Can't generate random numbers

I can't figure out how to generate random number.

Here is how I'm trying to do according to this:

main :: IO ()
main = do
  let
     g <- getStdGen
     bla = randomRs (0, 360) g

Also tried:

main :: IO ()
main = do
  let
    r <- randomIO

then:

parse error on input `<-' Perhaps this statement should be within a 'do' block?