On Wed, Feb 23, 2011 at 6:30 AM,
I'm working through the "Write Yourself a Scheme" wikibook and having trouble with one of the exercises. For the function parseNumber :: Parser LispVal, both
parseNumber = liftM (Number . read) $ many1 digit parseNumber' = do digits <- many1 digit return $ (Number . read) digits
work. But
parseNumber'' = many1 digit >>= liftM read >>= liftM Number
You misunderstand liftM : liftM takes an ordinary function (a -> b) and "lift" it so that it acts inside the monad and become (m a -> m b) thus the parameter of "liftM f" must be a monadic action but through (>>=) you feed it an ordinary value since (>>=) bind a monadic action (m a) to a function that takes an ordinary value and return an action (a -> m b). You used liftM as if it was (return .) The correct usage would have been :
parseNumber'' = liftM (Number . read) (many1 digit)
like your parseNumber or (using Applicative if you wish to do so) :
parseNumber'' = Number . read <$> many1 digit
-- Jedaï