[Haskell-beginners] monad question

[mike h]

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