On Feb 2, 2008 5:28 PM, Antoine Latter
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