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

[Michael Litchard]

Specifically, I will be running this database query as one step of
many in the process of generating the menu in a selectField.

On Thu, Nov 3, 2011 at 4:57 PM, Michael Litchard <michael at schmong.org> wrote:
> Going over the Yesod Book again, I realize I had it right the first
> time. The database query needs to stay in the Handler. The question
> then becomes, how do I get data structures inside a form from a
> handler, when the only acceptable parameter seems to be an Html type?
>
> On Thu, Nov 3, 2011 at 4:52 PM, Michael Litchard <michael at schmong.org> wrote:
>> 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.
>>
>