如何将 JSON 数组解码为自定义类型的 Elm 列表

Question

我想将 JSON 负载对象数组解码为 List Payload，其中每个 Payload 都是自定义类型：

type Payload
     = PayloadP1 P1
     | PayloadP2 P2

使用下面的解码器解码 PayloadP1 和 PayloadP2 后，我该如何解码 Payload？

type alias P1  = 
    { id : Int
    , st : String
    }

type alias P2  = 
    { id : Int
    , s1 : String
    , s2 : String
    }

type Payload
     = PayloadP1 P1
     | PayloadP2 P2

type alias PayloadQueue = List Payload

decodeP1 : Jd.Decoder P1
decodeP1 =
    Jd.map2 P1
        (Jd.field "id" Jd.int)
        (Jd.field "st" Jd.string)

decodeP2 : Jd.Decoder P2
decodeP2 =
    Jd.map3 P2
        (Jd.field "id" Jd.int)
        (Jd.field "p1" Jd.string)
        (Jd.field "p2" Jd.string)

decodePayload =
    Jd.field ".type" Jd.string
    |> Jd.andThen decodePayload_

{-
decodePayload_ : String -> Jd.Decoder Payload
decodePayload_ ptype =
    case ptype of
        "P1" -> decodeP1
        "P2" -> decodeP2
-}

json_str = """[
   {".type" : "P1", "id" : 1, "st" : "st"},
   {".type" : "P2", "id" : 2, "p1" : "p1", "p2" : "p2"},
]"""

Answer 1

您需要将 P1 和 P2 分别包装在 PayloadP1 和 PayloadP2 中，以便 return 每个分支的通用类型，您可以使用 map. Then you also need to account for the possibility that the type field is neither P1 or P2. In that case you can either provide a default or return an error using fail。我在下面完成了后者。

decodePayload_ : String -> Jd.Decoder Payload
decodePayload_ ptype =
    case ptype of
        "P1" -> decodeP1 |> Jd.map PayloadP1
        "P2" -> decodeP2 |> Jd.map PayloadP2
        _ -> Jd.fail "invalid type"

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