Pass root field names together with resolvers

src/Data/GraphQL/Schema.hs

@@ -31,14 +31,16 @@ import Control.Monad.Trans.Class (lift)
 import Control.Monad.Trans.State ( get
                                  , put
                                  )
-import Data.Foldable (fold)
+import Data.Foldable ( find
+                     , fold
+                     )
 import Data.GraphQL.Error
 import Data.List.NonEmpty (NonEmpty)
 import Data.Maybe (fromMaybe)
-import Data.Monoid (Alt(..))
 import qualified Data.Aeson as Aeson
 import Data.HashMap.Strict (HashMap)
 import qualified Data.HashMap.Strict as HashMap
+import Data.Text (Text)
 import qualified Data.Text as T
 
 import Data.GraphQL.AST.Core
@@ -48,10 +50,10 @@ import Data.GraphQL.AST.Core
 type Schema m = NonEmpty (Resolver m)
 
 -- | Resolves a 'Field' into an @Aeson.@'Aeson.Object' with error information
---   (or 'empty'). @f@ is usually expected to be an instance of 'Alternative'.
-type Resolver m = Field -> CollectErrsT m Aeson.Object
-
-type Resolvers m = [Resolver m]
+--   (or 'empty'). @m@ is usually expected to be an instance of 'MonadPlus'.
+data Resolver m = Resolver
+    Text -- ^ Name
+    (Field -> CollectErrsT m Aeson.Object) -- ^ Resolver
 
 type Fields = [Field]
 
@@ -61,7 +63,7 @@ type Arguments = [Argument]
 type Subs = Name -> Maybe Value
 
 -- | Create a new 'Resolver' with the given 'Name' from the given 'Resolver's.
-object :: MonadPlus m => Name -> Resolvers m -> Resolver m
+object :: MonadPlus m => Name -> [Resolver m] -> Resolver m
 object name resolvers = objectA name $ \case
   [] -> resolvers
   _  -> empty
@@ -69,12 +71,14 @@ object name resolvers = objectA name $ \case
 -- | Like 'object' but also taking 'Argument's.
 objectA
   :: MonadPlus m
-  => Name -> (Arguments -> Resolvers m) -> Resolver m
-objectA name f fld@(Field _ _ args flds) = withField name (resolve (f args) flds) fld
+  => Name -> (Arguments -> [Resolver m]) -> Resolver m
+objectA name f = Resolver name go
+  where
+    go fld@(Field _ _ args flds) = withField name (resolve (f args) flds) fld
 
 
 -- | Create a named 'Resolver' from a list of 'Resolver's.
-object' :: MonadPlus m => Name -> m (Resolvers m) -> Resolver m
+object' :: MonadPlus m => Name -> m [Resolver m] -> Resolver m
 object' name resolvs = objectA' name $ \case
      [] -> resolvs
      _  -> empty
@@ -82,10 +86,12 @@ object' name resolvs = objectA' name $ \case
 -- | Like 'object'' but also taking 'Argument's.
 objectA'
   :: MonadPlus m
-  => Name -> (Arguments -> m (Resolvers m)) -> Resolver m
-objectA' name f fld@(Field _ _ args flds) = do
-    resolvs <- lift $ f args
-    withField name (resolve resolvs flds) fld
+  => Name -> (Arguments -> m [Resolver m]) -> Resolver m
+objectA' name f = Resolver name go
+  where
+    go fld@(Field _ _ args flds) = do
+        resolvs <- lift $ f args
+        withField name (resolve resolvs flds) fld
 
 -- | A scalar represents a primitive value, like a string or an integer.
 scalar :: (MonadPlus m, Aeson.ToJSON a) => Name -> a -> Resolver m
@@ -97,10 +103,12 @@ scalar name s = scalarA name $ \case
 scalarA
   :: (MonadPlus m, Aeson.ToJSON a)
   => Name -> (Arguments -> m a) -> Resolver m
-scalarA name f fld@(Field _ _ args []) = withField name (lift $ f args) fld
-scalarA _ _ _ = empty
+scalarA name f = Resolver name go
+  where
+    go fld@(Field _ _ args []) = withField name (lift $ f args) fld
+    go _ = empty
 
-array :: MonadPlus m => Name -> [Resolvers m] -> Resolver m
+array :: MonadPlus m => Name -> [[Resolver m]] -> Resolver m
 array name resolvers = arrayA name $ \case
     [] -> resolvers
     _  -> empty
@@ -108,12 +116,13 @@ array name resolvers = arrayA name $ \case
 -- | Like 'array' but also taking 'Argument's.
 arrayA
   :: MonadPlus m
-  => Name -> (Arguments -> [Resolvers m]) -> Resolver m
-arrayA name f fld@(Field _ _ args sels) =
-     withField name (traverse (`resolve` sels) $ f args) fld
+  => Name -> (Arguments -> [[Resolver m]]) -> Resolver m
+arrayA name f = Resolver name go
+  where
+    go fld@(Field _ _ args sels) = withField name (traverse (`resolve` sels) $ f args) fld
 
