scale: Int
-  ): Codec.Aux[BigDecimal, ByteBuffer] = {
+  ): Codec.Aux[ByteBuffer, BigDecimal] = {
     val conversion = new Conversions.DecimalConversion()
     val logicalType = LogicalTypes.decimal(precision, scale)
     val schema = AvroError.catchNonFatal {
@@ -309,7 +309,7 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group General
     */
-  implicit final val double: Codec.Aux[Double, java.lang.Double] =
+  implicit final val double: Codec.Aux[java.lang.Double, Double] =
     Codec.instance(
       Right(SchemaBuilder.builder().doubleType()),
       java.lang.Double.valueOf(_).asRight,
@@ -348,7 +348,7 @@ object Codec extends CodecCompanionCompat {
   implicit final def either[A, B](
     implicit codecA: Codec[A],
     codecB: Codec[B]
-  ): Codec.Aux[Either[A, B], AnyRef] =
+  ): Codec.Aux[AnyRef, Either[A, B]] =
     Codec.union(alt => alt[Left[A, B]] |+| alt[Right[A, B]])
 
   /**
@@ -374,7 +374,7 @@ object Codec extends CodecCompanionCompat {
     doc: Option[String] = None,
     aliases: Seq[String] = Seq.empty,
     props: Props = Props.empty
-  ): Codec.Aux[A, AnyRef] = {
+  ): Codec.Aux[AnyRef, A] = {
     val typeName = if (namespace.isEmpty) name else s"$namespace.$name"
     val schema = AvroError.catchNonFatal {
       props.toChain.map { props =>
@@ -431,7 +431,7 @@ object Codec extends CodecCompanionCompat {
     doc: Option[String] = None,
     aliases: Seq[String] = Seq.empty,
     props: Props = Props.empty
-  ): Codec.Aux[A, AnyRef] =
+  ): Codec.Aux[AnyRef, A] =
     enumeration(name, namespace, symbols, encode, decode, default, doc, aliases, props)
 
   /**
@@ -453,7 +453,7 @@ object Codec extends CodecCompanionCompat {
     doc: Option[String] = None,
     aliases: Seq[String] = Seq.empty,
     props: Props = Props.empty
-  ): Codec.Aux[A, GenericFixed] = {
+  ): Codec.Aux[GenericFixed, A] = {
     val typeName = if (namespace.isEmpty) name else s"$namespace.$name"
     val schema = AvroError.catchNonFatal {
       props.toChain.map { props =>
@@ -502,7 +502,7 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group General
     */
-  implicit final val float: Codec.Aux[Float, java.lang.Float] =
+  implicit final val float: Codec.Aux[java.lang.Float, Float] =
     Codec.instance(
       Right(SchemaBuilder.builder().floatType()),
       java.lang.Float.valueOf(_).asRight,
@@ -575,7 +575,7 @@ object Codec extends CodecCompanionCompat {
     schema: Either[AvroError, Schema],
     encode: A => Either[AvroError, Repr0],
     decode: (Any, Schema) => Either[AvroError, A]
-  ): Codec.Aux[A, Repr0] = {
+  ): Codec.Aux[Repr0, A] = {
     val _schema = schema
     val _encode = encode
     val _decode = decode
@@ -606,7 +606,7 @@ object Codec extends CodecCompanionCompat {
     schema: Either[AvroError, Schema],
     encode: A => Either[AvroError, Repr],
     decode: PartialFunction[(Any, Schema), Either[AvroError, A]]
-  ): Codec.Aux[A, Repr] = {
+  ): Codec.Aux[Repr, A] = {
     instance(
       schema,
       encode,
@@ -635,7 +635,7 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group JavaTime
     */
-  implicit final val instant: Codec.Aux[Instant, java.lang.Long] =
+  implicit final val instant: Codec.Aux[java.lang.Long, Instant] =
     Codec.instanceForTypes(
       "Long",
       "Instant",
@@ -652,7 +652,7 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group General
     */
-  implicit final val int: Codec.Aux[Int, java.lang.Integer] =
+  implicit final val int: Codec.Aux[java.lang.Integer, Int] =
     Codec.instanceForTypes(
       "Int",
       "Int",
@@ -666,7 +666,7 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group General
     */
-  implicit final def left[A, B](implicit codec: Codec[A]): Codec.Aux[Left[A, B], codec.Repr] =
+  implicit final def left[A, B](implicit codec: Codec[A]): Codec.Aux[codec.Repr, Left[A, B]] =
     codec.imap(Left[A, B](_))(_.value)
 
