Deprecate internal error generation functions

The functions generating errors in the executor should be changed anyway
when we provide better error messages from the executor, with the error
location and response path. So public definitions of these functions are
deprecated now and they are replaced by more generic functions in the
executor code.

src/Language/GraphQL/Execute/Execution.hs

@@ -43,7 +43,7 @@ resolveFieldValue result args resolver =
         => ResolverException
         -> CollectErrsT m Type.Value
     handleFieldError e =
-        addErr (Error (Text.pack $ displayException e) [] []) >> pure Type.Null
+        addError Type.Null $ Error (Text.pack $ displayException e) [] []
     context = Type.Context
         { Type.arguments = Type.Arguments args
         , Type.values = result
@@ -98,7 +98,7 @@ executeField fieldResolver prev fields
         let Out.Field _ fieldType argumentDefinitions = fieldDefinition
         let (Transform.Field _ _ arguments' _ :| []) = fields
         case coerceArgumentValues argumentDefinitions arguments' of
-            Nothing -> addErrMsg "Argument coercing failed."
+            Nothing -> addError null $ Error "Argument coercing failed." [] []
             Just argumentValues -> do
                 answer <- resolveFieldValue prev argumentValues resolver
                 completeValue fieldType fields answer
@@ -124,7 +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 addError $ Error "Enum value completion failed." [] []
+        else addError null $ Error "Enum value completion failed." [] []
 completeValue (Out.ObjectBaseType objectType) fields result =
     executeSelectionSet result objectType $ mergeSelectionSets fields
 completeValue (Out.InterfaceBaseType interfaceType) fields result
@@ -134,7 +134,8 @@ completeValue (Out.InterfaceBaseType interfaceType) fields result
         case concreteType of
             Just objectType -> executeSelectionSet result objectType
                 $ mergeSelectionSets fields
-            Nothing -> addErrMsg "Interface value completion failed."
+            Nothing -> addError null
+                $ Error "Interface value completion failed." [] []
 completeValue (Out.UnionBaseType unionType) fields result
     | Type.Object objectMap <- result = do
         let abstractType = Internal.AbstractUnionType unionType
@@ -142,8 +143,9 @@ completeValue (Out.UnionBaseType unionType) fields result
         case concreteType of
             Just objectType -> executeSelectionSet result objectType
                 $ mergeSelectionSets fields
-            Nothing -> addErrMsg "Union value completion failed."
-completeValue _ _ _ = addErrMsg "Value completion failed."
+            Nothing -> addError null
+                $ Error "Union value completion failed." [] []
+completeValue _ _ _ = addError null $ Error "Value completion failed." [] []
 
 mergeSelectionSets :: MonadCatch m
     => NonEmpty (Transform.Field m)
@@ -159,7 +161,7 @@ coerceResult :: (MonadCatch m, Serialize a)
     -> CollectErrsT m a
 coerceResult outputType result
     | Just serialized <- serialize outputType result = pure serialized
-    | otherwise = addErrMsg "Result coercion failed."
+    | otherwise = addError null $ Error "Result coercion failed." [] []
 
 -- | Takes an 'Out.ObjectType' and a list of 'Transform.Selection's and applies
 -- each field to each 'Transform.Selection'. Resolves into a value containing

src/Language/GraphQL/Execute/Internal.hs

@@ -3,23 +3,34 @@
    obtain one at https://mozilla.org/MPL/2.0/. -}
 
 {-# LANGUAGE DuplicateRecordFields #-}
+{-# LANGUAGE ExplicitForAll #-}
 {-# LANGUAGE NamedFieldPuns #-}
 
 module Language.GraphQL.Execute.Internal
     ( addError
+    , singleError
     ) where
 
 import Control.Monad.Trans.State (modify)
 import Control.Monad.Catch (MonadCatch)
 import Data.Sequence ((|>))
-import Language.GraphQL.Error
+import Data.Text (Text)
 import Language.GraphQL.Execute.Coerce
+import Language.GraphQL.Error
+    ( CollectErrsT
+    , Error(..)
+    , Resolution(..)
+    , Response(..)
+    )
 import Prelude hiding (null)
 
-addError :: (Serialize a, MonadCatch m) => Error -> CollectErrsT m a
-addError error' = modify appender >> pure null
+addError :: MonadCatch m => forall a. a -> Error -> CollectErrsT m a
+addError returnValue error' = modify appender >> pure returnValue
   where
    appender :: Resolution m -> Resolution m
    appender resolution@Resolution{ errors } = resolution
        { errors = errors |> error'
        }
+
+singleError :: Serialize b => forall a. Text -> Either a (Response b)
+singleError message = Right $ Response null $ pure $ Error message [] []

src/Language/GraphQL/Execute/Subscribe.hs

@@ -28,8 +28,6 @@ import qualified Language.GraphQL.Type as Type
 import qualified Language.GraphQL.Type.Out as Out
 import Language.GraphQL.Type.Schema
 
--- This is actually executeMutation, but we don't distinguish between queries
--- and mutations yet.
 subscribe :: (MonadCatch m, Serialize a)
     => HashMap Name (Type m)
     -> Out.ObjectType m