 -- | Like 'object'' but taking lists of 'Resolver's instead of a single list.
-array' :: MonadPlus m => Name -> m [Resolvers m] -> Resolver m
+array' :: MonadPlus m => Name -> m [[Resolver m]] -> Resolver m
 array' name resolvs = arrayA' name $ \case
     [] -> resolvs
     _  -> empty
@@ -121,46 +130,50 @@ array' name resolvs = arrayA' name $ \case
 -- | Like 'array'' but also taking 'Argument's.
 arrayA'
   :: MonadPlus m
-  => Name -> (Arguments -> m [Resolvers m]) -> Resolver m
-arrayA' name f fld@(Field _ _ args sels) = do
-     resolvs <- lift $ f args
-     withField name (traverse (`resolve` sels) resolvs) fld
+  => Name -> (Arguments -> m [[Resolver m]]) -> Resolver m
+arrayA' name f = Resolver name go
+  where
+    go fld@(Field _ _ args sels) = do
+        resolvs <- lift $ f args
+        withField name (traverse (`resolve` sels) resolvs) fld
 
 -- | Represents one of a finite set of possible values.
 --   Used in place of a 'scalar' when the possible responses are easily enumerable.
-enum :: MonadPlus m => Name -> m [T.Text] -> Resolver m
+enum :: MonadPlus m => Name -> m [Text] -> Resolver m
 enum name enums = enumA name $ \case
      [] -> enums
      _  -> empty
 
 -- | Like 'enum' but also taking 'Argument's.
-enumA :: MonadPlus m => Name -> (Arguments -> m [T.Text]) -> Resolver m
-enumA name f fld@(Field _ _ args []) = withField name (lift $ f args) fld
-enumA _ _ _ = empty
+enumA :: MonadPlus m => Name -> (Arguments -> m [Text]) -> Resolver m
+enumA name f = Resolver name go
+  where
+    go fld@(Field _ _ args []) = withField name (lift $ f args) fld
+    go _ = empty
 
 -- | Helper function to facilitate 'Argument' handling.
 withField :: (MonadPlus m, Aeson.ToJSON a)
-          => Name -> CollectErrsT m a -> Field -> CollectErrsT m (HashMap T.Text Aeson.Value)
-withField name v (Field alias name' _ _)
-    | name == name' = do
-        collection <- HashMap.singleton aliasOrName . Aeson.toJSON <$> runAppendErrs v
-        errors <- get
-        if null errors
-           then return collection
-           -- TODO: Report error when Non-Nullable type for field argument.
-           else put [] >> return (HashMap.singleton aliasOrName Aeson.Null)
-    | otherwise = empty
+          => Name -> CollectErrsT m a -> Field -> CollectErrsT m (HashMap Text Aeson.Value)
+withField name v (Field alias _ _ _) = do
+    collection <- HashMap.singleton aliasOrName . Aeson.toJSON <$> runAppendErrs v
+    errors <- get
+    if null errors
+        then return collection
+        -- TODO: Report error when Non-Nullable type for field argument.
+        else put [] >> return (HashMap.singleton aliasOrName Aeson.Null)
   where
     aliasOrName = fromMaybe name alias
 
 -- | Takes a list of 'Resolver's and a list of 'Field's and applies each
 --   'Resolver' to each 'Field'. Resolves into a value containing the
 --   resolved 'Field', or a null value and error information.
-resolve :: MonadPlus m => Resolvers m -> Fields -> CollectErrsT m Aeson.Value
-resolve resolvers =
-    fmap (Aeson.toJSON . fold)
-  . traverse (\fld -> getAlt (foldMap (Alt . ($ fld)) resolvers) <|> errmsg fld)
+resolve :: MonadPlus m
+        => [Resolver m] -> Fields -> CollectErrsT m Aeson.Value
+resolve resolvers = fmap (Aeson.toJSON . fold) . traverse tryResolvers
   where
+    tryResolvers fld = maybe empty (tryResolver fld) (find (compareResolvers fld) resolvers) <|> errmsg fld
+    compareResolvers (Field _ name _ _) (Resolver name' _) = name == name'
+    tryResolver fld (Resolver _ resolver)  = resolver fld
     errmsg (Field alias name _ _) = do
         addErrMsg $ T.unwords ["field", name, "not resolved."]
         return $ HashMap.singleton aliasOrName Aeson.Null