   /**
@@ -674,7 +674,7 @@ object Codec extends CodecCompanionCompat {
     */
   implicit final def list[A](
     implicit codec: Codec[A]
-  ): Codec.Aux[List[A], java.util.List[codec.Repr]] =
+  ): Codec.Aux[java.util.List[codec.Repr], List[A]] =
     Codec.instanceForTypes(
       "Collection",
       "List",
@@ -688,7 +688,7 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group JavaTime
     */
-  implicit final val localDate: Codec.Aux[LocalDate, java.lang.Integer] =
+  implicit final val localDate: Codec.Aux[java.lang.Integer, LocalDate] =
     Codec.instanceForTypes(
       "Integer",
       "LocalDate",
@@ -705,7 +705,7 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group General
     */
-  implicit final val long: Codec.Aux[Long, java.lang.Long] =
+  implicit final val long: Codec.Aux[java.lang.Long, Long] =
     Codec.instance(
       Right(SchemaBuilder.builder().longType()),
       java.lang.Long.valueOf(_).asRight,
@@ -739,7 +739,7 @@ object Codec extends CodecCompanionCompat {
     */
   implicit final def map[A](
     implicit codec: Codec[A]
-  ): Codec.Aux[Map[String, A], java.util.Map[Utf8, codec.Repr]] =
+  ): Codec.Aux[java.util.Map[Utf8, codec.Repr], Map[String, A]] =
     Codec.instanceForTypes(
       "java.util.Map",
       "Map",
@@ -769,7 +769,7 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group General
     */
-  implicit final val none: Codec.Aux[None.type, Null] =
+  implicit final val none: Codec.Aux[Null, None.type] =
     Codec.instanceForTypes(
       "null",
       "None",
@@ -783,7 +783,7 @@ object Codec extends CodecCompanionCompat {
     */
   implicit final def nonEmptyChain[A](
     implicit codec: Codec[A]
-  ): Codec.Aux[NonEmptyChain[A], java.util.List[codec.Repr]] =
+  ): Codec.Aux[java.util.List[codec.Repr], NonEmptyChain[A]] =
     Codec
       .chain[A]
       .imapError(
@@ -796,7 +796,7 @@ object Codec extends CodecCompanionCompat {
     */
   implicit final def nonEmptyList[A](
     implicit codec: Codec[A]
-  ): Codec.Aux[NonEmptyList[A], java.util.List[codec.Repr]] =
+  ): Codec.Aux[java.util.List[codec.Repr], NonEmptyList[A]] =
     Codec
       .list[A]
       .imapError(
@@ -810,7 +810,7 @@ object Codec extends CodecCompanionCompat {
   implicit final def nonEmptySet[A](
     implicit codec: Codec[A],
     ordering: Ordering[A]
-  ): Codec.Aux[NonEmptySet[A], java.util.List[codec.Repr]] =
+  ): Codec.Aux[java.util.List[codec.Repr], NonEmptySet[A]] =
     Codec
       .list[A]
       .imapError(
@@ -826,7 +826,7 @@ object Codec extends CodecCompanionCompat {
     */
   implicit final def nonEmptyVector[A](
     implicit codec: Codec[A]
-  ): Codec.Aux[NonEmptyVector[A], java.util.List[codec.Repr]] =
+  ): Codec.Aux[java.util.List[codec.Repr], NonEmptyVector[A]] =
     Codec
       .vector[A]
       .imapError(
@@ -851,7 +851,7 @@ object Codec extends CodecCompanionCompat {
     doc: Option[String] = None,
     aliases: Seq[String] = Seq.empty,
     props: Props = Props.empty
-  )(f: FieldBuilder[A] => FreeApplicative[Field[A, *], A]): Codec.Aux[A, GenericRecord] = {
+  )(f: FieldBuilder[A] => FreeApplicative[Field[A, *], A]): Codec.Aux[GenericRecord, A] = {
     val typeName = if (namespace.isEmpty) name else s"$namespace.$name"
     val free = f(FieldBuilder.instance)
     val schema = AvroError.catchNonFatal {
@@ -965,7 +965,7 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group General
     */
-  implicit final def right[A, B](implicit codec: Codec[B]): Codec.Aux[Right[A, B], codec.Repr] =
+  implicit final def right[A, B](implicit codec: Codec[B]): Codec.Aux[codec.Repr, Right[A, B]] =
     codec.imap(Right[A, B](_))(_.value)
 
