Parsing json with lens-aeson

Question

Trying out Edward Kmett's mind expanding lens and lens-aeson package. Here's a warmup

s = "{ \"somekey\" : [ { \"deeperkey\" : 1} , {\"deeperkey\": 2, \"otherkey\":3}]}

The desired result is

[(1, Nothing), (2, Just 3)]

I can make a little progress with

import Control.Lens.Aeson
import Control.Lens
import Control.Monad
import qualified Data.Vector as V

λ> s ^. key "somekey" . _Array . to V.toList >>= (^.. key "deeperkey" . _Number)
[1,2]

But I so far haven't figure out how to work the combinators hard enough to get the correct answer. I most probably involves the fragment ' key "otherkey" '. Any ideas?

Answer 1

Try this

s ^.. key "somekey" . _Array . traverse 
                    . to (\o -> ( o ^?! key "deeperkey" . _Number
                                , o ^?  key "otherkey"  . _Number
                                )
                         )

With (^..) to get multiple elements from the traverse Traversal and Array to match on the multiple values in the "somekey" slot. Technically the key "deeperkey" should be allowed to fail, but based on your return type you're making strong assumptions that it doesn't---so we use (^?!) to do the failing (^?) without a Maybe monad for protection.