Qnikst blog - Logging all exceptions in ghci

On one resource there was a question how to log all exception in GHCI. There was a use case where user could start a background processes and some exceptions could be lost.

In a normal program you can use -prof compilation option and run program with +RTS -xc in order to get exceptions messages and stack-traces logged. However I assume that most of the ghc users doesn’t have ghc compiled with profiling flags. So we need to find another option.

And there is a solution. Every forked thread installs a exceptions handler:

From GHC.Cons.Sym

forkIO :: IO () -> IO ThreadId
forkIO action = IO $ \ s ->
   case (fork# action_plus s) of (# s1, tid #) -> (# s1, ThreadId tid #)
    where
      action_plus = catchException action childHandler

From GHC.IO

{-
catchException used to handle the passing around of the state to the
action and the handler.  This turned out to be a bad idea - it meant
that we had to wrap both arguments in thunks so they could be entered
as normal (remember IO returns an unboxed pair...).

Now catch# has type

catch# :: IO a -> (b -> IO a) -> IO a

(well almost; the compiler doesn't know about the IO newtype so we
have to work around that in the definition of catchException below).
-}
catchException :: Exception e => IO a -> (e -> IO a) -> IO a

In order to set hander we can use:

setUncaughtExceptionHandler :: (SomeException -> IO ()) -> IO ()

In order to add something to ghci we can load a script by passing it with -ghci-script option

:m GHC.Conc.Sync Control.Exception

:{
let uncaughtExceptionHandler :: SomeException -> IO ()
    uncaughtExceptionHandler e = do
        putStrLn $ "Unhandled exception: " ++ show e
:}

setUncaughtExceptionHandler uncaughtExceptionHandler

:m -GHC.Conc.Sync Control.Exception

And load it with ghci -ghci-script script.hs.

qnikst@localhost ~ $ ghci -ghci-script script.hs 
GHCi, version 7.8.4: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Prelude> :m +Control.Exception Control.Concurrent
Prelude Control.Exception Control.Concurrent> forkIO $ threadDelay 10000 >> error "yo!"
ThreadId 28
Prelude Control.Exception Control.Concurrent> Unhandled exception: yo!

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