Select field populated from database

{-# LANGUAGE TypeFamilies, TemplateHaskell, MultiParamTypeClasses,
GADTs, QuasiQuotes, OverloadedStrings, FlexibleContexts #-}
import Yesod
import Database.Persist
import Database.Persist.Sqlite
import Control.Applicative ((<$>), (<*>))
import Data.Text

share [mkPersist sqlSettings, mkMigrate "migrateAll"] [persist|
Category
    name Text
    deriving Show
Product
    name Text
    category CategoryId Maybe
    deriving Show
|]

data App = App ConnectionPool

mkYesod "App" [parseRoutes|
/ HomeR GET POST
|]

instance Yesod App
instance YesodPersist App where
    type YesodPersistBackend App = SqlPersist
    runDB action = do
        App pool <- getYesod
        runSqlPool action pool

instance RenderMessage App FormMessage where
    renderMessage _ _ = defaultFormMessage

-- load Twitter Bootstrap styles
addStyle :: Widget
addStyle = addStylesheetRemote "http://netdna.bootstrapcdn.com/twitter-bootstrap/2.1.0/css/bootstrap-combined.min.css"

getHomeR :: Handler RepHtml
getHomeR = do
    rows <- productsAndCategories
    ((result, formWidget), enctype) <- runFormGet $ productForm Nothing

    defaultLayout $ do
        addStyle
        [whamlet|$newline never
<div .container>
    <div .row>
        <h2>
            Add new product
        <form method=post enctype=#{enctype}>
            ^{formWidget}
            <input type=submit .btn .btn-primary value="Save">
        <h2>
            Products
        <table .table>
            <tr>
                <th>
                    Product name
                <th>
                    Category
            $forall row <- rows
                <tr>
                    <td>
                        #{productName $ fst row}
                    <td>
                        $maybe category <- snd row
                            #{categoryName $ category}
|]

productsAndCategories :: GHandler App App [(Product, Maybe Category)]
productsAndCategories = runDB $ selectList [ProductName !=. ""] [Asc ProductName] >>= mapM (\(Entity _ p) -> do
    category <- case (productCategory p) of
        Just c -> do
            get c
        Nothing -> return Nothing
    return (p, category))                                       

postHomeR :: Handler RepHtml
postHomeR = do
    ((result, _), _) <- runFormPostNoToken $ productForm Nothing
    case result of 
        FormSuccess product -> do
            _ <- runDB $ insert product
            redirect HomeR
        _ -> do
            setMessage "Failure adding"
            redirect HomeR

productForm :: Maybe Product -> Html -> MForm App App (FormResult Product, Widget)
productForm mproduct = renderBootstrap $ Product
    <$> areq textField "Name" (productName <$> mproduct)
    <*> aopt (selectField categories) "Category" (productCategory <$> mproduct)
    where
        categories :: GHandler App App (OptionList CategoryId)
        categories = do
            entities <- runDB $ selectList [] [Asc CategoryName]
            optionsPairs $ Prelude.map (\cat -> (categoryName $ entityVal cat, entityKey cat)) entities

openConnectionCount :: Int
openConnectionCount = 10

main :: IO ()
main = withSqlitePool ":memory:" openConnectionCount $ \pool -> do
    flip runSqlPool pool $ do
        runMigration migrateAll

        -- add some example data
        home <- insert $ Category "Home, Garden & Tools"
        insert $ Product "Vinyl chair" (Just home)

        kitchen <- insert $ Category "Kitchen & Dining"
        insert $ Product "Coffeemaker" (Just kitchen)

        toys <- insert $ Category "Toys & Games"
        insert $ Product "Nerf Blaster" (Just toys)

        clothing <- insert $ Category "Clothing"
        insert $ Product "Urban Sprawl Print Hi-low Dress" (Just clothing)

        insert $ Product "Milkshake" Nothing

        return ()

    warpDebug 3000 $ App pool