diff --git a/frontend/src/Core/Decoder.elm b/frontend/src/Core/Decoder.elm
index ca8bdbe..8b269ba 100644
--- a/frontend/src/Core/Decoder.elm
+++ b/frontend/src/Core/Decoder.elm
@@ -100,3 +100,8 @@ decodeOneType = D.succeed T.OneType
     |> required "guests" (D.list decodeGuest)
     |> required "userRequest" decodeUserRequest
     |> required "nonEmpty" (elmStreetDecodeNonEmpty decodeMyUnit)
+
+decodeCustomCodeGen : Decoder T.CustomCodeGen
+decodeCustomCodeGen = D.succeed T.CustomCodeGen
+    |> required "customFunTestString" D.string
+    |> required "customFunTestInt" D.int
diff --git a/frontend/src/Core/Encoder.elm b/frontend/src/Core/Encoder.elm
index ace7983..dddf3a0 100644
--- a/frontend/src/Core/Encoder.elm
+++ b/frontend/src/Core/Encoder.elm
@@ -95,3 +95,10 @@ encodeOneType x = E.object
     , ("userRequest", encodeUserRequest x.userRequest)
     , ("nonEmpty", (elmStreetEncodeNonEmpty encodeMyUnit) x.nonEmpty)
     ]
+
+encodeCustomCodeGen : T.CustomCodeGen -> Value
+encodeCustomCodeGen x = E.object
+    [ ("tag", E.string "CustomCodeGen")
+    , ("customFunTestString", E.string x.customFunTestString)
+    , ("customFunTestInt", E.int x.customFunTestInt)
+    ]
diff --git a/frontend/src/Core/Types.elm b/frontend/src/Core/Types.elm
index 5ba8f5c..54373cc 100644
--- a/frontend/src/Core/Types.elm
+++ b/frontend/src/Core/Types.elm
@@ -119,3 +119,8 @@ type alias OneType =
     , userRequest : UserRequest
     , nonEmpty : (MyUnit, List MyUnit)
     }
+
+type alias CustomCodeGen =
+    { customFunTestString : String
+    , customFunTestInt : Int
+    }
diff --git a/frontend/tests/Tests/Golden.elm b/frontend/tests/Tests/Golden.elm
index 2267b0d..4ee1d53 100644
--- a/frontend/tests/Tests/Golden.elm
+++ b/frontend/tests/Tests/Golden.elm
@@ -36,7 +36,7 @@ goldenOneTypeJson =
             "bool": true,
             "unit": [],
             "nonEmpty": [1],
-            "value" : {
+            "value": {
                 "boolField": true,
                 "numberField": 1,
                 "stringField": "hi",
@@ -67,9 +67,9 @@ goldenOneTypeJson =
             "limit": 123
         },
         "age": 18,
-        "newtype":  666,
+        "newtype": 666,
         "newtypeList": [123],
-        "oneConstructor":  "OneConstructor",
+        "oneConstructor": "OneConstructor",
         "user": {
             "status": "Approved",
             "tag": "User",
diff --git a/types/Types.hs b/types/Types.hs
index 890e4a1..ebf33dc 100644
--- a/types/Types.hs
+++ b/types/Types.hs
@@ -33,7 +33,7 @@ import Data.Text (Text)
 import Data.Time.Calendar (fromGregorian)
 import Data.Time.Clock (UTCTime (..))
 import Data.Word (Word32)
-import Elm (Elm (..), ElmStreet (..), elmNewtype, elmStreetParseJson, elmStreetToJson)
+import Elm (Elm (..), ElmStreet (..), elmNewtype)
 import Elm.Generic (CodeGenOptions (..), ElmStreetGenericConstraints, GenericElmDefinition(..))
 import Elm.Aeson (elmStreetParseJsonWith, elmStreetToJsonWith)
 import GHC.Generics (Generic, Rep)
@@ -86,20 +86,14 @@ newtype NewtypeList = NewtypeList [Int]
 
