[Haskell-cafe] JSON querying

[Oliver Charles]

On 02/17/2013 03:01 PM, Sergey Mironov wrote:
> Hi folks. Hackage contains several JSON packages but as far as I see,
> they all provide 'static' conversion from JSON format to Haskell data
> type. Is there a method of converting object containing optional filed
> 'a' to for example Maybe a.
Assuming you have some sort of 'path' to the key in question, aeson-lens 
might be exactly what you want: 
http://hackage.haskell.org/package/aeson-lens

I use aeson-lens to turn a list of strings of the form ["foo", "bar", 
"baz"] into a query into first the 'foo' object, then the 'bar' object, 
then the 'baz' object, using the following:

pathToLens :: Functor f
            => [T.Text]
            -> (Maybe Value -> f (Maybe Value))
            -> Maybe Value
            -> f (Maybe Value)
pathToLens [p] = key p
pathToLens ps = let ps' = filter (not . T.null) ps
                 in key (head ps') . (foldl (.) id . map pathElem $ tail 
ps')
   where
     pathElem p = maybe (key p) nth (readMay $ T.unpack p)

So as you can see - I turn any arbitrary path into a query into JSON, 
with a chance of failure.

Hope this helps!
- Ollie