@hackage control-monad-exception0.4.5

This package provides explicitly typed, checked exceptions as a library. Example

data Expr = Add Expr Expr | Div Expr Expr | Val Double
eval (Val x)     = return x
eval (Add a1 a2) = do
   v1 <- eval a1
   v2 <- eval a2
   let sum = v1 + v2
   if sum < v1 || sum < v2 then throw SumOverflow else return sum
eval (Div a1 a2) = do
   v1 <- eval a1
   v2 <- eval a2
   if v2 == 0 then throw DivideByZero else return (v1 / v2)

data DivideByZero = DivideByZero deriving (Show, Typeable)
data SumOverflow  = SumOverflow  deriving (Show, Typeable)

instance Exception DivideByZero
instance Exception SumOverflow

GHCi infers the following types

eval :: (Throws DivideByZero l, Throws SumOverflow l) => Expr -> EM l Double
eval `catch` \ (e::DivideByZero) -> return (-1)  :: Throws SumOverflow l => Expr -> EM l Double
runEM(eval `catch` \ (e::SomeException) -> return (-1))  :: Expr -> Double

New in version 0.4:

• Support for unchecked exceptions (with UncaughtException)

• Support for exception stack traces (with WithSrcLoc). Example

      f () = $withLocTH $ throw MyException
      g a  = $withLocTH $ f a

      main = runEMT $ $withLocTH $ do
      g () `catchWithSrcLoc` \loc MyException ->
             lift $ putStrLn (showExceptionWithTrace loc MyException)

-- Running main produces the output:

      *Main> main
      MyException
       in Main(example.hs): (12,6)
          Main(example.hs): (11,7)