[Haskell-cafe] Can't get my head round monad transformers

[Colin Adams]

Thanks. That was all I needed to sort it out.

2009/9/2 Martijn van Steenbergen <martijn at van.steenbergen.nl>:
> Hi Colin,
>
> Colin Adams wrote:
>>
>> I'm trying to add a state monad onto the IO monad for use in a
>> happstack application. I thought this should involve using StateT and
>> Happstack.Server.SimpleHTTP.simpleHTTP', but I can't figure out how to
>> plumb it all together. Any tips will be welcome.
>
> I'm not familiar with Happstack, but looking at the documentation:
>
>> simpleHTTP' :: (Monad m, ToMessage b) =>
>>  (m  (Maybe (Either Response a, FilterFun Response)) ->
>>   IO (Maybe (Either Response b, FilterFun Response))) ->
>>  Conf ->
>>  ServerPartT m a ->
>>  IO ()
>
> Do I understand correctly you will be working with a
>>
>> myAction :: ServerPartT (StateT S IO) A
>
> ?
>
> Then it seems that you may call simpleHTTP' like this:
>>
>> simpleHTTP'
>>  (\action -> evalStateT action initState)
>>  myConf
>>  myAction
>
> The first argument (the lambda) has type
>>
>> forall a m. Monad m => StateT S m a -> m a
>
> which can be specialised to what simpleHTTP' is expecting.
>
> I hope this helps!
>
> Martijn.
>



-- 
Colin Adams
Preston,
Lancashire,
ENGLAND