[Haskell-cafe] Re: Problem with lazy IO

[Maciej Piechotka]

Maciej Piechotka <uzytkownik2 <at> gmail.com> 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