[Haskell-beginners] monad question

I have cap :: String -> String cap = toUpper rev :: String -> String rev = reverse then I make tupled :: String -> (String, String) tupled = do r <- rev c <- cap return (r, c) and to be honest, yes it’s been a long day at work, and this is coding at home rather than coding (java) at work but I’m not sure how tupled works!!! My first shot was supplying a param s like this tupled :: String -> (String, String) tupled s = do r <- rev s c <- cap s return (r, c) which doesn’t compile. But how does the first version work? How does the string to be processed get into the rev and cap functions?? Thanks Mike