go - 当参数与工作示例相同时，为什么 Elm 不会为 JSON 数据示例编译这个 HTTP 请求？

Question

我正在研究一个基本的解码 json Elm 示例以供练习，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 FYI 本地编译时却没有。

目标是进行一个简单的调用以从 Go 服务器获取 JSON，但仅编译我从文档中获得的用于解码 JSON 的示例 Elm 并没有成功，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
榆树制作./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?

Answer 1

看起来您已经安装了 elm/http 的 1.0.0 版。模块，而示例需要 elm/http 2.0.0 版。我可以使用此软件包的 1.0.0 版本重现您的错误，但使用 2.0.0 成功编译的代码。

编辑您的 elm.json文件以更改 elm/http 的版本至2.0.0并尝试运行 elm make ...再次。