1 files changed, 13 insertions, 7 deletions
diff --git a/src/Text/Pandoc/Lua/Filter.hs b/src/Text/Pandoc/Lua/Filter.hs
index f6a0aea5b..e626356d5 100644
--- a/src/Text/Pandoc/Lua/Filter.hs
+++ b/src/Text/Pandoc/Lua/Filter.hs
@@ -18,14 +18,15 @@ module Text.Pandoc.Lua.Filter ( LuaFilterFunction
                               ) where
 import Control.Applicative ((<|>))
 import Control.Monad (mplus, (>=>))
-import Control.Monad.Catch (finally)
+import Control.Monad.Catch (finally, try)
 import Data.Data (Data, DataType, dataTypeConstrs, dataTypeName, dataTypeOf,
                   showConstr, toConstr, tyconUQname)
 import Data.Foldable (foldrM)
 import Data.Map (Map)
 import Data.Maybe (fromMaybe)
-import Foreign.Lua (Lua, Peekable, Pushable)
+import Foreign.Lua (Lua, Peekable, Pushable, StackIndex)
 import Text.Pandoc.Definition
+import Text.Pandoc.Error (PandocError)
 import Text.Pandoc.Lua.Marshaling ()
 import Text.Pandoc.Lua.Marshaling.List (List (..))
 import Text.Pandoc.Lua.Walk (SingletonsList (..))
@@ -102,7 +103,7 @@ elementOrList x = do
   if elementUnchanged
     then [x] <$ Lua.pop 1
     else do
-       mbres <- Lua.peekEither topOfStack
+       mbres <- peekEither topOfStack
        case mbres of
          Right res -> [res] <$ Lua.pop 1
          Left _    -> Lua.peekList topOfStack `finally` Lua.pop 1
@@ -234,11 +235,16 @@ singleElement x = do
   if elementUnchanged
     then x <$ Lua.pop 1
     else do
-    mbres <- Lua.peekEither (-1)
+    mbres <- peekEither (-1)
     case mbres of
       Right res -> res <$ Lua.pop 1
       Left err  -> do
         Lua.pop 1
-        Lua.throwException $
-          "Error while trying to get a filter's return " ++
-          "value from lua stack.\n" ++ err
+        Lua.throwMessage
+          ("Error while trying to get a filter's return " <>
+           "value from Lua stack.\n" <> show err)
+
+-- | Try to convert the value at the given stack index to a Haskell value.
+-- Returns @Left@ with an error message on failure.
+peekEither :: Peekable a => StackIndex -> Lua (Either PandocError a)
+peekEither = try . Lua.peek