Re: [Haskell-cafe] Re: Parsec, updateState, and failure

On Sat, May 31, 2008 at 12:12 PM, Achim Schneider wrote:

"Dimitry Golubovsky" wrote:

...

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

I have no idea, I gave up before investigating that far.

You want to avoid state at any cost, even more so if the state would influence parsing: spaghetti and headaches lay ahead. In the end I did three passes: two times parsec and one time something similar to accumMap.

If you really really wanted state that doesn't un-roll when parsing fails, this should work (but is untested):

type MyParser = ParsecT String () (State MyState)

runMyParser :: MyParser a -> String -> SourceName -> MyState -> (Either ParseError a, MyState)

runMyParser m in name state = flip runState state $ runPT m in name

You can then use 'get' and 'set'. But as you suggest, that way may be madness. -Antoine