Hi, So far, I figured it out like this: updateState ... mbx <- (p3 >>= return . Just) <|> (return Nothing) updateState ... case mbx of Just x -> return x Nothing -> pzero but this seems a bit clumsy - is there a more elegant solution? ---------- Forwarded message ---------- From: Dimitry Golubovsky Date: Sat, May 31, 2008 at 10:12 AM Subject: Parsec, updateState, and failure To: haskell 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? Will it be state after the first updateState, or will failure of p1 roll the update back? Is there any bracket- or try-catch-finally-like mechanism for Parsec? Thanks. -- Dimitry Golubovsky Anywhere on the Web -- Dimitry Golubovsky Anywhere on the Web