Bad error message when sending the wrong thing through a port: [object Object] #1043

lydell · 2019-09-16T07:36:00Z

SSCCE

port module Main exposing (main)


main : Program () () ()
main =
    Platform.worker
        { init = always ( (), Cmd.none )
        , update = \msg model -> ( model, Cmd.none )
        , subscriptions = always (test (always ()))
        }


port test : (String -> msg) -> Sub msg

<body>
  <script>
    var app = Elm.Main.init()
    app.ports.test.send(1337)
  </script>
</body>

Result:

Error: Trying to send an unexpected type of value through port `test`: [object Object]

Expected something like this:

Error: Trying to send an unexpected type of value through port `test`: Expected a STRING, but got: 1337

Story

If I have a port like this…

port test : (String -> msg) -> Sub msg

…and accidentally send a number instead of a string…

app.ports.test.send(1337)

…I get this error:

Error: Trying to send an unexpected type of value through port `test`: [object Object]

It seems like no matter what type of faulty value I send, it always says [object Object]. Which isn’t very helpful :)

The browser devtools shows not only the error message, but also which piece of code caused the error. If I click the error location, I’m taken to code looking like this in the devtools debugger:

		case 4:
			var portName = fact1;
			var problem = fact2;
			throw new Error('Trying to send an unexpected type of value through port `' + portName + '`:\n' + problem);

That comes from here:

core/src/Elm/Kernel/Debug.js

Lines 257 to 260 in fc9d96b

    
           case 4: 
        
           	var portName = fact1; 
        
           	var problem = fact2; 
        
           	throw new Error('Trying to send an unexpected type of value through port `' + portName + '`:\n' + problem);

If I put a breakpoint on the throw line, it turns out that problem/fact2 isn’t a string, but an object looking like this:

{
  "$": "Failure",
  "a": "Expecting a STRING",
  "b": {
    "$": 0,
    "a": 1337
  }
}

That explains where [object Object] is coming from. So it looks like it would be possible to say something like Expecting a STRING, but got: 1337 instead.

Background

The other day I was trying to make a port like this:

port clickedOutsideDropdown : (() -> msg) -> Sub msg

The JavaScript code only has to tell Elm that a click occurred, but does not need to pass any extra information. That’s why I used unit/() in the port.

When it was time to implement the JavaScript part, I was thinking: “Since this port does not take any value, do I need to pass anything to .send?” So I tried this…

app.ports.clickedOutsideDropdown.send()

…which gave me this error:

Error: Trying to send an unexpected type of value through port `test`: [object Object]

I was confused. I didn’t send anything, so what [object Object] is the error message talking about?

After using the good old JavaScript debugger I finally figured out that I was supposed to pass null:

app.ports.clickedOutsideDropdown.send(null)

The text was updated successfully, but these errors were encountered:

lydell mentioned this issue Feb 9, 2021

Incoming ports that take unit as an argument causes a crash at runtime elm/compiler#1367

Closed

lue-bird mentioned this issue Aug 8, 2022

port sending an unexpected value → uninformative error message gren-lang/core#26

Open

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Bad error message when sending the wrong thing through a port: [object Object] #1043

Bad error message when sending the wrong thing through a port: [object Object] #1043

lydell commented Sep 16, 2019

Bad error message when sending the wrong thing through a port: [object Object] #1043

Bad error message when sending the wrong thing through a port: [object Object] #1043

Comments

lydell commented Sep 16, 2019

SSCCE

Story

Background