Question

I get an error which I can’t resolve.
The snap application compiles without a problem and everything seems to be ok.
But when I render the relevant page in a browser I get this error:

A web handler threw an exception. Details:
expected (“code” :: Integer) in [ _id: 50b56f19208c2e9a09dccc2b, id: 1.0, code: “hdg435”, name: “froggy”]

The code value is just a rendom string I picked for testing. I am not sure why an integer is expected?

These are the relevant parts of an example snap application.

getData :: IO [Document]
getData = do
  pipe <- runIOE $ connect $ host "127.0.0.1"
  let run act = access pipe master "test" act
  result <- run (find (select [] "pcs") >>= rest)
  close pipe
  return $ either (const []) id result

mkSplice :: Document -> Splice AppHandler
mkSplice d = runChildrenWithText [dtp "id" d
                                 ,dtp "code" d
                                 ,dtp "name" d
                                 ]


dtp :: Text -> Document -> (Text,Text)
dtp tag d = (tag, T.pack $ show $ at tag d)

recSplice :: Splice AppHandler
recSplice = mapSplices mkSplice =<< liftIO getData

table :: Handler App App ()
table = heistLocal (bindSplice "rec" recSplice) $ render "table"

The relevant Heist template part of table.tpl is here:

<table>
  <tbody>
    <rec>
      <tr><td><id/></td><td><code/></td><td><name/></td></tr>
    </rec>
  </tbody>
</table>

Please let me know what other parts of code need to be posted.

Answer 1

When I compile your dtp function I get:

import Data.Bson
import Data.Text (Text)
import qualified Data.Text as T

dtp :: Text -> Document -> (Text,Text)
dtp tag d = (tag, T.pack $ show $ at tag d)

Ambiguous type variable `a0' in the constraints:
  (Show a0)
  [...]

which makes perfect sense. It seems like in your case it’s defaulting to Integer when you really want String. You can try adding a signature or better yet, just:

dtp tag d = (tag, at tag d)

If you want this to work for other types, you’ll have to work harder.

UPDATE

Here’s a GHCi session that illustrates the problem and how GHCi seems to default the Show instance to Integer:

Prelude Data.Bson> show $ at "hello" ["hello" =: "world"]
"*** Exception: expected ("hello" :: Integer) in [ hello: "world"]