[Haskell-cafe] [Haskell Cafe] Troubles with StateT and Parsec

[Daniel van den Eijkel]

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
> ------------------------------------------------------------------------
>
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