Update error messages for enum and union support

src/Language/GraphQL/Class.hs

@@ -322,7 +322,7 @@ deriveFromGraphQL typeName = do
         NewtypeD _ _ _ _ cons' _
             | RecC dataConName varBangTypes <- cons' ->
                 withRecordConstructor dataConName varBangTypes
-        _ -> error "Only records with a single data constructor are supported"
+        _ -> error "Only input objects and enums are supported if all member types have a FromGraphQL instance"
   where
     enumMemberPattern (NormalC normalName []) =
         let fromGraphQLF = conP (mkName "Type.Enum") [litP $ stringL $ nameBase normalName]
@@ -376,7 +376,7 @@ deriveToGraphQL typeName = do
         NewtypeD _ _ _ _ cons' _
             | RecC dataConName varBangTypes <- cons' ->
                 withRecordConstructor dataConName varBangTypes
-        _ -> error "Only records with a single data constructor are supported"
+        _ -> error "Only objects, unions and enums are supported if all member types have a ToGraphQL instance"
   where
     conTName = conT typeName
     collectEnumMemberNames (NormalC normalName []) = Just normalName
@@ -397,7 +397,7 @@ deriveToGraphQL typeName = do
             enumMemberPattern <$> enumMemberNames
         | Just unionMembers <- traverse collectUnionMembers cons' =
             unionMemberPattern <$> unionMembers
-        | otherwise =  error "Enum member should be a normal constructor without parameters"
+        | otherwise =  error "All data constructors should have either no parameters (Enum) or one parameter (Union)"
     withRecordConstructor dataConName varBangTypes = do
         fieldAliases <- traverse newFieldAliases varBangTypes
         let fBody =