Create a "withField" encoder

[sebsheep]

Hi,

My (weird) json contains either one of 3 possibles fields (say fA, fB and fC). I would like to write a custom coder for this but I'm stucked in the encoder part :

weirdCoder =
    let
        encoder source =
            case source of
                A a -> ??
                    
                B b -> ??

                C c -> ??
   in
      ( encoder
      ,  Decode.oneOf
        [ field "fA" (Decode.map A aDecoder)
        , field "fB" (Decode.map B bDecoder)
        , field "fC" (Decode.map C cDecoder)
        ])

I think it would be easy if the decoders were exposed but there arent...