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haskellhaskell-lenslenses

How can I reconstruct a JSON array from a slice using lens?

I would like to extract a slice of a JSON array using lenses, and get a Value back. More specifically, I am trying to do the following:

$ import qualified Data.ByteString.Lazy as LBS
$ import Data.Aeson.Lens
$ import Control.Lens
$ let e = "{\"headers\":[[\"Host\",\"localhost:9090\"],[\"Accept-Encoding\",\"gzip\"]]}" :: LBS.ByteString
$ e ^? key "headers" . nth 0 . _Array . sliced 0 2
$ Just [String "Host",String "localhost:9090"]

It works but I would like the result to be a Value and not a List. How can I "reconstruct" a JSON array as part of the lens expression?

Solution

  • _Array is a Prism. This means you can invert it to get re _Array :: AsValue t => Getter (Vector Value) t.

    e ^? key "headers" . nth 0 . _Array . sliced 0 2 . re _Array

    You can also use over (or its alias (%~)):

    e ^? key "headers" . nth 0 . to (over _Array (slice 0 2))
e ^? key "headers" . nth 0 . to (_Array %~ slice 0 2)