Receive message from an Elm process

user_1886510 0
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I'm toying around with Elm processes in order to learn more about how they work. In parts of this, I'm trying to implement a timer.

I bumped into an obstacle, however: I can't find a way to access the result of a process' task in the rest of the code.

For a second, I hoped that if I make the task resolve with a Cmd, the Elm runtime would be kind enough to perform that effect for me, but that was a naive idea:

type Msg
  = Spawned Process.Id
  | TimeIsUp

init _ =
  ( Nothing
  , Task.perform Spawned (Process.spawn backgroundTask)
  )

backgroundTask : Task.Task y (Platform.Cmd.Cmd Msg)
backgroundTask =
  Process.sleep 1000
    -- pathetic attempt to send a Msg starts here
    |> Task.map ( always
                  <| Task.perform (always TimeIsUp)
                  <| Task.succeed ()
                )
    -- and ends here
    |> Task.map (Debug.log "Timer finished") -- logs "Timer finished: <internals>"

update msg state =
  case msg of
    Spawned id ->
      (Just id, Cmd.none)

    TimeIsUp ->
      (Nothing, Cmd.none)

view state =
  case state of
    Just id ->
      text "Running"

    Nothing ->
      text "Time is up"

The docs say

there is no public API for processes to communicate with each other.

I'm not sure if that implies that a process can't cummunicate with the rest of the app.

Is there any way to have update function receive a TimeIsUp once the process exits?

elm ipc process
2 Answers

There is one way but it requires a port of hell:

  1. make a fake HTTP request from the process,
  2. then intercept it via JavaScript
  3. and pass it back to Elm.
port ofHell : (() -> msg) -> Sub msg

subscriptions _ =
  ofHell (always TimeIsUp)

backgroundTask : Task.Task y (Http.Response String)
backgroundTask =
  Process.sleep 1000
    -- nasty hack starts here 
    |> Task.andThen ( always
                      <| Http.task { method = "EVIL"
                                   , headers = []
                                   , url = ""
                                   , body = Http.emptyBody
                                   , resolver = Http.stringResolver (always Ok "")
                                   , timeout = Nothing 
                                   }
                    )

Under the hood, Http.task invokes new XMLHttpRequest(), so we can intercept it by redefining that constructor.

<script src="elm-app.js"></script>
<div id=hack></div>
<script>
  var app = Elm.Hack.init({
    node: document.getElementById('hack')
  })

  var orig = window.XMLHttpRequest
  window.XMLHttpRequest = function () {
    var req = new orig()
    var orig = req.open
    req.open = function (method) {
      if (method == 'EVIL') {
        app.ports.ofHell.send(null)
      }
      return orig.open.apply(this, arguments)
    }
    return req
  }
</script>

The solution is not production ready, but it does let you continue playing around with Elm processes.

user_18865100

Elm Processes aren't a fully fledged API at the moment. It's not possible to do what you want with the Process library on its own.

See the notes in the docs for Process.spawn:

Note: This creates a relatively restricted kind of Process because it cannot receive any messages. More flexibility for user-defined processes will come in a later release!

and the whole Future Plans section, eg.:

Right now, this library is pretty sparse. For example, there is no public API for processes to communicate with each other.

user_4037020
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