[Haskell-beginners] Parsing 'A's and then ('A's or 'B's)

[Francesco Ariis]

On Wed, Jan 27, 2016 at 01:34:29PM +0100, Ulrik Rasmussen wrote:
> The language is recognized by a relatively simple DFA (attached), so the
> simplest solution (I think) is to just encode that:
> 
>     module Main where
> 
>     import Text.Parsec
>     import Text.Parsec.String
> 
>     p :: Parser ()
>     p = char 'A' >> ((char 'A' >> sA) <|> (char 'B' >> sB))
>       where
>         sA = (char 'A' >> sA) <|> (char 'B' >> sB) <|> return ()
>         sB = (char 'B' >> sB) <|> return ()

I am probably missing something: say we have an "AAB" string, how does
this check that it is `compatible` with [x,y] or [x,x,y] or [x,y,y]
(or not compatible with [x,x,x], etc.)?