List(Decoder a) -> Decoder (List a).elm

-- Sometimes we want to decode value of type a from a json list of a.. but each with some little differences..
-- Like server sends json: [{"name" : "a"},{"name" : "b"}]
-- and we have

type alias Player = 
    { name : String
    , color : Color
    }

-- color is view specific and server is not sending it..

-- so we have a function :

decodePlayer : Color -> Decoder Player
decodePlayer color =
    Decode.object (\name -> Player name color) ("name" : Decode.string)

-- so.. now we can decode above json array with decodeList

decodeList (List.map decodePlayer [Color.red, Color.blue, Color.green])`

 -- where decodeList is below..

decodeList : List (Decoder a) -> Decoder (List a)
decodeList decoders =
    Decode.customDecoder
        (Decode.list Decode.value)
        (\jsonList ->
             List.foldr (Result.map2 (::)) (Ok [])
                 (List.map2 (\decoder json -> Decode.decodeValue decoder json)
                      decoders jsonList))

-- see documentation of Decode.customDecoder / Decode.value for more details.