Lua.Util: avoid altering the stack if peeking fails

The stack now remains unaltered if `getRawInt` or `getTable` fail.  This
is important when those functions are used in an operation that is part
of an Alternative.

Change: minor

1 files changed, 12 insertions, 4 deletions

diff --git a/src/Text/Pandoc/Lua/Util.hs b/src/Text/Pandoc/Lua/Util.hs
index e688ad255..28d09d339 100644
--- a/src/Text/Pandoc/Lua/Util.hs
+++ b/src/Text/Pandoc/Lua/Util.hs
@@ -67,7 +67,7 @@ getTable :: (ToLuaStack a, FromLuaStack b) => StackIndex -> a -> Lua b
 getTable idx key = do
   push key
   rawget (idx `adjustIndexBy` 1)
-  peek (-1) <* pop 1
+  popValue
 
 -- | Add a key-value pair to the table at the top of the stack.
 addValue :: (ToLuaStack a, ToLuaStack b) => a -> b -> Lua ()
@@ -86,10 +86,9 @@ addFunction name fn = do
 
 -- | Get value behind key from table at given index.
 getRawInt :: FromLuaStack a => StackIndex -> Int -> Lua a
-getRawInt idx key =
+getRawInt idx key = do
   rawgeti idx key
-  *> peek (-1)
-  <* pop 1
+  popValue
 
 -- | Set numeric key/value in table at the given index
 setRawInt :: ToLuaStack a => StackIndex -> Int -> a -> Lua ()
@@ -106,6 +105,15 @@ raiseError e = do
   Lua.push e
   fromIntegral <$> Lua.lerror
 
+-- | Get, then pop the value at the top of the stack.
+popValue :: FromLuaStack a => Lua a
+popValue = do
+  resOrError <- Lua.peekEither (-1)
+  pop 1
+  case resOrError of
+    Left err -> Lua.throwLuaError err
+    Right x -> return x
+
 -- | Newtype wrapper intended to be used for optional Lua values. Nesting this
 -- type is strongly discouraged and will likely lead to a wrong result.
 newtype OrNil a = OrNil { toMaybe :: Maybe a }