Add exception handler newtype

[original issue 65 by @turion]

catchE and throwE make ExceptT into a monad in the exception type:

newtype HandleExcept a m e = HandleExcept { runHandleExcept :: ExceptT e m a }


instance Functor (HandleExcept a m) where

  fmap f = HandleExcept . withExceptT f . runHandleExcept

instance Monad (HandleExcept a m) where

  return = HandleExcept . throwE

  he >>= f = HandleExcept $ runHandleExcept he `catchE` (runHandleExcept . f)

try :: ExceptT e m a -> HandleExcept a m e

try = HandleExcept

safe :: m a -> HandleExcept a m Void

safe = HandleExcept . lift

safely :: HandleExcept a m Void -> m a

safely = fmap (fromLeft $ error "safely: Got Right value") . runExceptT . runHandleExcept

This monad interface is very useful. We can write programs like this:

finallyCalculateThatValue = safely $ do

  reasonItMightNotHaveWorked <- try $ calculateValue

  someBackupPlan <- handler reasonItMightNotHaveWorked

  safe $ thisWillAlwaysWork someBackupPlan

It will execute all the statements after each other until they throw exceptions (or succeed in returning a value). For example:

rightToMaybe :: ExceptT m a e -> m (Maybe a)

rightToMaybe mae = safely $ do

  e <- try mae

  safe $ return Nothing

HandleExcept is also a functor in the category of monads (http://hackage.haskell.org/package/mmorph-1.1.2/docs/Control-Monad-Morph.html#t:MFunctor).