尝试解码较大的json值时,我遇到了Json-Decode-Extra
库中的以下代码。 (位于here)
import Date (Date)
type alias User =
{ id : Int
, createdAt : Date
, updatedAt : Date
, deletedAt : Maybe Date
, username : Maybe String
, email : Maybe String
, fullname : Maybe String
, avatar : Maybe String
, isModerator : Bool
, isOrganization : Bool
, isAdmin : Bool
}
metaDecoder : (Int -> Date -> Date -> Maybe Date -> b) -> Decoder b
metaDecoder f = f
`map` ("id" := int)
`apply` ("createdAt" := date)
`apply` ("updatedAt" := date)
`apply` ("deletedAt" := maybe date)
userDecoder : Decoder User
userDecoder = metaDecoder User
`apply` ("username" := maybe string)
`apply` ("email" := maybe string)
`apply` ("fullname" := maybe string)
`apply` ("avatar" := maybe string)
`apply` ("isModerator" := bool)
`apply` ("isOrganization" := bool)
`apply` ("isAdmin" := bool)
但是,我经常遇到:=
运算符的编译器错误。这定义在哪里? JSON解码教程不会在任何地方明确导入此运算符。
答案 0 :(得分:5)
在Elm 0.18中,:=
运算符被Json.Decode.field
替换,并且删除了使用反引号运算符的反引号。
您正在使用尚未更新为Elm 0.18的软件包(circuithub / elm-json-extra)。
考虑切换使用由Elm社区维护的包:elm-community/json-extra
。您可以使用andMap
代替apply
。以下是升级到新库和Elm 0.18的示例代码:
import Date exposing (Date)
import Json.Decode exposing (..)
import Json.Decode.Extra exposing (andMap, date)
metaDecoder : (Int -> Date -> Date -> Maybe Date -> b) -> Decoder b
metaDecoder f =
succeed f
|> andMap (field "id" int)
|> andMap (field "createdAt" date)
|> andMap (field "updatedAt" date)
|> andMap (field "deletedAt" (maybe date))
userDecoder : Decoder User
userDecoder =
metaDecoder User
|> andMap (field "username" (maybe string))
|> andMap (field "email" (maybe string))
|> andMap (field "fullname" (maybe string))
|> andMap (field "avatar" (maybe string))
|> andMap (field "isModerator" bool)
|> andMap (field "isOrganization" bool)
|> andMap (field "isAdmin" bool)
请注意,elm-community/json-extra
包还会导出中缀运算符|:
,这是andMap
的中缀版本。您可以使用它来使您的代码更简洁。例如:
metaDecoder : (Int -> Date -> Date -> Maybe Date -> b) -> Decoder b
metaDecoder f =
succeed f
|: field "id" int
|: field "createdAt" date
|: field "updatedAt" date
|: field "deletedAt" (maybe date)