New combinator "fixed :: Int -> Parser a -> Parser a"

[merijn]

I frequently find myself wanting/needing to parse complex fields of a fixed size, which is currently rather tricky using attoparsec. For example a 20 byte field consisting of a zero padded ASCII string. This field can be easily described as manyTill anyWord8 (word8 0) <* many (word8 0), but taking care of the size limiting is non-trivial.

I would like to have a combinator as the one proposed in the issue name, with the semantics that it tries to match it's argument parser completely against the next N bytes. An example implementation (although probably rather inefficient) would be:

fixed :: Int -> Parser a -> Parser a
fixed i p = do
    intermediate <- take i
    case parseOnly (p <* endOfInput) intermediate of
        Left _ -> empty
        Right x -> return x

[bos]

Your implementation seems reasonable, but I haven't seen any other requests for this, so I'm not inclined to add it in response to a once-off request. I'll reconsider if someone else shows up. Thanks.