.hs

rollDie :: State StdGen Int
rollDie = do 
            generator <- get
            let (value, newGenerator) = randomR (1,6) generator
            put newGenerator
            return value

rollDie = get >>= 
  -- pr@(St (\st -> (st, st))) >>= f
  -- -- St $ \st ->
  -- -- let (st, st) = runState pr st
  -- -- --             (\st -> (st, st)) st -- get's runState is \st -> (st, st)
  -- -- in runState (f st) st
  -- -- --          chunk below[1]...
  -- -- -- (\st -> (value, st)) st
--[1] f st
\generator ->
  let (value, newGenerator) = randomR (1,6) generator 
  in  put newGenerator >> return value
  -- --   St (\newGenerator -> ((), newGenerator) >> St (\st -> (value, st))
  -- -- -- _putState >>= \_ -> St (\st -> (value, st))
  -- -- -- -- St $ \st ->
  -- -- -- -- let (v, st') = runState _putState st -- Note: v = (), st' = st
  -- -- -- -- in runState (f v) st' -- v is ignored by the lambda f
  -- -- -- -- -- in runState (St (\st' -> (value, st'))) st
  -- -- -- -- -- -- in (value, st')
  -- -- -- -- St $ \st -> (value, st')
  -- Therefore, rollDie evaluates to St $ \st -> (value, st')


main = print $ runState (rollDie >> rollDie) getStdGen
  -- -- (rollDie >>= \_ -> rollDie)
  -- -- pr@(St $ \st -> (v1, st) >>= \_ -> rollDie
  -- -- -- St $ \st' ->
  -- -- -- let (v2, st'') = runState pr st'
  -- -- -- in runState rollDie st''
  -- -- -- -- in St $ (\st'' -> {- do stuff in \generator lambda -} (value, st''') st''