A simple workaround that *should* work: xs <- runDB $ selectList [UserNameEq x] [] 0 0 defaultLayout $ do setTitle $ string $ "homepage" addHamlet $(hamletFile "homepage") The rest of this email is a little technical: One of the changes in WAI 0.3 is that the application itself runs in the Iteratee monad instead of the IO monad. This works fine for the most part, since Iteratee is an instance of MonadIO. However, Iteratee is *not* an instance of MonadPeelIO. Persistent, on the other hand, *must* run in a MonadPeelIO since it uses MonadPeelIO's error handling code to ensure resource cleanup. Therefore, in Yesod 0.7, GHandler and GWidget were given more generic siblings, GGHandler and GGWidget (I'm so creative with naming, I know). These allow any arbitrary inner monad, including Iteratee and IO. When running Persistent code through runDB, the inner monad is IO, and the runDB lifts the inner IO into an Iteratee. Overall, this works without a hitch. The only downside is that you cannot call functions that require an Iteratee inner monad from inside Persistent calls anymore. The practical ramification is that you cannot parse POST parameters inside of Persistent (the Iteratee is how WAI passes in the request body). defaultLayout itself could in theory want to parse those parameters, and therefore its type signature is GGHandler Iteratee. Therefore, it cannot be lifted into a Persistent call. The workaround I gave simply returns the data from Persistent to outside of the runDB call. There shouldn't be any performance impact, it's merely a style change. Hope that was clear, Michael On Mon, Feb 21, 2011 at 2:29 AM, Katsutoshi Itoh wrote:

Hi,

i'm trying to convert my apps from yesod-0.6.7 to yesod-0.7.1.

If you do the following: How would I change?

on yesod-0.6.7.

RootR = do   :   :  runDB $ do    xs <- selectList [UserNameEq x] [] 0 0    lift $ defaultLayout $ do      setTitle $ string $ "homepage"      addHamlet $(hamletFile "homepage")

on yesod-0.7.1, an error occurred like as:

Couldn't match expected type `YesodDB                                                             MyApp (GGHandler MyApp MyApp IO) RepHtml              against inferred type `t (GGHandler                                                             sub                                                             master

(Data.Enumerator.Iteratee

Data.ByteString.Internal.ByteString IO)) RepHtml'     NB: `YesodDB' is a type function, and may nnot be injective  In the expression:           lift      $     defaultLayout           $ do {  setTitle                     $ ......

I guess convert `lift' to somewhat... would you teach me how to convert this?

_______________________________________________ web-devel mailing list web-devel@haskell.org http://www.haskell.org/mailman/listinfo/web-devel