[Haskell-cafe] Re: Problem with lazy IO

19 Jul 2009


      Maciej Piechotka  writes:
...
Hello.
I've tried to combine lazy IO and parsec. The hole process is done by
network.
Currently I have implemented 'short parsers' so I enter them on need. To
update state I have following code:
parser2nntp :: Monad m => NntpParser m a -> NntpT m a
parser2nntp p = do s <- NntpT (gets $ input . connection)
                   e <- runParserT (do v <- p
                                       i <- getInput
                                       return (v, i)) () "" s
                   case e of
                     Left er -> error $ show er
                     Right (v, i') -> (NntpT (modify (pNI i')) >> return
v)
                where pNI :: Monad m => ByteString ->NntpState m ->
NntpState m
                      pNI i s = s {connection = (connection s) {input =
i}}
However the 4 line (i <- getInput) blocks the execution as trace
indicated. String returned have no input available so it should block on
evaluation - and here I pass only a reference to it (or rather I think
so). What's wrong?
PS. Full code is in darsc nntp repository http://code.haskell.org/nntp/
- please note that it seems to require network compiled against parsec 3
- not 2.
This code seems to work:

parser2nntp :: Monad m => NntpParser m a -> NntpT m a
parser2nntp p = do s <- NntpT (gets $ input . connection)
                   r <- parserRep =<< runParsecT p (State s (initialPos "") ())
                   case r of
                     Ok v (State i _ _) _ -> NntpT (modify $ pNI i) >> return v
                     Error e -> error $ show e
                where parserRep (Consumed x) = x
                      parserRep (Empty x) = x
                      pNI i s = s {connection = (connection s) {input = i}}

Regards