[Haskell-cafe] Parsec and network data

John Van Enk vanenkj at gmail.com
Tue Aug 26 16:38:40 EDT 2008

Are you doing this all in a single thread?

On Tue, Aug 26, 2008 at 4:35 PM, brian <brianchina60221 at gmail.com> wrote:

> Hi, I've been struggling with this problem for days and I'm dying. Please
> help.
> I want to use Parsec to parse NNTP data coming to me from a handle I
> get from connectTo.
> One unworkable approach I tried is to get a lazy String from the
> handle with hGetContents. The problem: suppose the first message from
> the NNTP server is "200 OK\r\n". Parsec parses it beautifully. Now I
> need to discard the parsed part so that Parsec will parse whatever the
> server sends next, so I use Parsec's getInput to get the remaining
> data. But there isn't any, so it blocks. Deadlock: the client is
> inappropriately waiting for server data and the server is waiting for
> my first command.
> Another approach that doesn't quite work is to create an instance of
> Parsec's Stream with timeout functionality:
> instance Stream Handle IO Char where
>  uncons h =
>    do r <- hWaitForInput h ms
>       if r
>         then liftM (\c -> Just (c, h)) (hGetChar h)
>         else return Nothing
>    where ms = 5000
> It's probably obvious to you why it doesn't work, but it wasn't to me
> at first. The problem: suppose you tell parsec you're looking for
> (many digit) followed by (string "\r\n"). "123\r\n" won't match;
> "123\n" will. My Stream has no backtracking. Even if you don't need
> 'try', it won't work for even basic stuff.
> Here's another way:
> http://www.mail-archive.com/haskell-cafe@haskell.org/msg22385.html
> The OP had the same problem I did, so he made a variant of
> hGetContents with timeout support. The problem: he used something from
> unsafe*. I came to Haskell for rigor and reliability and it would make
> me really sad to have to use a function with 'unsafe' in its name that
> has a lot of wacky caveats about inlining, etc.
> In that same thread, Bulat says a timeout-enabled Stream could help.
> But I can't tell what library that is. 'cabal list stream' shows me 3
> libraries none of which seems to be the one in question. Is Streams a
> going concern? Should I be checking that out?
> I'm not doing anything with hGetLine because 1) there's no way to
> specify a maximum number of characters to read 2) what is meant by a
> "line" is not specified 3) there is no way to tell if it read a line
> or just got to the end of the data. Even using something like hGetLine
> that worked better would make the parsing more obscure.
> Thank you very very much for *any* help.
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