type IOE a = IO a

IOE used to be a monad with extra error handling built on top of the IO monad,
But the IO monad already supports error handling, so this construction was a
superfluous.

The new 'instance ErrorMonad IOE' is defined to preserve the previous error
handling behaviour, i.e. the function 'handle' only catches errors thrown with
'raise' (or 'fail') and not other errors in the IO monad.

src/compiler/GF/CompileInParallel.hs

@@ -79,7 +79,7 @@ batchCompile1 lib_dir (opts,filepaths) =
      deps <- newMVar M.empty
      toLog <- newLog runIOE
      let --logStrLn = toLog . ePutStrLn
-         ok :: CollectOutput IOE a -> IO a
+       --ok :: CollectOutput IO a -> IO a
          ok (CO m) = err bad good =<< appIOE m
            where
               good (o,r) = do toLog o; return r