CODEWITHSUNDEEP
jsonhaskellhaskell-lens
T_S_
T_S_

Reputation: 173

Parsing json with lens-aeson

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?

Upvotes: 4

Views: 1305

Answers (1)

J. Abrahamson
J. Abrahamson

Reputation: 74344

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.

Upvotes: 3

Related Questions