 data OneConstructor = OneConstructor
     deriving stock (Generic, Eq, Show)
-    deriving anyclass (Elm)
-
-instance ToJSON   OneConstructor where toJSON = elmStreetToJson
-instance FromJSON OneConstructor where parseJSON = elmStreetParseJson
+    deriving (Elm, FromJSON, ToJSON) via ElmStreet OneConstructor
 
 data RequestStatus
     = Approved
     | Rejected
     | Reviewing
     deriving (Generic, Eq, Show)
-    deriving anyclass (Elm)
-
-instance ToJSON   RequestStatus where toJSON = elmStreetToJson
-instance FromJSON RequestStatus where parseJSON = elmStreetParseJson
+    deriving (Elm, FromJSON, ToJSON) via ElmStreet RequestStatus
 
 data User = User
     { userId     :: !(Id User)
@@ -107,10 +101,7 @@ data User = User
     , userAge    :: !Age
     , userStatus :: !RequestStatus
     } deriving (Generic, Eq, Show)
-      deriving anyclass (Elm)
-
-instance ToJSON   User where toJSON = elmStreetToJson
-instance FromJSON User where parseJSON = elmStreetParseJson
+      deriving (Elm, FromJSON, ToJSON) via ElmStreet User
 
 data Guest
     = Regular Text Int
@@ -118,20 +109,14 @@ data Guest
     | Special (Maybe [Int])
     | Blocked
     deriving (Generic, Eq, Show)
-    deriving anyclass (Elm)
-
-instance ToJSON   Guest where toJSON = elmStreetToJson
-instance FromJSON Guest where parseJSON = elmStreetParseJson
+    deriving (Elm, FromJSON, ToJSON) via ElmStreet Guest
 
 data UserRequest = UserRequest
     { userRequestIds     :: ![Id User]
     , userRequestLimit   :: !Word32
     , userRequestExample :: !(Maybe (Either User Guest))
     } deriving (Generic, Eq, Show)
-      deriving anyclass (Elm)
-
-instance ToJSON   UserRequest where toJSON = elmStreetToJson
-instance FromJSON UserRequest where parseJSON = elmStreetParseJson
+      deriving (Elm, FromJSON, ToJSON) via ElmStreet UserRequest
 
 data MyUnit = MyUnit ()
     deriving stock (Show, Eq, Ord, Generic)
@@ -142,10 +127,7 @@ data MyResult
     = Ok
     | Err Text
     deriving (Generic, Eq, Show)
-    deriving anyclass (Elm)
-
-instance ToJSON   MyResult where toJSON = elmStreetToJson
-instance FromJSON MyResult where parseJSON = elmStreetParseJson
+    deriving (Elm, FromJSON, ToJSON) via ElmStreet MyResult
 
 -- | All test types together in one type to play with.
 data OneType = OneType
@@ -163,10 +145,7 @@ data OneType = OneType
     , oneTypeUserRequest    :: !UserRequest
     , oneTypeNonEmpty       :: !(NonEmpty MyUnit)
     } deriving (Generic, Eq, Show)
-      deriving anyclass (Elm)
-
-instance ToJSON   OneType where toJSON = elmStreetToJson
-instance FromJSON OneType where parseJSON = elmStreetParseJson
+      deriving (Elm, FromJSON, ToJSON) via ElmStreet OneType
 
 data CustomCodeGen = CustomCodeGen
     { customCodeGenString :: String
@@ -178,7 +157,6 @@ data CustomCodeGen = CustomCodeGen
 customCodeGenOptions :: CodeGenOptions
 customCodeGenOptions = CodeGenOptions (Text.replace "CodeGen" "FunTest")
 
-
 -- Newtype whose Elm/ToJSON/FromJSON instance use custom CodeGenOptions
 newtype CustomElm a = CustomElm {unCustomElm :: a}
 
@@ -207,6 +185,7 @@ type Types =
     , Guest
     , UserRequest
     , OneType
+    , CustomCodeGen
     ]