His example is intentionally contrived to show a minimal example of StateT. This is who you'd really do it in Haskell:

for strings:

    main = repl ""

    repl :: String -> IO ()
    repl a = getLine >>= \c -> print (a++c) >> repl (a++c)

and it's not that much harder for Ints

    main = repl 0

    repl :: Int -> IO ()
    repl a = fmap read getLine >>= \b -> print (a+b) >> repl (a+b)

or using do notation

    repl a = do $
       b <- getLine 
       print sum
       repl sum
       where   
           sum = a + read b

which is still concise and easy to read.