Provide an internal function to add errors

The old function, addErrMsg, takes only a string with an error
description, but more information is required for the execution errors:
locations and path. addErrMsg should be deprecated after the switching
to the new addError.

src/Language/GraphQL/Execute/Execution.hs

@@ -21,6 +21,7 @@ import qualified Data.Text as Text
 import Language.GraphQL.AST (Name)
 import Language.GraphQL.Error
 import Language.GraphQL.Execute.Coerce
+import Language.GraphQL.Execute.Internal
 import Language.GraphQL.Execute.OrderedMap (OrderedMap)
 import qualified Language.GraphQL.Execute.OrderedMap as OrderedMap
 import qualified Language.GraphQL.Execute.Transform as Transform
@@ -123,7 +124,7 @@ completeValue outputType@(Out.EnumBaseType enumType) _ (Type.Enum enum) =
     let Type.EnumType _ _ enumMembers = enumType
      in if HashMap.member enum enumMembers
         then coerceResult outputType $ Enum enum
-        else addErrMsg "Enum value completion failed."
+        else addError $ Error "Enum value completion failed." [] []
 completeValue (Out.ObjectBaseType objectType) fields result =
     executeSelectionSet result objectType $ mergeSelectionSets fields
 completeValue (Out.InterfaceBaseType interfaceType) fields result

src/Language/GraphQL/Execute/Internal.hs

@@ -0,0 +1,25 @@
+{- 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 DuplicateRecordFields #-}
+{-# LANGUAGE NamedFieldPuns #-}
+
+module Language.GraphQL.Execute.Internal
+    ( addError
+    ) where
+
+import Control.Monad.Trans.State (modify)
+import Control.Monad.Catch (MonadCatch)
+import Data.Sequence ((|>))
+import Language.GraphQL.Error
+import Language.GraphQL.Execute.Coerce
+import Prelude hiding (null)
+
+addError :: (Serialize a, MonadCatch m) => Error -> CollectErrsT m a
+addError error' = modify appender >> pure null
+  where
+   appender :: Resolution m -> Resolution m
+   appender resolution@Resolution{ errors } = resolution
+       { errors = errors |> error'
+       }