[Haskell-beginners] Value from Monad

Hi, Going through the '<-' description in 'learnouahaskell' it says - *" If we have a Maybe String and we bind it with <- to a variable, that variable will be a String"* so if you write do str <- Just "3" str should be of type 'String'. My question is how do I return "34" by concatenating it to "4" because the following line of code fails - --- do str <- Just "3" return str++"4"::Maybe String --- or for that matter how can I return/print just the value of str from this do block ? Thanks, Shishir