Haskell: Reusing FromJSON instances with lenses, lens-aeson, and nested JSON

Question

I have been playing with Aeson and the lens package (lens-aeson, migrated from the core lens package), and have been sruggling to get them to work together.

As a toy exa

Answer 1

You can use the _JSON prism. You only need to write a ToJSON instance for Colour:

instance ToJSON Colour where
  toJSON Yellow = String "yellow"
  toJSON Blue   = String "blue"
  toJSON Green  = String "green"

and then you can use it like this:

parseColour :: String -> Maybe Colour
parseColour j = j ^? key "info" . key "colour" . _JSON
-- point-free: parseColor = preview $ key "info" . key "colour" . _JSON

-- In GHCi
λ: parseColour "{ \"info\": { \"colour\": \"yellow\" } }"
Just Yellow