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

[Michael Litchard]

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.
>