Re: [Haskell-cafe] Attoparsec: Limiting Parsers to N Bytes, or Combing Parsers?

I agree on all counts! The hex-handling logic here is so straightforward that it's hardly worth bothering with. In fact, my application's code as it stands looks very similar to what you wrote. I'm really asking because I want to be more "fluent" in attoparsec. So the question remains: is there a way to limit a parser to a finite chunk of input? Perhaps a way to run the 'take n' parser on the input and then run another parser on its result? This smells like monadic behavior, but of course with different semantics than the Monad instance for Parser. Mike S Craig (908) 328 8030 On Fri, Sep 23, 2011 at 11:23 PM, Evan Laforge wrote:

BTW you probably want 'data Color = Color !Word8 !Word8 !Word8'

On Fri, Sep 23, 2011 at 8:21 PM, Evan Laforge wrote:

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On Fri, Sep 23, 2011 at 8:04 PM, Michael Craig wrote:

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Suppose we want to parse a 24-bit hex color value: input :: ByteString input = "af093c blah blah blah" type Color = (Word8, Word8, Word8)

Attoparsec.Char8 exports a nice hexadecimal parser, but it consumes all available hex-flavored input. I'd like to make it consume exactly two bytes, so I could write my color parser like this:

color :: Parser Color color = do r <- hex2 g <- hex2 b <- hex2 return $ Color (r, g, b) hex2 :: Parser Word8 hex2 = ???

So my question is "how do I write hex2?" I could easily rewrite hexadecimal, but it would be nicer to reuse its hex-handling logic.

If it's easy enough to write inline, might as well do so. And it's fun with Applicative :)

hex2 = (+) <$> ((*16) <$> higit) <*> higit higit = subtract (fromEnum '0') <$> satisfy isHexDigit color = Color <$> hex2 <*> hex2 <*> hex2