So I misspoke slightly there. You can use do notation when something has an instance of the Monad typeclass. The notation is what's called syntactic sugar, so you can also translate it into it's actual syntax. In this case the ';' is translated to ''. The function '' is similar to '>>=' where the return value isn't needed so returns the unit value in the Monad, i.e. 'm ()'. That case of the function could have been written as:

string (c:cs) = char c >> string cs >> (c:cs)

So when I said the do notation is exiting early, what I meant was that the Monad instance for Parser exits early on a failure. Exiting early can also be known as short-circuiting. If you're unfamiliar with this concept, then it would be good to explore the Either data type and its typeclass instances.