On Feb 2, 2008 5:28 PM, Antoine Latter <[EMAIL PROTECTED]> 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. -Antoine _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
