Allow resolvers to return arbitrary nested lists

src/Language/GraphQL/Type.hs

@@ -0,0 +1,57 @@
+-- | Definitions for @GraphQL@ type system.
+module Language.GraphQL.Type
+    ( Wrapping(..)
+    ) where
+
+import Data.Aeson as Aeson ( ToJSON
+                           , toJSON
+                           )
+import qualified Data.Aeson as Aeson
+
+-- | GraphQL distinguishes between "wrapping" and "named" types. Each wrapping
+--   type can wrap other wrapping or named types. Wrapping types are lists and
+--   Non-Null types (named types are nullable by default).
+--
+--   This 'Wrapping' type doesn\'t reflect this distinction exactly but it is
+--   used in the resolvers to take into account that the returned value can be
+--   nullable or an (arbitrary nested) list.
+data Wrapping a
+    = List [Wrapping a] -- ^ Arbitrary nested list
+    | Named a -- ^ Named type without further wrapping
+    | Null -- ^ Null
+    deriving (Eq, Show)
+
+instance Functor Wrapping where
+    fmap f (List list) = List $ fmap (fmap f) list
+    fmap f (Named named) = Named $ f named
+    fmap _ Null = Null
+
+instance Foldable Wrapping where
+    foldr f acc (List list) = foldr (flip $ foldr f) acc list
+    foldr f acc (Named named) = f named acc
+    foldr _ acc Null = acc
+
+instance Traversable Wrapping where
+    traverse f (List list) = List <$> traverse (traverse f) list
+    traverse f (Named named) = Named <$> f named
+    traverse _ Null = pure Null
+
+instance Applicative Wrapping where
+    pure = Named
+    Null <*> _ = Null
+    _ <*> Null = Null
+    (Named f) <*> (Named x) = Named $ f x
+    (List fs) <*> (List xs) = List $ (<*>) <$> fs <*> xs
+    (Named f) <*> list = f <$> list
+    (List fs) <*> named = List $ (<*> named) <$> fs
+
+instance Monad Wrapping where
+    return = pure
+    Null >>= _ = Null
+    (Named x) >>= f = f x
+    (List xs) >>= f = List $ fmap (>>= f) xs
+
+instance ToJSON a => ToJSON (Wrapping a) where
+    toJSON (List list) = toJSON list
+    toJSON (Named named) = toJSON named
+    toJSON Null = Aeson.Null