Pulling a record (ie naked JSON object) of one of two types out of a JSON string using purescript-argonaut and Alt instance of Either

ArgonautAlt.purs

module Main where

import Prelude
import Control.Alt ((<|>))
import Data.Argonaut (Json, decodeJson, (.?))
import Data.Either (Either)

data Response = A { commandResponse :: String, value :: String }
              | B { command         :: String, value :: Int }

decodeResponse :: Json -> Either String Response
decodeResponse json = decodeA json <|> decodeB json

decodeA :: Json -> Either String Response
decodeA json = do
  obj <- decodeJson json
  r   <- obj .? "commandResponse"
  v   <- obj .? "value"
  pure (A { commandResponse: r, value: v })

decodeB :: Json -> Either String Response
decodeB json = do
  obj <- decodeJson json
  r   <- obj .? "command"
  v   <- obj .? "value"
  pure (B { command: r, value: v })