# Calling HTTP.Post from submodule

**URL:** <https://discourse.elm-lang.org/t/calling-http-post-from-submodule/6547>\
**Category:** Learn\
**Created:** [November 23, 2020, 3:44am UTC](https://discourse.elm-lang.org/t/calling-http-post-from-submodule/6547 "2020-11-23T03:44:13Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![anotherhale](https://yyz1.discourse-cdn.com/flex035/user_avatar/discourse.elm-lang.org/anotherhale/32/3634_2.png) [@anotherhale](https://discourse.elm-lang.org/u/anotherhale)\
**Post date:** [November 23, 2020, 3:44am UTC](https://discourse.elm-lang.org/t/calling-http-post-from-submodule/6547/1 "2020-11-23T03:44:13Z")

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I am having the same issue as outlined in this SO question. [https://stackoverflow.com/questions/60513194/how-to-avoid-expect-msg-with-http-2-0-0-in-elm](https://stackoverflow.com/questions/60513194/how-to-avoid-expect-msg-with-http-2-0-0-in-elm)

I am trying to make a `HTTP Post` from an authentication submodule and not sure how to resolve the `Msg` vs `Auth.Msg`.  
I don’t get the suggestion of `Why don't you just pass an a -> msg function to your Api functions?`  
Can someone provide a more detailed response?

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**Author:** ![Lucas\_Payr](https://yyz1.discourse-cdn.com/flex035/user_avatar/discourse.elm-lang.org/lucas_payr/32/4993_2.png) [@Lucas\_Payr](https://discourse.elm-lang.org/u/Lucas_Payr)\
**Post date:** [November 23, 2020, 6:57am UTC](https://discourse.elm-lang.org/t/calling-http-post-from-submodule/6547/2 "2020-11-23T06:57:47Z")

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Here is an example for the solution given in StackOverflow

```elm
myApi: 
  (a->msg) 
  -> { method : String
     , headers : List Header
     , url : String
     , body : Body
     , expect : Expect a
     , timeout : Maybe Float
     , withCredentials : Maybe String
  }
  -> Cmd msg
myApi msgMapper {method, headers,url,body,expect,timeout,withCredentials} =
  request
    { method = method
    , headers = headers
    , url = url
    , body = body
    , expect = expect
    , timeout = timeout
    , withCredentials = withCredentials
    }
    |> Cmd.map msgMapper

```

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**Author:** ![Jeremy\_Hunt](https://yyz1.discourse-cdn.com/flex035/user_avatar/discourse.elm-lang.org/jeremy_hunt/32/3636_2.png) [@Jeremy\_Hunt](https://discourse.elm-lang.org/u/Jeremy_Hunt)\
**Post date:** [November 23, 2020, 8:03am UTC](https://discourse.elm-lang.org/t/calling-http-post-from-submodule/6547/3 "2020-11-23T08:03:23Z")

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The point is to **avoid** using `Cmd.map`, right?

What we want is `Api.elm` knows how to get a `Thing` from a server, but doesn’t want to deal with what to do with the response.

And `Main.elm`, and other clients of `Api.elm`, want to get a `Thing` from a remote server so they can do awesome things with it, but they don’t want to deal with how to get it.

So `Api.elm` can be lazy about doing things with `Things`:

```auto
module Api exposing (getSomething)

{-| I'll get a Thing for you, just tell me what you want done with it. 
For example, I could put it into some kind of msg for you, then I'll give 
you a Cmd you can invoke to get *that kind of msg* eventually.
-}
getThing : (Result Http.Error Thing -> msg) -> Cmd msg  
getThing constructor =
    Http.request 
        { method : "GET"
        , headers : []
        , url : "http://thingServer.someplace.com/"
        , body : Http.emptyBody
        , expect : Http.expectJson constructor thingDecoder
        , timeout : Nothing
        , tracker : Nothing
        }

```

And `Main.elm` is lazy about how to get things:

```auto
{-| When I init, the Api is going to do some work for me, and then 
I'll get a GotThing message that hopefully has a Thing in it!
-}
main : Program () MyAwesomeModel MyMainMessageType
main = 
    Browser.element 
        { init : () -> ( init, Api.getThing GotThing ) 
        , view : view
        , update : update
        , subscriptions : always Sub.none
        }

type MyMainMessageType
    = GotThing (Result Http.Error Thing)

update : 
    MyMainMessageType 
    -> MyAwesomeModel  
    -> ( MyAwesomeModel, Cmd MyMainMessageType )
update msg model = 
    case msg of
        GotThing (Ok thing) ->
            Thing.doAllTheThings thing

        GotThing (Err error) ->
            dealWith error

```

When they said suggested passing `( a -> msg)`, that translated to my example as passing the type constructor `GotThing` in `Api.getThing GotThing`. Note that `GotThing` by itself has the type `( Result Http.Error Thing -> MyMainMessageType)`, in other words its a constructor for values of `MyMainMessageType`. And that `Api.getThing` takes as its first argument a `( Result Http.Error Thing -> msg)`, or in other words, it just wants a message constructor for some type of `msg` so that it can cram the `Thing` it gets somewhere. It returns a `Cmd` that could produce that type of `msg` eventually.

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<div class="post-metadata">

**Author:** ![anotherhale](https://yyz1.discourse-cdn.com/flex035/user_avatar/discourse.elm-lang.org/anotherhale/32/3634_2.png) [@anotherhale](https://discourse.elm-lang.org/u/anotherhale)\
**Post date:** [November 24, 2020, 12:34am UTC](https://discourse.elm-lang.org/t/calling-http-post-from-submodule/6547/4 "2020-11-24T00:34:03Z")

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Thanks Jeremy. That does clarify things for me and I was able to get my code to work. 🤙

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**Author:** ![system](https://canada1.discourse-cdn.com/flex035/uploads/elm_lang/original/1X/50a05e53677a2c3b47776d7abd0f113eb50193a1.png) [@system](https://discourse.elm-lang.org/u/system)\
**Post date:** [December 4, 2020, 12:34am UTC](https://discourse.elm-lang.org/t/calling-http-post-from-submodule/6547/5 "2020-12-04T00:34:09Z")

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