Elm-watch succeeds in compiling, but in browser shows "Uncaught ReferenceError: _Platform_enqueueEffects is not defined" error and fails to start / validate elm/core version is recent enough

[casperschipper]

One of my Elm apps shows the following error in the browser console when I try to use elm-watch:

Uncaught ReferenceError: _Platform_enqueueEffects is not defined _Platform_initialize http://127.0.0.1:8080/elm.js:4788 _Browser_element http://127.0.0.1:8080/elm.js:6978 F4 http://127.0.0.1:8080/elm.js:2892 init http://127.0.0.1:8080/elm.js:5364 <anonymous> http://127.0.0.1:8080/:22

The widget shows ❓ and reports "It looks like no Elm apps were initialized by elm-watch. Check the console in the browser developer tools to see potential errors!"

Elm-watch.json is as follows:
{ "targets": { "My target name": { "inputs": [ "src/Main.elm" ], "output": "elm.js" } } }

Elm-watch itself reports a success compilation, however the browser shows the above error. I have tried a "clean slate" by removing the complete elm-watch folder and even elm-stuff, but the error keeps happening. As far as I can see, I haven't done anything unusual, it is a very simple Browser.element app. Building the app with elm-watch make works fine.
My other apps also run on elm-watch without problems. So I am somewhat mystified.

Anyhow, thank you for making elm-watch, it has been very useful despite this error.

[lydell]

Hi! Can you create a repo that reproduces this problem, that I can clone and debug?

[casperschipper]

Of course, I think I managed to make a minimum example:
https://github.com/casperschipper/elm-watch-issue-101/tree/main

[lydell]

Awesome, thank you!

I turns out the error was because your elm/core version is too old. If you change your elm.json like this it starts working:

         "direct": {
             "billstclair/elm-sortable-table": "1.2.0",
             "elm/browser": "1.0.1",
-            "elm/core": "1.0.2",
+            "elm/core": "1.0.5",
             "elm/html": "1.0.0",
             "elm/http": "2.0.0",
             "elm/json": "1.1.3",

There is a (pretty hidden and vague) note about this at https://lydell.github.io/elm-watch/hot-reloading/:

You need a recent enough elm/core version. Otherwise some regexes don’t match. Perfect time to update, though!

elm-watch already parses elm.json, so while doing that I could also validate that you have a recent enough version of elm/core and give a specific error message if this happens. I’m keeping the issue open for that!

[casperschipper]

Amazing, and indeed, problem solved!