In Haskell, is there a way to do IO in a function guard?

Question

For example:

newfile :: FilePath -> IO Bool
newfile x | length x <= 0 = return False
          | doesFileExist x == True = return False
          | otherwise = return True

Can this be made to work?

Answer 1

This works and does what's needed:

newfile :: FilePath -> IO Bool
newfile fn = do 
    x <- runErrorT $ do
        when ((length fn) <= 0) (throwError "Empty filename")
        dfe <- liftIO $ doesFileExist fn
        when (dfe) (throwError "File already exists")
        return True
    return $ either (\_ -> False) id x