BTW you probably want 'data Color = Color !Word8 !Word8 !Word8' On Fri, Sep 23, 2011 at 8:21 PM, Evan Laforge <qdun...@gmail.com> wrote: > On Fri, Sep 23, 2011 at 8:04 PM, Michael Craig <mks...@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 >
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