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Today's Topics:
1. Re: Parsing 'A's and then ('A's or 'B's) (Ulrik Rasmussen)
----------------------------------------------------------------------
Message: 1
Date: Wed, 27 Jan 2016 13:34:29 +0100
From: Ulrik Rasmussen <hask...@utr.dk>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] Parsing 'A's and then ('A's or 'B's)
Message-ID: <56a8b955.5070...@utr.dk>
Content-Type: text/plain; charset="utf-8"; Format="flowed"
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:
>
> ?> "AABB" `represents` [x, y]
> True
> ?> "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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