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

[Evan Laforge]

On Fri, Sep 23, 2011 at 8:04 PM, Michael Craig <mkscrg at gmail.com> wrote:
> 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