   /**
@@ -973,7 +973,7 @@ object Codec extends CodecCompanionCompat {
     */
   implicit final def seq[A](
     implicit codec: Codec[A]
-  ): Codec.Aux[Seq[A], java.util.List[codec.Repr]] =
+  ): Codec.Aux[java.util.List[codec.Repr], Seq[A]] =
     Codec
       .list[A]
       .imap[Seq[A]](_.toSeq)(_.toList)
@@ -984,7 +984,7 @@ object Codec extends CodecCompanionCompat {
     */
   implicit final def set[A](
     implicit codec: Codec[A]
-  ): Codec.Aux[Set[A], java.util.List[codec.Repr]] =
+  ): Codec.Aux[java.util.List[codec.Repr], Set[A]] =
     Codec
       .list[A]
       .imap(_.toSet)(_.toList)
@@ -993,7 +993,7 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group General
     */
-  implicit final val short: Codec.Aux[Short, java.lang.Integer] = {
+  implicit final val short: Codec.Aux[java.lang.Integer, Short] = {
     val min: Int = Short.MinValue.toInt
     val max: Int = Short.MaxValue.toInt
     Codec.int
@@ -1008,13 +1008,13 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group General
     */
-  implicit final def some[A](implicit codec: Codec[A]): Codec.Aux[Some[A], codec.Repr] =
+  implicit final def some[A](implicit codec: Codec[A]): Codec.Aux[codec.Repr, Some[A]] =
     codec.imap(Some(_))(_.value)
 
   /**
     * @group General
     */
-  implicit final val string: Codec.Aux[String, Utf8] =
+  implicit final val string: Codec.Aux[Utf8, String] =
     Codec.instance(
       Right(SchemaBuilder.builder().stringType()),
       new Utf8(_).asRight,
@@ -1091,7 +1091,7 @@ object Codec extends CodecCompanionCompat {
     *
     * @group Create
     */
-  final def union[A](f: AltBuilder[A] => Chain[Alt[A]]): Codec.Aux[A, AnyRef] = {
+  final def union[A](f: AltBuilder[A] => Chain[Alt[A]]): Codec.Aux[AnyRef, A] = {
     val alts = f(AltBuilder.instance)
     val schema = AvroError.catchNonFatal {
       alts.toList
@@ -1166,7 +1166,7 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group General
     */
-  implicit final val unit: Codec.Aux[Unit, Null] =
+  implicit final val unit: Codec.Aux[Null, Unit] =
     Codec.instanceForTypes(
       "null",
       "Unit",
@@ -1178,7 +1178,7 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group JavaUtil
     */
-  implicit final val uuid: Codec.Aux[UUID, Utf8] =
+  implicit final val uuid: Codec.Aux[Utf8, UUID] =
     Codec.instanceForTypes(
       "Utf8",
       "UUID",
@@ -1199,7 +1199,7 @@ object Codec extends CodecCompanionCompat {
     */
   implicit final def vector[A](
     implicit codec: Codec[A]
-  ): Codec.Aux[Vector[A], java.util.List[codec.Repr]] =
+  ): Codec.Aux[java.util.List[codec.Repr], Vector[A]] =
     Codec.instanceForTypes(
       "Collection",
       "Vector",
@@ -1228,7 +1228,7 @@ object Codec extends CodecCompanionCompat {
   /**
     * @group Cats
     */
-  implicit final def codecAuxShow[A, Repr <: AnyRef]: Show[Codec.Aux[A, Repr]] =
+  implicit final def codecAuxShow[Repr <: AnyRef, A]: Show[Codec.Aux[Repr, A]] =
     Show.fromToString
 
   /**

From 753afa8c06f5212ad29c83ab8b3870855a31f7b9 Mon Sep 17 00:00:00 2001
From: Ben Plommer <ben.plommer@itv.com>
Date: Thu, 28 Jan 2021 15:41:51 +0000
Subject: [PATCH 6/8] cleanup

