Passing multiple arguments to Javascript via port - elm

Question

I need to pass a few parameters to Javascript, but for a strange reason it does not compile. I started with:
port check : String -> Cmd msg
this works fine (as taken directly from JavaScript Interop). But when I am adding another parameter
port check : Int -> String -> Cmd msg
I am getting
1| port check : Int -> String -> Cmd msg
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
You are saying it should be:
Int -> String -> Platform.Cmd.Cmd msg
But you need to use the particular format described here:
http://guide.elm-lang.org/effect_managers/
I solved this simply by reducing arguments back to one
type alias Bundle = (Int, String)
port check : Bundle -> Cmd msg
but that would be cleaner if I could simply do
app.ports.check.subscribe(function(arg1, arg2) {
});
Why it doesn't compile?

Answer 1

ports can only have one parameter. If you need to pass multiple parameters then your only options are to pass a tuple (like you did with Bundle) or a record.
On the js side, you'll have to take one parameter but you can destructure it after with whatever var names you want
app.ports.check.subscribe(function(arg) {
var [arg1, arg2] = arg;
});
If you're using ES6, then you do have some nicer options for destructuring in the function params if you use a record like this:
app.ports.check.subscribe(function({arg1: arg1, arg2: arg2}) {
});

Answer 2

I noticed you don't really have to "decode" much on the Javascript side. So, you can package your single parameter up as a nice type in ELM.
Do something like this in your elm:
type alias MyObject =
{ name: String
, displayDate: String
, subTitle: String
, hashTag: String
}
port check : MyObject -> Cmd msg
Then in the javascript you can just do this:
app.ports.check.subscribe(function(myObject) {
alert( myObject.name);
alert( myObject.displayDate);
alert( myObject.subTitle);
alert( myObject.hashTag);
});