haskell - An easy way to change the type of Parsec user state? -

i'm looking easy way combine 2 parts of parsect code have same stream , monad, different user state , outcome. function nice:

withuserstate :: u -> parsect s u m -> parsect s v m 

the thing is, user state helpful in cases, need different states @ different times , don't want make states type bigger. have modify state somehow achieve this, or there function can't find @ moment?

edit:

i think alternative like

changeuserstate :: (u -> v) -> parsect s u m -> parsect s v m 

parsec doesn't let directly out of box, can achieve using parsec's public api follows:

{-# language scopedtypevariables #-}  import text.parsec  changestate   :: forall m s u v . (functor m, monad m)   => (u -> v)   -> (v -> u)   -> parsect s u m   -> parsect s v m changestate forward backward = mkpt . transform . runparsect       mapstate :: forall u v . (u -> v) -> state s u -> state s v     mapstate f st = st { stateuser = f (stateuser st) }      mapreply :: forall u v . (u -> v) -> reply s u -> reply s v     mapreply f (ok st err) = ok (mapstate f st) err     mapreply _ (error e) = error e      fmap3 = fmap . fmap . fmap      transform       :: (state s u -> m (consumed (m (reply s u a))))       -> (state s v -> m (consumed (m (reply s v a))))     transform p st = fmap3 (mapreply forward) (p (mapstate backward st)) 

note requires both forward , backward conversion between u , v. reason first need translate ambient state local state, run inner parser, convert back.

scopedtypevariables , local type signatures there clarity — feel free remove them if like.