---
 modules/core/src/main/scala/vulcan/Codec.scala                | 4 ++--
 modules/generic/src/test/scala/vulcan/generic/CodecSpec.scala | 2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/modules/core/src/main/scala/vulcan/Codec.scala b/modules/core/src/main/scala/vulcan/Codec.scala
index ecb49d8b..6134cdc3 100644
--- a/modules/core/src/main/scala/vulcan/Codec.scala
+++ b/modules/core/src/main/scala/vulcan/Codec.scala
@@ -571,7 +571,7 @@ object Codec extends CodecCompanionCompat {
     *
     * @group Create
     */
-  final def instance[A, Repr0 <: AnyRef](
+  final def instance[Repr0 <: AnyRef, A](
     schema: Either[AvroError, Schema],
     encode: A => Either[AvroError, Repr0],
     decode: (Any, Schema) => Either[AvroError, A]
@@ -600,7 +600,7 @@ object Codec extends CodecCompanionCompat {
     }
   }
 
-  private[vulcan] final def instanceForTypes[A, Repr <: AnyRef](
+  private[vulcan] final def instanceForTypes[Repr <: AnyRef, A](
     expectedValueType: String,
     decodingTypeName: String,
     schema: Either[AvroError, Schema],
diff --git a/modules/generic/src/test/scala/vulcan/generic/CodecSpec.scala b/modules/generic/src/test/scala/vulcan/generic/CodecSpec.scala
index b54ada79..3078cfa3 100644
--- a/modules/generic/src/test/scala/vulcan/generic/CodecSpec.scala
+++ b/modules/generic/src/test/scala/vulcan/generic/CodecSpec.scala
@@ -61,7 +61,7 @@ final class CodecSpec extends AnyFunSpec with ScalaCheckPropertyChecks with Eith
             coproductCodec[Int, CNil](
               Codec.int,
               shapeless.Lazy {
-                Codec.instance[CNil, Null](
+                Codec.instance[Null, CNil](
                   Right(SchemaBuilder.builder().nullType()),
                   _ => Left(AvroError("encode")),
                   (_, _) => Left(AvroError("decode"))

From 19329c424ed1a0165b9099746c38b853af8ca24c Mon Sep 17 00:00:00 2001
From: Ben Plommer <ben.plommer@itv.com>
Date: Fri, 29 Jan 2021 10:58:32 +0000
Subject: [PATCH 7/8] refactoring

---
 modules/core/src/main/scala/vulcan/Codec.scala              | 4 +---
 modules/generic/src/main/scala/vulcan/generic/package.scala | 6 +++---
 2 files changed, 4 insertions(+), 6 deletions(-)

diff --git a/modules/core/src/main/scala/vulcan/Codec.scala b/modules/core/src/main/scala/vulcan/Codec.scala
index 6134cdc3..50724887 100644
--- a/modules/core/src/main/scala/vulcan/Codec.scala
+++ b/modules/core/src/main/scala/vulcan/Codec.scala
@@ -753,12 +753,10 @@ object Codec extends CodecCompanionCompat {
         }
         .map(_.toMap.asJava), {
         case (map: java.util.Map[_, _], schema) =>
-          val valueSchema = schema.getValueType()
-
           map.asScala.toList
             .traverse {
               case (key: Utf8, value) =>
-                codec.decode(value, valueSchema).tupleLeft(key.toString)
+                codec.decode(value, schema.getValueType()).tupleLeft(key.toString)
               case (key, _) =>
                 Left(AvroError.decodeUnexpectedMapKey(key))
             }
diff --git a/modules/generic/src/main/scala/vulcan/generic/package.scala b/modules/generic/src/main/scala/vulcan/generic/package.scala
index 29e66e01..5106347c 100644
--- a/modules/generic/src/main/scala/vulcan/generic/package.scala
+++ b/modules/generic/src/main/scala/vulcan/generic/package.scala
@@ -18,7 +18,7 @@ import vulcan.internal.converters.collection._
 import vulcan.internal.tags._
 
