Question

I’m trying to write a form that allows users to upload a file to my server.
I saw Yesod uses fileAFormReq for such a functionality but was unable to get it to work, running into compilation errors, the latest of which was:
“No instance for (RenderMessage MySite t)”
Any stripped down example for how to use it will be highly appreciated.
Thanks,
Uri

Answer 1

Update 13-Sep-2012:

There is an official maintained help page for file uploading here

---

Googling on the function fileAFormReq gave me this example.

I Made a minimal version of it with only the relevant parts.

{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE QuasiQuotes, TypeFamilies, TemplateHaskell, MultiParamTypeClasses #-}
import Yesod.Core
import Yesod.Form
import Yesod.Form.MassInput
import Control.Applicative
import Data.Text (Text, pack)
import Network.Wai.Handler.Warp (run)
import Data.Time (utctDay, getCurrentTime)
import qualified Data.Text as T
import Control.Monad.IO.Class (liftIO)

mkYesod "HelloForms" [parseRoutes|
/file FileR GET POST
|]

data HelloForms = HelloForms

instance RenderMessage HelloForms FormMessage where
    renderMessage _ _ = defaultFormMessage

instance Yesod HelloForms where
    approot _ = ""


main = toWaiApp HelloForms >>= run 3000

fileForm = renderTable $ pure (,)
    <*> fileAFormReq "Required file"
    <*> fileAFormOpt "Optional file"

getFileR = do
    ((res, form), enctype) <- runFormPost fileForm
    defaultLayout [whamlet|
<p>Result: #{show res}
<form method=post enctype=#{enctype}>
    <table>
        ^{form}
    <tr>
        <td>
            <input type=submit>
|]

postFileR = getFileR

runhaskell this and then visit http://localhost:3000/file in your browser.

Hope this helps. 🙂

---

Edit:

Oh wait, it’s obvious what you’re missing. As the compilation error just said, you’re missing a RenderMessage instance for your Foundation.

I know recent efforts for i18n have changed the forms-package slightly. If you’re using the latest version of yesod, check this out.

The code I pasted uses the old non-i18n (default means english) version of yesod-forms package.