{-# LANGUAGE TemplateHaskell #-}

import Control.Lens
import Control.Lens.Combinators

data Container = Container { _slot1 :: Maybe String
                           , _slot2 :: Maybe Int
                           }
    deriving (Show)

makeLenses ''Container

setOrError g s val cont = case (cont ^. g) of
                              Nothing -> Right (set s val cont)
                              Just  x -> Left "Error: Already defined"

setSlot1 val = setOrError slot1 slot1 val
setSlot2 val = setOrError slot2 slot2 val

cont = Container (Just "") (Just 0)

test = setOrError slot1 slot1 (Just "Foo") cont