In case of exception, report the offending function

src/runtime/haskell/PGF2/FFI.hsc

@@ -219,12 +219,15 @@ utf8Length s = count 0 s
 -----------------------------------------------------------------------
 -- Exceptions
 
-newtype PGFError = PGFError String
-     deriving (Show, Typeable)
+data PGFError = PGFError String String
+     deriving Typeable
+
+instance Show PGFError where
+  show (PGFError loc msg) = loc++": "++msg
 
 instance Exception PGFError
 
-withPgfExn f =
+withPgfExn loc f =
   allocaBytes (#size PgfExn) $ \c_exn -> do
     res <- f c_exn
     ex_type <- (#peek PgfExn, type) c_exn :: IO (#type PgfExnType)
@@ -236,13 +239,13 @@ withPgfExn f =
          mb_fpath <- if c_msg == nullPtr
                        then return Nothing
                        else fmap Just (peekCString c_msg)
-         ioError (errnoToIOError "readPGF" (Errno errno) Nothing mb_fpath)
+         ioError (errnoToIOError loc (Errno errno) Nothing mb_fpath)
       (#const PGF_EXN_PGF_ERROR) -> do
          c_msg <- (#peek PgfExn, msg) c_exn
          msg <- peekCString c_msg
          free c_msg
-         throwIO (PGFError msg)
-      _ -> throwIO (PGFError "An unidentified error occurred")
+         throwIO (PGFError loc msg)
+      _ -> throwIO (PGFError loc "An unidentified error occurred")
 
 -----------------------------------------------------------------------
 -- Marshalling

src/runtime/haskell/PGF2/Transactions.hsc

@@ -75,7 +75,7 @@ branchPGF_ :: Ptr PgfText -> PGF -> Transaction a -> IO PGF
 branchPGF_ c_name p (Transaction f) =
   withForeignPtr (a_db p) $ \c_db ->
   withForeignPtr (revision p) $ \c_revision ->
-  withPgfExn $ \c_exn ->
+  withPgfExn "branchPGF" $ \c_exn ->
   mask $ \restore -> do
     c_revision <- pgf_clone_revision c_db c_revision c_name c_exn
     ex_type <- (#peek PgfExn, type) c_exn
@@ -103,7 +103,7 @@ checkoutPGF :: PGF -> String -> IO (Maybe PGF)
 checkoutPGF p name =
   withForeignPtr (a_db p) $ \c_db ->
   withText name $ \c_name -> do
-    c_revision <- withPgfExn (pgf_checkout_revision c_db c_name)
+    c_revision <- withPgfExn "checkoutPGF" (pgf_checkout_revision c_db c_name)
     if c_revision == nullPtr
       then return Nothing
       else do fptr2 <- C.newForeignPtr c_revision (withForeignPtr (a_db p) (\c_db -> pgf_free_revision c_db c_revision))