By coincidence, today I found a bug in the parsec 3.0.[01]. Probably due to changes introduced the bug is absent in parsec 3.1.0. I think it is worth to release 3.0.2 with this bug fixed. The bug itself is demonstrated in the following code. It gives Right (False,True) with parsec-3.0.x while should be giving Right (False,False). And, by the way, does parsec have any code repository and/or bug tracker?
import Text.Parsec hiding (parse) import Control.Monad.Reader
type Parser = ParsecT String () (Reader Bool)
change :: Parser a -> Parser a change p = local (const False) p
p = change $ do was <- ask anyChar now <- ask return (was,now)
parse :: Parser a -> SourceName -> String -> Either ParseError a parse p name s = runReader (runPT p () name s) True
main = print $ parse p "" "a"
* Derek Elkins
Changes: - the changes to the core of Parsec lead to some changes to when things get executed when it is used as a monad transformer "In the new version bind, return and mplus no longer run in the inner monad, so if the inner monad was side-effecting for these actions the behavior of existing code will change." - notFollowedBy p now behaves like notFollowedBy (try p) which changes the behavior slightly when p consumes input, though the behavior should be more natural now. - the set of names exported from Text.Parsec.Prim has changed somewhat
-- Roman I. Cheplyaka :: http://ro-che.info/ "Don't let school get in the way of your education." - Mark Twain