 package object generic {
-  implicit final val cnilCodec: Codec[CNil] =
+  implicit final val cnilCodec: Codec.Aux[Nothing, CNil] =
     Codec.instance(
       Right(Schema.createUnion()),
       cnil => Left(AvroError.encodeExhaustedAlternatives(cnil, Some("Coproduct"))),
@@ -298,7 +298,7 @@ package object generic {
     symbols: Seq[String],
     encode: A => String,
     decode: String => Either[AvroError, A]
-  )(implicit tag: WeakTypeTag[A]): Codec[A] =
+  )(implicit tag: WeakTypeTag[A]): Codec.Aux[AnyRef, A] =
     Codec.enumeration(
       name = nameFrom(tag),
       symbols = symbols,
@@ -319,7 +319,7 @@ package object generic {
     size: Int,
     encode: A => Array[Byte],
     decode: Array[Byte] => Either[AvroError, A]
-  )(implicit tag: WeakTypeTag[A]): Codec[A] =
+  )(implicit tag: WeakTypeTag[A]): Codec.Aux[GenericFixed, A] =
     Codec.fixed(
       name = nameFrom(tag),
       size = size,

From d6f974a3eb1b45eeae1cfb8a7105b60597c960b3 Mon Sep 17 00:00:00 2001
From: Ben Plommer <ben.plommer@itv.com>
Date: Fri, 29 Jan 2021 11:09:59 +0000
Subject: [PATCH 8/8] Fix compiler errors in scala 2.12, use more Aux types

---
 modules/core/src/main/scala-2/vulcan/CodecCompat.scala      | 5 +++--
 modules/core/src/main/scala/vulcan/Codec.scala              | 2 +-
 modules/generic/src/main/scala/vulcan/generic/package.scala | 4 ++--
 3 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/modules/core/src/main/scala-2/vulcan/CodecCompat.scala b/modules/core/src/main/scala-2/vulcan/CodecCompat.scala
index d6837d4c..f826a5c4 100644
--- a/modules/core/src/main/scala-2/vulcan/CodecCompat.scala
+++ b/modules/core/src/main/scala-2/vulcan/CodecCompat.scala
@@ -6,6 +6,7 @@
 
 package vulcan
 
+import org.apache.avro.generic.GenericFixed
 import scala.reflect.runtime.universe.WeakTypeTag
 import vulcan.internal.tags._
 
@@ -20,7 +21,7 @@ private[vulcan] trait CodecCompanionCompat {
     symbols: Seq[String],
     encode: A => String,
     decode: String => Either[AvroError, A]
-  )(implicit tag: WeakTypeTag[A]): Codec[A] =
+  )(implicit tag: WeakTypeTag[A]): Codec.Aux[AnyRef, A] =
     Codec.enumeration(
       name = nameFrom(tag),
       symbols = symbols,
@@ -39,7 +40,7 @@ private[vulcan] trait CodecCompanionCompat {
     size: Int,
     encode: A => Array[Byte],
     decode: Array[Byte] => Either[AvroError, A]
-  )(implicit tag: WeakTypeTag[A]): Codec[A] =
+  )(implicit tag: WeakTypeTag[A]): Codec.Aux[GenericFixed, A] =
     Codec.fixed(
       name = nameFrom(tag),
       size = size,
diff --git a/modules/core/src/main/scala/vulcan/Codec.scala b/modules/core/src/main/scala/vulcan/Codec.scala
index 50724887..c18d0265 100644
--- a/modules/core/src/main/scala/vulcan/Codec.scala
+++ b/modules/core/src/main/scala/vulcan/Codec.scala
@@ -1097,7 +1097,7 @@ object Codec extends CodecCompanionCompat {
         .map(schemas => Schema.createUnion(schemas.asJava))
     }
 
-    Codec.instance(
+    Codec.instance[AnyRef, A](
       schema,
       a =>
         alts
diff --git a/modules/generic/src/main/scala/vulcan/generic/package.scala b/modules/generic/src/main/scala/vulcan/generic/package.scala
index 5106347c..b5933c74 100644
--- a/modules/generic/src/main/scala/vulcan/generic/package.scala
+++ b/modules/generic/src/main/scala/vulcan/generic/package.scala
@@ -221,9 +221,9 @@ package object generic {
     final def derive[A]: Codec[A] =
       macro Magnolia.gen[A]
 
-    final def dispatch[A](sealedTrait: SealedTrait[Codec, A]): Codec[A] = {
+    final def dispatch[A](sealedTrait: SealedTrait[Codec, A]): Codec.Aux[AnyRef, A] = {
       val typeName = sealedTrait.typeName.full
-      Codec.instance(
+      Codec.instance[AnyRef, A](
         AvroError.catchNonFatal {
           sealedTrait.subtypes.toList
             .traverse(_.typeclass.schema)