{-# LANGUAGE ExistentialQuantification #-}

{-# LANGUAGE GADTs #-}

module WhySomeException where

import Control.Exception (ArithException (..), AsyncException (..))

import Data.Typeable

data MyException = forall e . (Show e, Typeable e) => MyException e

instance Show MyException where

showsPrec p (MyException e) = showsPrec p e

multiError :: Int -> Either MyException Int

multiError n = case n of

0 -> Left (MyException DivideByZero)

1 -> Left (MyException StackOverflow)

_ -> Right n

data SomeError = Arith ArithException

| Async AsyncException

| SomethingElse

deriving Show

discriminateError :: MyException -> SomeError

discriminateError (MyException e) =

case cast e of

Just arith -> Arith arith

Nothing ->

case cast e of

Just async -> Async async

Nothing -> SomethingElse

runDisc :: Int -> SomeError

runDisc n = either discriminateError (const SomethingElse) (multiError n)