Elm: How can I send extra params to a Msg from "expect" when calling Http.post?-CodePudding

I'm trying to create an APP in Elm at the first time. And I need to interact with a server application using Http.post.

I have Msg type like this:

type Msg =
    …
    | Send (String, String)
    | Recv (String, String)

and update function is like this:

update : Msg -> Model -> (Model, Cmd Msg)
update msg model =
    case msg of
        …
        Send (name, data) ->
            (newModel, Http.post
                { url = url
                , body = Http.multipartBody [Http.stringPart "data" data]
                , expect = Http.expectString (Recv name)
                })
        Recv (name, data) -> … -- process data

But when I run this, it produces a type mismatch error in Http.post. So how can I pass name and newData to Msg "Recv" at the same time?

CodePudding user response：

Your type constructor Recv is expecting a single argument which is a tuple of string. So essentially Recv : ( String, String ) -> Msg.

So when you call Recv name, where name : String, you will get a type error.

There are two solutions here:

You change the definition of Recv to be
```
| Recv String String
```
This will now mean that you can partially apply the Recv constructor and get back a function String -> Msg, which is what Http.expectString wants.
You change the expect call to get the right shape:
```
, expect = Http.expectString (\body -> Recv ( name, body ))
```
Here you explicitly make the function that Http.expectString wants.