I’ve run into strange behavior of text input elements in combination with “double update” - i.e. process Msg from input, issue a Cmd Msg and update model in the subsequent update, like this: | user input | -- Msg1 value --> | update (model unchanged yet) | -- Cmd (Msg2 value) --> | update (model ch…

I’ve made an example where the cursor does not jump. https://ellie-app.com/bbMMCjXngNba1 In general I think one should avoid passing the Model in a Msg since this can cause tricky concurrency behaviours that are hard to reason about.

[image] lazurski: In my app I have a complex type representing a document. There are many possible transformations that can be applied to it. For versioning purposes every time any transformation is applied I need to capture and store the current time. I want my type to be opaque and transformat…

Thank you for your input. I really appreciate it! [image] albertdahlin: In general I think one should avoid passing the Model in a Msg since this can cause tricky concurrency behaviours that are hard to reason about. You (and @rupert ) are right about this. Please consider that my Ellie is a …

[image] rupert: I would second this - I made an Ellie to show what can happen if multiple events fire in quick succession. A later update can overwrite the model with stale data from an earlier state: https://ellie-app.com/yVC9wPk33xa1 That is a good example of tricky concurrency that is not…

[image] lazurski: It would solve the issue of double updates, but it has two downsides. (1) If two users update the document concurrently there might be a version clash. This is very unlikely with a timestamp. The update function in Elm is single threaded - as javascript in the browser is sin…

[image] rupert: The update function in Elm is single threaded - as javascript in the browser is single threaded. This means that it cannot have a race condition where 2 updates get the same value - the increments will be ‘atomic’ Right. That’s the good part of your solution - it’s atomic. But…

If you really want to go with time stamps, you might also try going with a hybrid approach where you first assign a numerical version number, so you can update the model immediately and at the same time request a time stamp for it. Once the time stamp arrives, you replace the version number by the t…

[image] lazurski: If I won’t find a better way I will do it like you suggested, but I hope to get keep the updates atomic. As I see it, there are two problems here: Atomic updates The cursor jumps My (partial) understanding of your problem with atomic updates is that you need both a string…

[image] albertdahlin: My (partial) understanding of your problem with atomic updates is that you need both a string and a time. In other words, you need to unambiguously pair a text with the corresponding timestamp. Yes! I think your technical explanation is also correct. Basically if I type b…

[image] lazurski: Right. That’s the good part of your solution - it’s atomic. But I was thinking about a scenario where two users open the document (version 1). Each of them makes one change (maybe one removes a section and the other inserts a comma). Each of them have version 2 now, but they ar…

Trouble with text inputs and double update roundtrips

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rupert October 8, 2020, 12:26pm 11

It occurs to me that you might find this Elm Europe talk interesting:

Topic		Replies	Views
Preventing invalid states in update function Learn	23	1780	March 30, 2021
A different API design for elm web that removes the need for `Msg` and `update` Request Feedback	13	473	August 29, 2025
Weird behavior. Updating value of one input also updates value of another input Learn	7	1087	July 14, 2019
In Elm, What happens when? Learn	7	2357	August 25, 2020
Add timestamp also when updating a record Learn	9	870	April 3, 2022

Trouble with text inputs and double update roundtrips

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