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

[Paulo J. Matos]

On Dec 23, 2007 12:44 PM, Isaac Dupree <isaacdupree at charter.net> wrote:
>
> -- 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

Oh gee, I just noticed that my type sig is in fact not correct. How
come GHC doesn't complain?

> parseHeader3 bs = do
>    (x, rest) <- BS.readInt $ BS.dropWhile (not . isDigit) bs
>    (y, _) <- BS.readInt $ BS.dropWhile (not . isDigit) rest
>    return (x, y)

What happens then if the first BS.readInt return Nothing???

> --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 <pocm at soton.ac.uk> 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
> >>
> >
> >
> >
>
>
>
>



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