Yesod Subsites and Forms

Question

I'm working on a Yesod subsite. Basically, it is a blog. I am having problems attaching forms to the handlers. Consider:

getSubBlogR :: Yesod master
            => YesodPersist master
            => PersistQuery (YesodPersistBackend master (HandlerT master IO))
            => RenderMessage master FormMessage
            => HandlerT Blog (HandlerT master IO) Html
getSubBlogR = lift $ do
  articles              <- runDB $ selectList [] [Asc ArticleDate]
  day                   <- liftIO $ (utctDay <$> getCurrentTime)
  (formWidget, enctype) <- generateFormPost $ (articleForm day)

  defaultLayout $ [whamlet|
    <div .articles>
      $forall Entity _ article <- articles    
        ^{articleWidget article}     
  |]

As it stands, this does compile. But I'm not actually using the formWidget, and I would really like to. I'd like something "like"

getSubBlogR :: Yesod master
            => YesodPersist master
            => PersistQuery (YesodPersistBackend master (HandlerT master IO))
            => RenderMessage master FormMessage
            => HandlerT Blog (HandlerT master IO) Html
getSubBlogR = lift $ do
  articles              <- runDB $ selectList [] [Asc ArticleDate]
  day                   <- liftIO $ (utctDay <$> getCurrentTime)
  (formWidget, enctype) <- generateFormPost $ (articleForm day)

  defaultLayout $ [whamlet|
    <div .articles>
      $forall Entity _ article <- articles    
        ^{articleWidget article}
      <div .panel .panel-default>
        <div .panel-heading><h1>Add Article
        <div .panel-body>
          <form method="post" action=@{SubBlogR} enctype=#{enctype}>
            ^{formWidget}
  |]

But this does not compile. I get the error:

src/Yesod/Blog/Handler.hs:64:28:
    Could not deduce (master ~ Blog)
    from the context (Yesod master,
                      YesodPersist master,
                      PersistQuery (YesodPersistBackend master (HandlerT master IO)),
                      RenderMessage master FormMessage)
      bound by the type signature for
                 getSubBlogR
...
  Expected type: WidgetT
                 master IO (Route Blog -> [(Text, Text)] -> Text)
  Actual type: WidgetT
                 master
                 IO
                 (Route (HandlerSite (WidgetT master IO)) -> [(Text, Text)] -> Text)

Okay, fair enough. I understand that 'master' and 'Blog' are not the same types. But how do I get "the diagram" to commute?

Answer 1

Be sure to add type annotations to all your forms! This is why Michael Snoyman asked for the type of articleForm. My subsite refused to typecheck with similar errors because I didn't annotate this function:

simpleSourceForm = DataSourceInput
  <$> areq textField "Name" Nothing
  <*> areq intField "Start" Nothing
  <*> areq intField "End" Nothing

That gave me errors such as these:

Yesod\DataSource.hs:58:36:
Couldn't match type `m0' with `HandlerT m IO'
  because type variable `m' would escape its scope
This (rigid, skolem) type variable is bound by
  the type signature for
    postDataSourceInputR :: YesodDataSource m =>
                            HandlerT DataSource (HandlerT m IO) Html

Yesod\DataSource.hs:49:7:
No instance for (RenderMessage (HandlerSite m0) FormMessage)
  arising from a use of `areq'
In the second argument of `(<$>)', namely
  `areq textField "Name" Nothing'
In the first argument of `(<*>)', namely
  `DataSourceInput <$> areq textField "Name" Nothing'
In the first argument of `(<*>)', namely
  `DataSourceInput <$> areq textField "Name" Nothing
   <*> areq intField "Start" Nothing'

Yesod\DataSource.hs:30:22:
Could not deduce (m ~ HandlerSite m0)
from the context (YesodDataSource m)
  bound by the type signature for
             getDataSourceInputR :: YesodDataSource m =>
                                    HandlerT DataSource (HandlerT m IO) Html

Annotating the function fixed everything:

simpleSourceForm :: YesodDataSource m => AForm (HandlerT m IO) DataSourceInput
simpleSourceForm = DataSourceInput
  <$> areq textField "Name" Nothing
  <*> areq intField "Start" Nothing
  <*> areq intField "End" Nothing

(I'll also include the YesodDataSource typeclass for informational purposes)

class (RenderMessage master FormMessage, Yesod master) => YesodDataSource master