On 31 May 2008, at 7:12 AM, Dimitry Golubovsky wrote:

Hi,

If a parser which updated user state fails, will such update be reverted?

Suppose we have two parsers combined with <|>

p = p1 <|> p2

p1 has the following:

p1 = try $ do ... -- getting something from the input stream updateState (\st -> ...) -- updated state based on what gotten from the input x <- p3 -- p3 should see updated state and return something updateState (\st -> ...) -- updated state again (if p3 succeeded) return x

If p3 fails, p1 fails too (second updateState will not be reached). But what will p2 (tried next) see in the user state?

The same thing p1 saw. You can see the implementation in http:// darcs.haskell.org/ghc-6.8/libraries/parsec/Text/ParserCombinators/ Parsec/Prim.hs; you want the parsecPlus function. Furthermore, you might note that the GenParser type is defined as newtype GenParser tok st a = Parser (State tok st -> Consumed (Reply tok st a)) runP (Parser p) = p data Consumed a = Consumed a --input is consumed | Empty !a --no input is consumed data Reply tok st a = Ok !a !(State tok st) ParseError -- parsing succeeded with "a" | Error ParseError -- parsing failed data State tok st = State { stateInput :: [tok] , statePos :: !SourcePos , stateUser :: !st } A parser that fails doesn't deliver a new state for parsecPlus to consider using going forward. jcc