{-# LANGUAGE PatternSynonyms #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE ViewPatterns #-}

import Control.Exception
import System.IO.Unsafe

whnfEither :: Exception e => a -> Either e a
whnfEither x = unsafePerformIO $ try (evaluate x)

pattern IsException :: Exception e => e -> a
pattern IsException e <- (whnfEither -> Left e) where
  IsException e = throw e

pattern MkSomeException e = IsException (e :: SomeException)
pattern MkArithException e = IsException (e :: ArithException)

pattern Pure :: a -> a
pattern Pure x <- (whnfEither @SomeException -> Right x)

foo :: a -> String
foo (MkArithException e) = "ArithException: " ++ show e
foo (MkSomeException e) = show e
foo (Pure x) = "No exception thrown"