[Haskell-cafe] Problems with GHC API and error handling

Daniel F difrumin at gmail.com
Sat Jun 15 11:26:54 CEST 2013

Hello, everyone.

I am in need of setting up custom exception handlers when using GHC
API to compile modules. Right now I have the following piece of code:

* Main.hs:
import GHC
import GHC.Paths
import MonadUtils
import Exception
import Panic
import Unsafe.Coerce
import System.IO.Unsafe

handleException :: (ExceptionMonad m, MonadIO m)
                   => m a -> m (Either String a)
handleException m =
  ghandle (\(ex :: SomeException) -> return (Left (show ex))) $
  handleGhcException (\ge -> return (Left (showGhcException ge ""))) $
  flip gfinally (liftIO restoreHandlers) $
  m >>= return . Right

initGhc :: Ghc ()
initGhc = do
  dfs <- getSessionDynFlags
  setSessionDynFlags $ dfs { hscTarget = HscInterpreted
                           , ghcLink = LinkInMemory }
  return ()

test :: IO (Either String Int)
test = handleException $ runGhc (Just libdir) $ do
  setTargets =<< sequence [ guessTarget "./test/file1.hs" Nothing ]
  graph <- depanal [] False
  loaded <- load LoadAllTargets
  -- when (failed loaded) $ throw LoadingException
  setContext (map (IIModule . moduleName . ms_mod) graph)
  let expr = "main"
  ty <- exprType expr -- throws exception if doesn't typecheck
  output ty
  res <- unsafePerformIO . unsafeCoerce <$> compileExpr expr
  return res


* file1.hs:

module Main where

main = do
  return x


The problem is when I run the 'test' function above I receive the
following output:

h> test

test/file1.hs:4:10: Not in scope: `x'

Left "Cannot add module Main to context: not a home module"
it :: Either String Int

So, if I understand this correctly, my exception handler does indeed
catch an exception correctly,
however, I still receive some output which I want to be captured.
Is there a way to do this?

Sincerely yours,
-- Daniil Frumin

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