与递归相关的异常：无法获取未定义或空引用的属性“标记” [英] Recursion related exception: Unable to get property 'tag' of undefined or null reference

问题描述

执行HTTP发布后，我收到以下错误消息：

I receive the following error after performing an HTTP post:

无法获取未定义或空引用的属性标记

Unable to get property 'tag' of undefined or null reference

我相信执行以下解码器功能时会发生错误：

I believe the error occurs when executing the following decoder function:

sourceDecoder : Decoder JsonSource
sourceDecoder =
    Decode.map5 JsonSource
        ...
        (field "Links" providerLinksDecoder)

解码器依赖性：

providerLinksDecoder : Decoder JsonProviderLinks
providerLinksDecoder =
    Decode.map JsonLinkFields
        (field "Links" <| Decode.list (Decode.lazy (\_ -> linkDecoder)))
        |> Decode.map JsonProviderLinks

linkDecoder : Decoder JsonLink
linkDecoder =
    Decode.map6 JsonLink
        (field "Profile" profileDecoder)
        ...

profileDecoder : Decoder JsonProfile
profileDecoder =
    Decode.map7 JsonProfile
        ...
        (field "Sources" <| Decode.list (Decode.lazy (\_ -> sourceDecoder)))

附录：

type JsonProviderLinks
    = JsonProviderLinks JsonLinkFields


type alias JsonLinkFields =
    { links : List JsonLink
    }

源代码位于此处。

注意：我试图研究此错误，并遇到此页面。
结果，我尝试使用Decode.lazy函数。但是，我的尝试失败了。

Note: I attempted to research this error and came across this page. As a result, I attempted to use the Decode.lazy function. However, my attempt failed.

推荐答案

在您的示例中，有很多解码器依赖于其他解码器。您已经更改了其中一些使用 Decode.lazy 的功能，但不是全部，并且当某些不可控制的递归时，您会收到该错误。

There's a lot of decoders that rely on other decoders in your examples. You've changed some of them to use Decode.lazy, but not all, and that error you've received will happen when there's some out of control recursion.

您不需要列表即可使用懒惰。尝试-至少是调试的第一步-更改引用其他解码器的 all 解码器以使用 Decode.lazy 。例如：

You don't need a list to be able to use lazy. Try - as a first step towards debugging, at least - to change all decoders that reference other decoders to use Decode.lazy. For example:

sourceDecoder : Decoder JsonSource
sourceDecoder =
    Decode.map5 JsonSource
        ...
        (field "Links" (Decode.lazy (\_ -> providerLinksDecoder)))

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