explicit-rethrowing.hs

-- Error message with scoping annotation
newtype Error (s :: k) = Error Text
  deriving newtype IsString

-- Explicitly opt-in to bubbling up unhandled errors
rethrowError
  :: Functor f
  => ExceptT (Error s1) f a
  -> ExceptT (Error s2) f a
rethrowError = Except.withExceptT coerce

-- Handle errors (use instead of `catchError` from `MonadError`)
handleError
  :: Functor f
  => ExceptT (Error s1) f a
  -> (Error s -> ExceptT (Error s) f a)
  -> ExceptT (Error s2) f a
handleError = handleError . rethrowError

-- Example function which may throw
divide :: Monad m => Int -> Int -> ExceptT (Error "divide") m Int
divide x y = do
  when (y == 0) do
    Except.throwError "Cannot divide by zero"
  pure (x `div` y)

-- Example of explicitly opting-in to exception bubbling
-- (would not compile without wrapping in `rethrowError`)
showMyNumber :: Monad m => ExceptT (Error "showMyNumber") m String
showMyNumber = do
  number <- rethrowError (divide 42 0)
  pure (show number)

-- Example of being forced to handle errors if you opt not to bubble them up
-- (would not compile without `handleError`)
topLevel :: ExceptT (Error "topLevel") IO ()
topLevel = do
  string <- showMyNumber `handleError` \_ -> pure "handled failure"
  putStrLn string