Question

Let’s make the following assumptions:

• my program aborts due to an uncaught exception
• I have no idea what the type of that exception is
• the printed error message contains no hint on the exception type

How would I find out the type of that exception?

Minimal example:

main = error "foo"

(Here it’s of course ErrorCall, but you can’t tell from the error message.)

Answer 1

Yes. All Exception types must be instances of Typeable, assuming you use the new exceptions API.

import Control.Exception
import Data.Typeable
import Prelude hiding (catch)

realMain = error "example"
main = realMain `catch` h where
  h (SomeException e) = do
    putStrLn $ "Caught exception of type " ++ show (typeOf e)
    putStrLn $ show e

Results:

Caught exception of type GHC.Exception.ErrorCall
example