Re: [Haskell-cafe] Re: Comments on reading two ints off Bytestring

-- this should work too parseHeader3 :: BS.ByteString -> Maybe (Int, Int) --note accurate type signature, which helps us use Maybe failure-monad, --although losing your separate error messages parseHeader3 bs = do (x, rest) <- BS.readInt $ BS.dropWhile (not . isDigit) bs (y, _) <- BS.readInt $ BS.dropWhile (not . isDigit) rest return (x, y) --or to be clearer without syntactic sugar, that is parseHeader3 bs = (BS.readInt $ BS.dropWhile (not . isDigit) bs) >>= \(x, rest) -> (BS.readInt $ BS.dropWhile (not . isDigit) rest) >>= \(y, _) -> return (x, y) Isaac Paulo J. Matos wrote:

On Dec 23, 2007 12:32 PM, Paulo J. Matos wrote:

...

Hello all,

It is either too difficult to get two integers of a bytestring, in which case something should be done to ease the process or I should learn much more Haskell. I guess the latter is the correct guess.

I have a bytestring containing two naturals. I was to get them as efficiently as possible. Here's my code:

Just tried a better one, what do you think of this: parseHeader2 :: BS.ByteString -> (Int, Int) parseHeader2 bs = case (BS.readInt $ BS.dropWhile (not . isDigit) bs) of Nothing -> error "Couldn't find first natural." Just (x, rest) -> case (BS.readInt $ BS.dropWhile (not . isDigit) rest) of Nothing -> error "Couldn't find second natural." Just (y, _) -> (x, y)

...

parseHeader :: BS.ByteString -> (Int, Int) parseHeader bs = let first = BS.readInt $ BS.dropWhile (not . isDigit) bs in if(isNothing first) then error "Couldn't find first natural." else let second = BS.readInt $ BS.dropWhile (not . isDigit) $ snd $ fromJust first in if(isNothing second) then error "Couldn't find second natural." else (fst $ fromJust first, fst $ fromJust second)

This seems to work:

...

parseHeader $ BS.pack "hello 252 359" (252,359)

Is there a better way?

Cheers,

-- Paulo Jorge Matos - pocm at soton.ac.uk http://www.personal.soton.ac.uk/pocm PhD Student @ ECS University of Southampton, UK