Array has unwanted side effects - duplicates elements

[okkero]

Description

When using Array under certain circumstances, elements in the array are duplicated when they shouldn't be. This makes some array operations impure. I can only guess that this is due to a bug in the code that handles persistent data structures in the runtime (does Array map more or less directly to JavaScript's array under the hood?).

Example that exhibits the problem

Check out this example: https://ellie-app.com/3W8CQfWkjfSa1/4 (I tried making it as short as possible and still keeping the bug)

This is a small portion of an app that I'm working on. It's a relatively unstyled version of a dialog box for configuring a graph, wherein you can select which parameters are shown on which axis.

Steps to reproduce

1. Compile the example and click the Setup button
2. Select the abc parameter in the first list, by clicking on it
3. Move it to the second list by clicking the button labeled v
4. Click cancel. According to the Elm code the state of the setup dialog should now be reset and abc should be back in the first list.
5. Click Setup again. Observe how abc has been moved back to the first list, but that it has also been duplicated and still exists in the second list.
6. Weirder still, if you open Debug and click the ClickMoveTo message, abc will be duplicated again. Keep clicking this message and abc will keep duplicating. This is not expected nor correct behaviour.

Additional information

I have tested the same code with the difference that the Arrays have been replaced with Lists. Now the app works as expected.

I have only tested this on Windows 10, on Google Chrome Version 60.0.3112.90, but I am willing to bet that it happens on all browsers and OSes.

[process-bot]

Thanks for the issue! Make sure it satisfies this checklist. My human colleagues will appreciate it!

Here is what to expect next, and if anyone wants to comment, keep these things in mind.

[process-bot]

Thanks for the issue! Make sure it satisfies this checklist. My human colleagues will appreciate it!

Here is what to expect next, and if anyone wants to comment, keep these things in mind.

[sigurdsa]

Able to reproduce on Firefox 54.0.1 also

[sigurdsa]

Able to reproduce on Firefox 54.0.1 also

[Skinney]

This is fixed in the upcoming Elm 0.19.

If you need a working version NOW, then you can use the package Skinney/elm-array-exploration.

[Skinney]

This is fixed in the upcoming Elm 0.19.

If you need a working version NOW, then you can use the package Skinney/elm-array-exploration.