Error.hs

{-|
Module      : Foreign.Lua.Raw.Error
Copyright   : © 2017-2020 Albert Krewinkel
License     : MIT
Maintainer  : Albert Krewinkel <tarleb+hslua@zeitkraut.de>
Stability   : beta
Portability : portable

Lua exceptions and exception handling.
-}
module Foreign.Lua.Raw.Error
  ( -- * Passing Lua errors to Haskell
    errorMessage
  ) where

import Data.ByteString (ByteString)
import Foreign.Lua.Raw.Auxiliary (hsluaL_tolstring)
import Foreign.Lua.Raw.Functions (lua_pop)
import Foreign.Marshal.Alloc (alloca)
import Foreign.Ptr (nullPtr)

import qualified Data.ByteString as B
import qualified Data.ByteString.Char8 as Char8
import qualified Foreign.Storable as Storable
import qualified Foreign.Lua.Raw.Types as Lua

-- | Retrieve and pop the top object as an error message. This is very similar
-- to tostring', but ensures that we don't recurse if getting the message
-- failed.
errorMessage :: Lua.State -> IO ByteString
errorMessage l = alloca $ \lenPtr -> do
  cstr <- hsluaL_tolstring l (-1) lenPtr
  if cstr == nullPtr
    then return $ Char8.pack ("An error occurred, but the error object " ++
                              "cannot be converted into a string.")
    else do
      cstrLen <- Storable.peek lenPtr
      msg <- B.packCStringLen (cstr, fromIntegral cstrLen)
      lua_pop l 2
      return msg