WriteForeign for newtyped maps

src/Simple/JSON.purs

@@ -330,7 +330,10 @@ instance writeForeignMaybe :: WriteForeign a => WriteForeign (Maybe a) where
 instance writeForeignNullable :: WriteForeign a => WriteForeign (Nullable a) where
   writeImpl = maybe (unsafeToForeign $ toNullable Nothing) writeImpl <<< toMaybe
 
-instance writeForeignObject :: WriteForeign a => WriteForeign (Map String a) where
+instance writeForeignMap :: WriteForeign a => WriteForeign (Map String a) where
+  writeImpl = unsafeToForeign <<< map writeImpl
+
+else instance writeForeignNewtypeMap :: (Newtype b String, WriteForeign a) => WriteForeign (Map b a) where
   writeImpl = unsafeToForeign <<< map writeImpl
 
 instance recordWriteForeign ::

test/Main.purs

@@ -8,6 +8,7 @@ import Data.Either (Either(..), either, fromLeft, isRight)
 import Data.List (List(..), (:))
 import Data.List.NonEmpty (NonEmptyList(..))
 import Data.Maybe (Maybe)
+import Data.Newtype (class Newtype)
 import Data.NonEmpty (NonEmpty(..))
 import Data.Nullable (Nullable)
 import Data.Variant (Variant)
@@ -59,6 +60,14 @@ type MyTestStrMap =
   , b :: Map String Int
   }
 
+newtype AlsoAString = AlsoAString String
+derive instance alsoAStringNewtype :: Newtype AlsoAString _
+
+type MyTestStrMapNewtype =
+  { a :: Int
+  , b :: Map AlsoAString Int
+  }
+
 type MyTestMaybe =
   { a :: Maybe String
   }
@@ -167,6 +176,12 @@ main = do
     { "a": 1, "b": {"asdf": 1, "c": 2} }
   """
 
+  -- "works with JSON containing Map field with newtyped keys"
+  roundtrips (Proxy :: Proxy MyTestStrMapNewtype) """
+    { "a": 1, "b": {"asdf": 1, "c": 2} }
+  """
+
+
   -- "works with Maybe field and existing value"
   roundtrips (Proxy :: Proxy MyTestMaybe) """
     { "a": "foo" }