Elm：Json解码器时间戳记至今 [英] Elm: Json decoder timestamp to Date

问题描述

我正在尝试将时间戳（例如： 1493287973015）从JSON转换为日期类型。

I'm trying to convert a timestamp (ex: "1493287973015") from a JSON to a Date type.

到目前为止，我已经创建了此自定义解码器：

So far I created this custom decoder:

stringToDate : Decoder String -> Decoder Date
stringToDate decoder =
  customDecoder decoder Date.fromTime

无法工作，因为它已返回结果，而不是日期：

But it doesn't work because it has return a Result, not a Date:

Function `customDecoder` is expecting the 2nd argument to be:

    Time.Time -> Result String a

But it is:

    Time.Time -> Date.Date

是否可以进行转换？

推荐答案

假设您的JSON实际上是将数字值放在引号内（这意味着您正在解析JSON值 1493287973015 ，而不是 1493287973015 ），则您的解码器可能如下所示：

Assuming your JSON is actually placing the numeric value inside quotes (meaning you are parsing the JSON value "1493287973015" and not 1493287973015), your decoder could look like this:

import Json.Decode exposing (..)
import Date
import String

stringToDate : Decoder Date.Date
stringToDate =
  string
    |> andThen (\val ->
        case String.toFloat val of
          Err err -> fail err
          Ok ms -> succeed <| Date.fromTime ms)

请注意， stringToDate 没有传递任何参数，与您尝试将解码器字符串作为参数传递的示例相反。

Notice that stringToDate doesn't get passed any parameters, as opposed to your example in which you were attempting to pass a Decoder String as a parameter. That's not quite how decoders work.

相反，这可以通过在更原始的解码器上构建来完成，在这种情况下，我们从解码器 string from Json.Decode 。

Instead, this can be done by building upon more primitive decoders, in this case, we start with the decoder string from Json.Decode.

andThen 部分然后获取解码器给出的字符串值，并尝试将其解析为浮点型。如果它是有效的 Float ，则将其输入到 Date.fromTime 中，否则将失败。

The andThen portion then takes the string value given by the decoder, and tries to parse it to a float. If it is a valid Float, it is fed into Date.fromTime, otherwise, it's a failure.

失败 和 成功 函数将您要处理的正常值包装到 Decoder Date.Date中。 上下文，以便可以将其返回。

The fail and succeed functions wrap up the normal values you're dealing with into Decoder Date.Date contexts so they can be returned.

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