https://github.com/cmditch/elm-msg-bug
https://github.com/cmditch/elm-msg-bug
Last synced: 2 months ago
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- Host: GitHub
- URL: https://github.com/cmditch/elm-msg-bug
- Owner: cmditch
- Created: 2021-05-26T18:17:14.000Z (about 5 years ago)
- Default Branch: master
- Last Pushed: 2021-05-26T19:33:27.000Z (about 5 years ago)
- Last Synced: 2026-01-03T10:29:43.098Z (6 months ago)
- Language: Elm
- Size: 102 KB
- Stars: 1
- Watchers: 1
- Forks: 0
- Open Issues: 0
-
Metadata Files:
- Readme: README.md
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README
# Elm Msg mapping issue
A basic multi-page SPA app SSCCE
### What's happening here?
I think Elm is mapping a `Msg` from the Home page into the `Settings` page.
The compiler would recognize this as invalid, but the Elm runtime is allowing this.
Something very relevant here: This is only reproducable in Chrome due to how
the `focusout` event is fired off once the DOM element is removed. Other browsers
do not behave this way. `preventDefaultOn "mousedown"` is very important as
this prevents the textfield from losing focus when `mousedown` hits the "Go to Settings" link.
**Here is what I think the flow is:**
1. Focusing `input` element causes `Note.Msg (EditorFocused True)`.
2. Mousedown on "Go to Settings" link causes `Note.Msg NoOp` to fire. Input still has focus.
3. Mouseup on link causes the page to change to `/settings`.
4. The `input` is removed from the DOM on page change and the `focusout` event is fired (only in Chrome).
5. `EditorFocused False` is fired but caught by `Settings.Msg`
6. The settings update function does not recognize the `EditorFocused` Msg,
but the way the Elm compiler generates the final match in a case statements results in the Msg being caught in the `default` case,
aka `TheDefaultCase { wat : { hereIsAnElmException : Int } }`
7. The code attempts to access `constructorValue.wat.hereIsAnElmException`.
`constructorValue` is actually a `Boolean` off of `EditorFocused` so an exception is thrown.
**Note:** If you disable the offending code in `Settings` under `Msg.TheDefaultCase`, you an invalid `Msg` type propagating to `Settings.update`
Screen Shot 2021-05-26 at 1.28.36 PM
### Run
1. Install deps and start dev server
```
yarn install
yarn dev
```
2. Open http://localhost:1234 **in Chrome**
3. Click into textfield.
4. Click "Go to Settings" link.
5. Look at console for the exception.