apfelmus wrote:
Andrew Coppin wrote:
OK, I'm stuck now. :-S
1. How do I run the input through parseRLE and *then* through
parseHuffmanTable?
2. How do I get parseHuffmanPayload to continue from where parseRLE left
off? (How do I get parseRLE to not parse the entire input, for that
matter...)
Well, there's no way to do that with a monolithic parseRLE since it will
parse the input to the bitter end. But assuming that
parseRLE = concat `liftM` many parseRLEBlock
parseRLEBlock :: Parser Word8 [Word8]
you can write an RLE parser that repeatedly parses RLE blocks until it
has read equal or more than n Word8s
parseRLEAmount n
| n <= 0 = return []
| otherwise = do
xs <- parseRLEBlock
xss <- parseRLEAmount (n - length xs)
return (xs ++ xss)
To parse a huffman table, run the actual parser on the result of
parseRLEAmount:
parseHuffmanHeader =
runParser parseHuffmanTable `liftM` parseRLEAmount (2^8)
The use of lifeM is *inspired* - I would never have thought of writing
it infix!
Still, while this does solve the issue at hand, I'm still left wondering
whether there's any way to make this work for some arbitrary parser.
parseHuffmanTable always uses exactly N bytes of input, but what if it
didn't? What if it was impossible to tell how many bytes it should read
without actually running it? How would you implement that? Nice a since
general solution exist?
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