The documentation for Data.Dynamic is pretty clear for this and I believe with GHC6 you can derive Typeable.
{-# NOINLINE myExceptionTyCon #-} -- the documentation suggests a fully qualified name myExceptionTyCon = mkTyCon "MyException"
instance Typeable MyException where typeOf _ = mkAppTy myExceptionTyCon []
I disagree that the docs are clear. I could see that I needed a value of type TypeRep (a datatype with no constructors - so I can't create one directly), and also the mkApplyTy, mkFunTy, and applyTy functions, but it wasn't clear how to use them to get a TypeRep (which one do I use? and how?). An example like this is more helpful. Is there somewhere else I could have read about this? (I took a quick look at the wiki first.) I should have realised I could derive Typeable with ghc6; there was an itch in the back of my brain (now I remember reading about deriving Typeable in ghc6) that was trying to tell me there was an easy way... This is what I have at present, but it still doesn't catch the exception:
module Main where import Prelude hiding (catch) import Control.Exception import Data.Dynamic
data MyException = MkExc Int String deriving Typeable
mkErr :: Int -> String -> MyException mkErr n s = MkExc n s
temp :: IO () temp = do putStrLn "line 1" throwIO (DynException (toDyn (mkErr 2 "my temp")))
handler :: Dynamic -> IO () handler e = do case (fromDynamic e) of Nothing -> putStrLn ("dynamic cast failed") Just (MkExc n s) -> putStrLn ("exception: " ++ (show n) ++ " " ++ s)
main :: IO () main = catchDyn temp handler
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