当参数与工作示例相同时，为什么 Elm 不会编译此 JSON 数据示例的 HTTP 请求？

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我正在研究一个基本的解码 json elm 示例以进行练习，但无法找出 elm 编译错误。我也很困惑为什么它在 ellie 中运行（即使用远程 json url），但在使用 elm v 19.0 本地编译时却不能运行（仅供参考）。

目标是进行一个简单的调用以从 go 服务器获取 json，但仅编译我从解码 json 的文档中获得的示例 elm 并不起作用，所以我们在这里。

module helloworld exposing (..)


import browser
import html exposing (..)
import html.attributes exposing (..)
import html.events exposing (..)
import http
import json.decode exposing (decoder, field, string)



-- main


main =
  browser.element
    { init = init
    , update = update
    , subscriptions = subscriptions
    , view = view
    }



-- model


type model
  = failure
  | loading
  | success string


init : () -> (model, cmd msg)
init _ =
  (loading, getrandomcatgif)



-- update


type msg
  = moreplease
  | gotgif (result http.error string)


update : msg -> model -> (model, cmd msg)
update msg model =
  case msg of
    moreplease ->
      (loading, getrandomcatgif)

    gotgif result ->
      case result of
        ok url ->
          (success url, cmd.none)

        err _ ->
          (failure, cmd.none)



-- subscriptions


subscriptions : model -> sub msg
subscriptions model =
  sub.none



-- view


view : model -> html msg
view model =
  div []
    [ h2 [] [ text "random cats" ]
    , viewgif model
    ]


viewgif : model -> html msg
viewgif model =
  case model of
    failure ->
      div []
        [ text "i could not load a random cat for some reason. "
        , button [ onclick moreplease ] [ text "try again!" ]
        ]

    loading ->
      text "loading..."

    success url ->
      div []
        [ button [ onclick moreplease, style "display" "block" ] [ text "more please!" ]
        , img [ src url ] []
        ]



-- http


getrandomcatgif : cmd msg
getrandomcatgif =
  http.get
    { url = "http://127.0.0.1:8080/test"
    , expect = http.expectjson gotgif gifdecoder
    }


gifdecoder : decoder string
gifdecoder =
  field "data" (field "image_url" string)

现在是 go 服务器

// write json back to elm call

package main

import (
    "fmt"
    "log"
    "net/http"
    "os"

    "github.com/gorilla/schema"
)

type todo struct {
    title       string
    description string
}

func main() {

    http.handlefunc("/test", test)

    http.listenandserve(getport(), nil)

}

func getport() string {
    p := os.getenv("port")
    if p != "" {
        return ":" + p
    }
    return ":8080"
}

func test(w http.responsewriter, r *http.request) {
    if r.method == "get" {
        //allow cors here by * or specific origin
        w.header().set("access-control-allow-origin", "*")
        // say hi
        fmt.fprintf(w, "hello from go"+"\n")

    } else {
        //allow cors here by * or specific origin
        w.header().set("access-control-allow-origin", "*")

        todo := readform(r)
        fmt.fprintf(w, "updated todo to"+todo.title+"\n")
    }
}

func readform(r *http.request) *todo {
    r.parseform()
    todo := new(todo)
    decoder := schema.newdecoder()
    decodeerr := decoder.decode(todo, r.postform)
    if decodeerr != nil {
        log.printf("error mapping parsed form data to struct : ", decodeerr)
    }
    return todo
}

最后是 elm 错误 elm make ./src/helloworld.elm

Detected problems in 1 module.
-- TYPE MISMATCH -------------------------------------------- src/HelloWorld.elm

Something is off with the body of the `getRandomCatGif` definition:

110|>  Http.get
111|>    { url = "http://127.0.0.1:8080/test"
112|>    , expect = Http.expectJson GotGif gifDecoder
113|>    }

This `get` call produces:

    Decoder a -> Http.Request a

But the type annotation on `getRandomCatGif` says it should be:

    Cmd Msg

-- TYPE MISMATCH -------------------------------------------- src/HelloWorld.elm

The 1st argument to `get` is not what I expect:

110|   Http.get
111|>    { url = "http://127.0.0.1:8080/test"
112|>    , expect = Http.expectJson GotGif gifDecoder
113|>    }

This argument is a record of type:

    { expect : b, url : String }

But `get` needs the 1st argument to be:

    String

-- TYPE MISMATCH -------------------------------------------- src/HelloWorld.elm

The 1st argument to `expectJson` is not what I expect:

112|     , expect = Http.expectJson GotGif gifDecoder
                                    ^^^^^^
This `GotGif` value is a:

    Result Http.Error String -> Msg

But `expectJson` needs the 1st argument to be:

    Decoder a

-- TOO MANY ARGS -------------------------------------------- src/HelloWorld.elm

The `expectJson` function expects 1 argument, but it got 2 instead.

112|     , expect = Http.expectJson GotGif gifDecoder
                    ^^^^^^^^^^^^^^^
Are there any missing commas? Or missing parentheses?

解决方案

看起来您已经安装了 elm/http 模块的 1.0.0 版，而该示例需要 elm/http 2.0.0 版。我可以使用此包的 1.0.0 版本重现您的错误，但使用 2.0.0 版本可以成功编译代码。

编辑 elm.json 文件，将 elm/http 的版本更改为 2.0.0 并尝试再次运行 elm make ...。

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