How to send a parametrized message to update function?

[alfred]

Hi! I’m not capable of understanding how to send a parametrized message to the update function when I receive a service response.

I mean, having the next types and functions:

type Message = 
    ApiCall (Result Error ApiCallResult) 

type ApiCallResult = 
    GetSections (List Section)

api : (Result Error ApiCallResult) -> Model -> (Model, Cmd Message)
api result model = case result of
    Ok call -> case call of
       GetSections _ -> (model, Cmd.none)
    Err _ -> (model, Cmd.none)

update msg model = case msg of
    ApiCall result -> api result model

I resolve the service calling like this:

sectionDecoder : D.Decoder Section

getSections : Cmd Message
getSections = 
    Http.get {
        url = "http://127.0.0.1:8000/api/sections/",
        expect = H.expectJson (ApiCall GetSections) (Decoder.list sectionDecoder)
        }

However, it’s clear that the expect parameter for the Http.get is wrong. The compiler gives the next error.

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

45|         expect = Http.expectJson (ApiCall GetSections) (Decoder.list sectionDecoder)
                                   ^^^^^^^^^^^^^^^^^^^^^^^
This `ApiCall` call produces:

    T.Message

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

    Result H.Error a -> msg

-- TYPE MISMATCH --------------------------------------------------- src/Api.elm

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

45|         expect = Http.expectJson (ApiCall GetSections) (Decoder.list sectionDecoder)
                                             ^^^^^^^^^^^^^
This `GetSections` value is a:

    List Section -> ApiCallResult

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

    Result Error ApiCallResult

Detected problems in 1 module.

Could you make me understand how to do it? Thanks!

[brian]

It looks like you’re trying to construct a ApiCall with GetSections. Pretend for a minute those are just regular functions. You couldn’t call ApiCall with GetSections because ApiCall expects a result, and GetSections is a function that takes List Section and returns an ApiCallResult. In other words, they don’t compose the way you want here.

So there are two problems here:

1. expectJson expects you to handle the error. Looks like you’ve already got a type that accepts a Result: ApiCall. We can just use that directly.
2. the success value in the Result should be an ApiCallResult but currently it’s a List Section. Looks like you’re trying to wrap that in the first argument, but it needs to happen in the decoder instead.

So this code might set you straight:

expectJson ApiCall (Decode.map GetSections (Decode.list sectionDecoder))

[alfred]

Thanks! I’ll give some thoughts to it to fully understand!