[Haskell-cafe] Idiomatic Aeson

[Rune Harder Bak]

Hi

I'm trying to parse a nested structure using Aeson (from Facebook)

It looks something like this:

{ "education": [
        {
            "school": {
                "name": "A"
            },
        },
        {
            "concentration": [
                {
                    "name": "B"
                }
        }
]}

And besides this there is a lot of fields I don't need. (And a lot of
other fields I do need)

What is the idiomatic aeson way of doing this?

I'm using Aeson because it's fast, and it seams to have all the
relevant combinators, just can't get them to play nicely together.
I was thinking something like
getInfo (Object m) = (,) <$> m .: "school" .: "name" <*> mapM (.:
"name")  (m .: "concentracion")
but of cause the types don't add up.

the verbose style like
getInfo (Object m) = do
 school <- m .: "school"
 schoolName <- case school of
  Object m' -> m' .: "name"
  l -> typeMismatch "schoolName" l
 concentration <- m .: "concentration"
 concentrationName <- case concentration of
  Array l -> forM (V.toList l) $ \m' ->
    case m' of
      Object m' -> m' .: "name"
      l -> typeMismatch "concentrationName" l
      )
   o -> typeMismatch "parseList" o
return (schoolName,concentrationName)
quickly explodes.

So I wrote my own functions of the following sort

parseName name _ (Object m) = m .: name
parseName _ err l = typeMismatch err l
parseList p (Array l) = mapM p $ V.toList l
parseList _ o = typeMismatch "parseList" o

so I can do something like
getInfo (Object m) = (,)
     <$> parseName "school "school" m >>= parseName "name" "schoolName"
     <*> parseName "concentration" "concentration" m >>= parseList
(parseName "name" "concentrationName")

And then variants for for .:?, but then it gets messy with >>= and maybe.
Getting rid of the m in the above I moved to arrows, and.. it seems
I'm not understanding the standard combinators.

So.. what is the idiomatic way of doing this in Aeson?

It shouldn't be necessary to move to template-haskell/generics to do this!

Thanks!