How can I convert this function from ExceptT to Except in Haskell?

Question

I got very confused with Except, specially since there is no good tutorial around the web. I don't know how can I convert this function from ExceptT to Except:

data Error = Empty deriving (Show)

badFunction :: ExceptT Error IO ()
badFunction = throwError Empty

main :: IO ()
main = do
    caught_result <- runExceptT badFunction
    case caught_result of
      Left _ -> putStrLn "caught some error"
      Right _ -> putStrLn "no errors were caught"

Answer 1

You write

badFunction :: ExceptT Error IO ()
badFunction = throwError Empty

which suggests that it does IO. But it doesn't do any IO. As Willem Van Onsem points out, one option is to indicate that by using Except instead:

badFunction :: Except Error ()

By the definition of Except, this is identical to

badFunction :: ExceptT Error Identity ()

There is a third option:

badFunction :: Monad m => ExceptT Error m ()

This gives you full flexibility!

runExcept badFunction :: Either Error ()
runExceptT badFunction :: Monad m => m (Either Error ())

So if you choose, you can write

main :: IO ()
main = do
    caught_result <- runExceptT badFunction
    case caught_result of
      Left _ -> putStrLn "caught some error"
      Right _ -> putStrLn "no errors were caught"

Whatever you think is clearer.