[英]Elm JSON decoder for Union type with data
我的 json 看起來像這樣:
{"name": "providerWithVal", "value": "example"}
或者像這樣:
{"name": "provider2"}
要么
{"name": "provider3"}
我的 Elm 聯合類型定義如下:
type Provider
= ProviderWithVal String
| Provider2
| Provider3
我可以為沒有附加數據的聯合類型編寫解碼器。 但是ProviderWithVal
需要一個字符串,我不確定如何讓它全部工作。
這是我到目前為止所擁有的:
import Json.Decode as D
providerDecoder : D.Decoder Provider
providerDecoder =
D.field "name" D.string |> D.andThen providerNameDecoder
providerNameDecoder : String -> D.Decoder Provider
providerNameDecoder string =
case string of
"providerWithVal" -> D.succeed ProviderWithVal
"provider2" -> D.succeed Provider2
"provider3" -> D.succeed Provider3
_ -> D.fail <| "Invalid provider: " ++ string
您問題的快速解決方案是將D.succeed ProviderWithVal
替換為D.map ProviderWithVal (D.field "value" Decode.string)
但我會創建一個幫助器來匹配目標字符串,然后按以下方式使用它:
decoder =
Decode.oneOf [ decodeWithVal, decodeP2, decodeP3 ]
decodeWithVal =
exactMatch (Decode.field "name" Decode.string)
"providerWithVal"
(Decode.map ProviderWithVal <| Decode.field "value" Decode.string)
decodeP2 =
exactMatch (Decode.field "name" Decode.string) "provider2" (Decode.succeed Provider2)
decodeP3 =
exactMatch (Decode.field "name" Decode.string) "provider3" (Decode.succeed Provider3)
exactMatch : Decoder String -> String -> Decoder a -> Decoder a
exactMatch matchDecoder match dec =
matchDecoder
|> Decode.andThen
(\str ->
if str == match then
dec
else
Decode.fail <| "[exactMatch] tgt: " ++ match ++ " /= " ++ str
)
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