Hi Paul, the expression (lift parse $ parseSyslog "" message) has the same meaning as (lift parse (parseSyslog "" message)), so you are indeed applying lift to two arguments, while it expects one. Probably you forgot the $ after lift? Best regards, Daniel Paul Sujkov schrieb:

Hi haskellers,

I have a few problems using monad transformers. I have such two functions:

parseSyslog :: StateT Integer Parser TimeStamp parseString :: StateT Integer Parser LogString

and the following code: parseString = do -- string parse here, all in the form of lift $ <parser> stamp <- lift $ lexeme parseTimestamp -- <?> "timestamp" message <- lift $ manyTill anyToken eof -- <?> "message" return (LogString <...parsed values here...> (check stamp console message) <...more parsed values here...>) where check :: (Maybe TimeStamp) -> Console -> String -> Maybe TimeStamp check Nothing Syslog message = case (lift parse $ parseSyslog "" message) of Left err -> Nothing Right res -> Just res <...other clauses here...>

this code seems quite intuitive to me, however it doesn't compile with a king error:

Couldn't match kind `(* -> *) -> * -> *' against `?? -> ? -> *' When matching the kinds of `t :: (* -> *) -> * -> *' and `(->) :: ?? -> ? -> *' Probable cause: `lift' is applied to too many arguments In the first argument of `($)', namely `lift parse'

I'm not so familiar with monad transformers whatsoever, so I'll be very happy if someone can show me the right way. The code compile nicely if I use "parse" line in a such way:

check Nothing Syslog message = case (parse (evalStateT parseSyslog 0) "" message) of

but this is not what I really want. To be accurate, here is the sequence which I do want to have in the code:

some user state is initialized; parseString gets called many times and changes the state via call to the parseSyslog (that is the only function that really uses/affects user state, everything else is pure Parsec code with it's own internal state). Two main problems that I have now is:

1) impossibility to use parse/parseTest functions with the (StateT <state type> Parser <parse type>) argument. I want it to be lifted somehow, but cannot see how 2) too many lifts in the code. I have only one function that really affects state, but code is filled with lifts from StateT to underlying Parser

Sorry if the questions are silly; any help is appreciated

-- Regards, Paul Sujkov ------------------------------------------------------------------------

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe