Hack to get a nice looking checkbox field with renderBootstrap3

BootstrapCheckBoxFieldHack.hs

import           Control.Applicative   ((<$>), (<*>))
import           Data.Text             (Text)
import           Text.Blaze            (ToMarkup)
import           Yesod
import           Yesod.Form.Bootstrap3 (BootstrapFormLayout(..), renderBootstrap3, bfs)

bootstrapCheckBoxField :: (ToMarkup a, RenderMessage (HandlerSite m) FormMessage, Monad m) => a -> Field m Bool
bootstrapCheckBoxField label = checkBoxField
    { fieldView = \theId name attrs val _ -> [whamlet|\
        $newline never
        <div .checkbox style="margin-top: -20px; margin-bottom: 0">
            <label>
                <input id=#{theId} *{attrs} type="checkbox" name=#{name} value=yes :showVal id val:checked> #{label}
        |]
    }
  where
    showVal = either $ const False

-- Here's an example of using it in a form.  Note that you need to pass the label directly to the field
-- instead of the field settings.  An empty string needs to be passed for the field settings so the checkbox
-- can be repositioned over the label.

exampleForm = renderBootstrap3 BootstrapBasicForm $ (,)
    <$> areq textField (bfs ("Text Field"::Text)) Nothing
    <*> areq (bootstrapCheckBoxField ("Checked?"::Text)) "" Nothing