Yesod: Using GFormMonad without a type signature

formMonadWithoutSignature.hs

{-# LANGUAGE QuasiQuotes, OverloadedStrings, TemplateHaskell, TypeFamilies #-}
import Control.Applicative
import Yesod
import Yesod.Form.Core

data CommentForm = CommentForm 
    { user :: String 
    , comment :: Textarea 
    , isHtml :: Bool
    }

commentForm = do
    (user, userField) <- stringField "name:" Nothing
    (comment, commentField) <- textareaField "comment:" Nothing
    (isHtml , isHtmlField) <- boolField "html?" Nothing
    return (
        CommentForm <$> user <*> comment <*> isHtml
      , [$hamlet|
          %p
          ^fiInput.userField^
          %br ^fiInput.commentField^
          %br ^fiInput.isHtmlField^
        |])

commentForm2 params = CommentForm 
    <$> stringField "name:" Nothing
    <*> textareaField "comment:" Nothing
    <*> boolField "html?" Nothing

data Test = Test

mkYesod "Test" [parseRoutes|
/ RootR GET
|]

getRootR = do
  ((f,w),e) <- runFormMonadGet commentForm
  defaultLayout [$hamlet| 
    %form!method=GET 
      ^w^ 
      %input!type=submit
    |]

instance Yesod Test where approot _ = ""

main = basicHandler 3000 Test