[web-devel] [Yesod] Need help resolving this compiler error.

[Michael Litchard]

I'm doing a database action, inside a Form action. I had wanted to do
it inside a Handler, as I had figured out how to make it work. But I
could not figure out how to pass the data the Form needs from the
Handler. Here's the error, followed by the code I am trying to use.

>Handler/Manager.hs:68:16:
    No instance for (PersistBackend
                       (YesodPersistBackend master0) (GGHandler sub0
master0 IO)
                                                                )
  arising from a use of `isFree'
Possible fix:
  add an instance declaration for
  (PersistBackend
     (YesodPersistBackend master0) (GGHandler sub0 master0 IO))
In a stmt of a 'do' expression: testFree <- isFree testDay
In the expression:
  do { pInfo <- liftIO getUIdata;
       let products = V.toList $ V.map productACC $ fst pInfo
           versions = V.toList $ V.map versionsACC $ snd pInfo
           ....;
       testFree <- isFree testDay;
       (productRes, productView) <- mreq
                                      (radioField products)
"Placeholder" No
                                                              thing;
       .... }
In an equation for `productForm':
    productForm extra
      = do { pInfo <- liftIO getUIdata;
             let products = ...
                 ....;
             testFree <- isFree testDay;
             .... }


>Handler/Manager.hs:68:16:
Couldn't match expected type `Control.Monad.Trans.RWS.Lazy.RWST
                                (Maybe (Env, FileEnv), Scheduler,
[Yesod.For
                                                        m.Types.Lang])
                                Enctype
                                Ints
                                (GGHandler Scheduler Scheduler IO)
                                t0'
            with actual type `GGHandler sub0 master0 monad0 Bool'
In the return type of a call of `isFree'
In a stmt of a 'do' expression: testFree <- isFree testDay
In the expression:
  do { pInfo <- liftIO getUIdata;
       let products = V.toList $ V.map productACC $ fst pInfo
           versions = V.toList $ V.map versionsACC $ snd pInfo
           ....;
       testFree <- isFree testDay;
       (productRes, productView) <- mreq
                                      (radioField products)
"Placeholder" Nothing;

>postManagerR :: Handler RepHtml
postManagerR = do
    ((res, widget), enctype) <- runFormPost productForm
    dataInsert <- case (addEntry res) of
                     Left blank ->  blank
                     Right result -> result
--    let testDay = C.fromGregorian 2011 12 27 -- I would prefer to
have the code in the Handler if I could figure out how to get it to
the Form? State Monad?
--    testFree <- isFree testDay
        defaultLayout [whamlet|
    <p>Result:#{show dataInsert}
      <form enctype=#{enctype}>
          ^{widget}
    |]

>productForm :: Html
            -> Form Scheduler Scheduler (FormResult SelectedProduct, Widget)
productForm extra = do
   pInfo <- liftIO getUIdata
   let products = V.toList $ V.map productACC  $ fst pInfo
       versions = V.toList $ V.map versionsACC $ snd pInfo
       testDay = C.fromGregorian 2011 12 27
   testFree <- isFree testDay
   (productRes, productView) <- mreq (radioField products) "Placeholder" Nothing
   versionInfo <- mapM generateVersionSelectFields versions
--   (dateRes, dateView) <- mreq requestedDayField "Schedule" Nothing
   (dateRes, dateView) <- mreq (jqueryDayField def
                         { jdsChangeYear = True
                         , jdsYearRange = "2011:2012"
                         }) "Schedule" Nothing

   let versionRes = map fst versionInfo
       versionViews = map snd versionInfo
       widget = do
          toWidget [whamlet|
 #{extra}
 <p>
    ^{fvInput productView}
     $forall versionView <- versionViews
        ^{fvInput versionView}
    ^{fvInput dateView}
    <input type=submit value="Request Test">
 |]

   return (makeSelected productRes versionRes dateRes, widget)

>isFree day = do
   match <- runDB $ selectList [TestStartDate ==. day,
                        TestStatus !=. Passed,
                        TestStatus !=. Failed] []
   if (L.null match) then liftIOHandler (return True) else
liftIOHandler (return False)

I don't think the lack of an instance declaration is the real problem
but an indication I am just doing it wrong. What 'it' is, is a mystery
to me.

ghci tells me this

>isFree
      :: (YesodPersist master,
          PersistBackend
            (YesodPersistBackend master) (GGHandler sub master IO),
          Control.Monad.IO.Class.MonadIO monad) =>
         Day -> GGHandler sub master monad Bool

I believe I am using liftIOHandler correctly, I'm not sure what I am
doing wrong. What I am sure of is it probably has to do with not
understanding Yesod monads well enough. Feedback welcome.