Passing Label Values to a Form as Text/String - haskell

Question

This is a follow-up to my question here: Extracting database field values inside a Handler
I would like to extract some information from the database and pass it in as a label value for a form. However, I get a type error.
A simple demonstration below (shell code from the Yesod Book):
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE QuasiQuotes #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeFamilies #-}
import Control.Applicative ((<$>), (<*>))
import Data.Text (Text)
import Data.Time (Day)
import Yesod
import Yesod.Form.Jquery
data App = App
mkYesod "App" [parseRoutes|
/ HomeR GET
|]
instance Yesod App
instance RenderMessage App FormMessage where
renderMessage _ _ = defaultFormMessage
instance YesodJquery App
data Person = Person
{ personName :: Text
}
deriving Show
personForm :: Text -> Html -> MForm Handler (FormResult Person, Widget)
personForm n1 = renderDivs $ Person
<$> areq textField n1 Nothing -- Changing n1 to "Name" works just fine.
getHomeR :: Handler Html
getHomeR = do
(widget, enctype) <- generateFormPost $ personForm "test"
defaultLayout
[whamlet|
<p>
The widget generated contains only the contents
of the form, not the form tag itself. So...
|]
main :: IO ()
main = warp 3000 App
When I run the program with runhaskell, I get the following error:
Couldn't match expected type `FieldSettings site0'
with actual type `Text'
In the second argument of `areq', namely `n1'
In the second argument of `(<$>)', namely
`areq textField n1 Nothing'
In the second argument of `($)', namely
`Person <$> areq textField n1 Nothing'
I also tried (FieldSettings n1 Nothing Nothing Nothing []) but no luck. Any thoughts on how to pass in label values to areq?

Answer 1

Let us look at the type of areq
areq :: (RenderMessage master msg, RenderMessage master FormMessage) => Field sub master a -> FieldSettings msg -> Maybe a -> AForm sub master a
So the areq function doesn't expect a Text value as its second parameter, it expects a FieldSettings msg. So what is the reason that it works when you write "Name" in your source code?
Looking up the documentation for FieldSettings in the documentation for yesod-forms we see that it is a datatype that has an IsString instance. Looking at the specific instance in the source code
we see that:
instance (a ~ Text) => IsString (FieldSettings a) where
fromString s = FieldSettings (fromString s) Nothing Nothing Nothing []
So everytime you write a String in your source code (if the OverloadedStrings extension is enabled), the compiler will insert the contents of the fromString instance.
However, you don't want to enter a String, but instead you want to create a FieldSettings from a Text value. Looking further we see that the first part of a FieldSettings is a SomeMessage, checking the documentation yet again and the source we see that a SomeMessage can be created by using the SomeMessage constructor.
personForm :: Text -> Html -> MForm Handler (FormResult Person, Widget)
personForm n1 = renderDivs $ Person
<$> areq textField (FieldSettings (SomeMessage n1) Nothing Nothing Nothing []) Nothing