Receive message from an Elm process - process

Question

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?

Answer 1

There is one way but it requires a port of hell:
make a fake HTTP request from the process,
then intercept it via JavaScript
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.

Answer 2

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.