Add Resolver module with helpers and exceptions

src/Language/GraphQL/Resolver.hs

@@ -0,0 +1,61 @@
+{- This Source Code Form is subject to the terms of the Mozilla Public License,
+   v. 2.0. If a copy of the MPL was not distributed with this file, You can
+   obtain one at https://mozilla.org/MPL/2.0/. -}
+
+{-# LANGUAGE OverloadedStrings #-}
+
+-- | Helper functions and exceptions to write resolvers.
+module Language.GraphQL.Resolver
+    ( argument
+    , defaultResolver
+    ) where
+
+import Control.Monad.Catch (Exception(..), MonadCatch(..), MonadThrow(..))
+import Control.Monad.Trans.Reader (ReaderT, asks)
+import Data.HashMap.Strict ((!))
+import Data.Text (Text)
+import qualified Data.Text as Text
+import Data.Typeable (cast)
+import Language.GraphQL.AST.Document (Name)
+import Language.GraphQL.Error
+import qualified Language.GraphQL.Type as Type
+import Language.GraphQL.Class (FromGraphQL(..))
+
+-- | Exceptions thrown by the functions in this module.
+data ServerException
+    = FieldNotResolvedException !Text
+    | ErroneousArgumentTypeException !Text
+
+instance Show ServerException where
+    show (FieldNotResolvedException fieldName) =
+        Text.unpack $ Text.unwords ["Field", fieldName, "not resolved."]
+    show (ErroneousArgumentTypeException argumentName) =
+        Text.unpack $ Text.unwords
+            [ "Unable to convert the argument"
+            , argumentName
+            , "to a user-defined type."
+            ]
+
+instance Exception ServerException where
+    toException = toException . ResolverException
+    fromException x = do
+        ResolverException a <- fromException x
+        cast a
+
+-- | Default resolver expects that the field value is returned by the parent
+-- object. If the parent is not an object or it doesn't contain the requested
+-- field name, an error is thrown.
+defaultResolver :: MonadCatch m => Name -> Type.Resolve m
+defaultResolver fieldName = do
+    values' <- asks Type.values
+    case values' of
+        Type.Object objectValue -> pure $ objectValue ! fieldName
+        _nonObject -> throwM $ FieldNotResolvedException fieldName
+
+-- | Takes an argument name, validates that the argument exists, and optionally
+-- converts it to a user-defined type.
+argument :: (MonadCatch m, FromGraphQL a) => Name -> ReaderT Type.Context m a
+argument argumentName =
+    Type.argument argumentName >>= maybe throwError pure . fromGraphQL
+  where
+    throwError = throwM $ ErroneousArgumentTypeException argumentName