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

[Ulrik Rasmussen]

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 ()


Cheers,

Ulrik

On 2016-01-24 19:11, Simon Jakobi wrote:
> Hi!
>
> I want to test whether a sequence of the characters 'A' and 'B' can
> represent a sequence of the symbols x and y where x may be represented
> by one or more 'A's and y may be represented by one or more 'A's or one
> or more 'B's.
>
> In code, I would like to see the following:
>
> λ> "AABB" `represents` [x, y]
> True
> λ> "AA" `represents` [x, y]
> True
>
> But with my current implementation using attoparsec only the first
> example works as expected:
>
> import Control.Applicative
> import Data.Attoparsec.ByteString.Char8
> import Data.ByteString
> import Data.Either
> import Data.Foldable
> import Data.Word
>
> type Symbol = Parser [Word8]
>
> x :: Symbol
> x = many1 (char8 'A')
>
> y :: Symbol
> y = many1 (char8 'A') <|> many1 (char8 'B')
>
> represents :: ByteString -> [Symbol] -> Bool
> bs `represents` symbols =
>    isRight $ parseOnly ((sequenceA_ symbols) *> endOfInput) bs
>
>
> It seems that in
>     "AA" `represents` [x, y]
> x consumes all the 'A's, leaving none for y.
>
> Is it possible to solve this with attoparsec or are there other parsing
> libraries that I could use instead?
>
> Cheers,
> Simon
>
>
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