[Haskell-cafe] parsec3 pre-release [attempt 2]

[Derek Elkins]

On Sat, 2008-02-02 at 20:43 -0600, Antoine Latter wrote:
> On Feb 2, 2008 5:28 PM, Antoine Latter <aslatter at gmail.com> wrote:
> > I'm not a fan of parameterizing the "Stream" class over the monad parameter `m':
> >
> > > class Stream s m t | s -> t where
> > >    uncons :: s -> m (Maybe (t,s))
> >
> > which leads to instance declarations like so:
> >
> > > instance Monad m => Stream [tok] m tok where
> > >     uncons []     = return $ Nothing
> > >     uncons (t:ts) = return $ Just (t,ts)
> >
> 
> To expand on this point, side-effect instances of Stream don't play
> nice with the backtracking in Text.Parsec.Prim.try:
> 
> > import Text.Parsec
> > import Text.Parsec.Prim
> > import System.IO
> > import Control.Monad
> 
> > type Parser a = (Stream s m Char) => ParsecT s u m a
> 
> This particular instance was suggested by Derek.
> 
> > instance Stream Handle IO Char where
> >    uncons hdl = do
> >        b <- hIsEOF hdl
> >        if b then return Nothing
> >             else liftM (\c -> Just (c,hdl)) getChar
> 
> > testParser :: Parser String
> > testParser = try (string "hello1") <|> string "hello2"
> 
> > test1 = runPT testParser () "stdin" stdin >>= print
> > test2 = hGetLine stdin >>= print . runP testParser () "stdin"
> 
> "test1" uses the  (Stream Handle IO Char) instance, "test2" uses the
> (Monad m => Stream [a] m a) instance.
> 
> For input "hello2", test2 produces a valid parse while test1 does not.

Note that instance has a typo in it (which I fixed before testing this
myself): getChar should be (hGetChar hdl) (though that makes no
difference when you pass in stdin)