main:CollectionResource consists of CollectionItem s

src/Servant/Hateoas/ContentType/Collection.hs

@@ -27,23 +27,35 @@ data Collection (t :: Type)
 -- | Resource wrapper for Collection.
 data CollectionResource a = CollectionResource
   { href  :: Maybe String
-  , items :: [a]
+  , items :: [CollectionItem a]
   , links :: [(String, Link)]
-  } deriving (Generic)
+  } deriving (Show, Generic)
+
+-- | A single item inside a 'CollectionResource'.
+data CollectionItem a = CollectionItem
+  { item :: a
+  , itemLinks :: [(String, Link)]
+  } deriving (Show, Generic)
 
 instance HasResource (Collection t) where
   type Resource (Collection t) = CollectionResource
 
 instance Accept (Collection JSON) where
   contentType _ = "application" M.// "vnd.collection+json"
 
+instance ToJSON a => ToJSON (CollectionItem a) where
+  toJSON (CollectionItem (toJSON -> Object m) ls) = object ["data" .= itemData, "links" .= collectionLinks ls]
+    where
+      itemData = Array $ Foldable.foldl' (\xs (k, v) -> pure (object ["name" .= k, "value" .= v]) <> xs) mempty $ toList m
+  toJSON (CollectionItem (toJSON -> v) _) = v
+
 instance {-# OVERLAPPABLE #-} ToJSON a => ToJSON (CollectionResource a) where
   toJSON (CollectionResource mHref is ls) = object ["collection".= collection]
     where
-      collection = object $ ["version" .= ("1.0" :: String), "links" .= ls', "items" .= is'] <> maybe [] (pure . ("href" .=)) mHref
-      ls' = Array $ Foldable.foldl' (\xs (rel, l) -> pure (object ["name" .= rel, "value" .= linkURI l]) <> xs) mempty ls
-      is' = Array $ Foldable.foldl' (\xs i -> pure (object ["data" .= toCollectionData i, "links" .= (mempty :: Array)]) <> xs) mempty is
+      collection = object $ ["version" .= ("1.0" :: String), "links" .= collectionLinks ls, "items" .= is'] <> maybe [] (pure . ("href" .=)) mHref
+      is' = Array $ Foldable.foldl' (\xs i -> pure (toJSON i) <> xs) mempty is
+
+collectionLinks :: [(String, Link)] -> Value
+collectionLinks = Array . Foldable.foldl' (\xs (rel, l) -> pure (object ["name" .= rel, "value" .= linkURI l]) <> xs) mempty
 
-toCollectionData :: ToJSON a => a -> Value
-toCollectionData (toJSON -> Object m) = Array $ Foldable.foldl' (\xs (k, v) -> pure (object ["name" .= k, "value" .= v]) <> xs) mempty $ toList m
-toCollectionData (toJSON -> x)        = x
+-- instance ToResource ... - both for CollectionItem and CollectionResource...?