js files updated on compilation errors

[joprice]

While running fable in watch mode alongside Vite, I noticed that my app was restarting or hot reloading in cases where the fsharp code had either a syntax or type error. At first I thought this might be a misconfigured watcher, but checking the output, I do see that the js files are being generated in a way where there are "holes" with throw 1, defaultOf() or similar where the error occurred. I'm sure it's a large change and maybe not feasible, but I'm curious if it's possible to instead retain the previous compilation result, only generating or modifying files on successful compilation.

[joprice]

I hit another issue which I think is related to this one: on first compile, if there are errors, the process fails with an non-zero exit code and shows the errors. If the command is run again, Skipped compilation because all generated files are up-to-date! in printed and the command succeeds. I assume this is because it is doing a "shallow" check for the existence of output files and not maintaining some sort of build state to indicate that the file failed on last run and still needs to be built.

[MangelMaxime]

To workaround this issue, you can pass --noCache so it cleans the cache on the first run.

[MangelMaxime]

I am not sure what the action to take from here is. Because, we will need to check how it works from a Developer Experience and what is best.

I believe the file compilation is done in parallels and there writing too. If we don't write throw 1 etc. to notify that Fable compilation fails, we could perhaps end up, with FileA being updated but not FileB. However, because FileB is not modified with a throw 1, it could be possible that the applications seems to works because the old compilation result is somewhat compatible with the new FileA.

My scenario above is hypothetical, but worth checking and thinking about I believe.

From a personal experience, having hot reloading happening on a failed compilation never really posed problems to me. Because, once the application come back in a successful compilation state then the application state is restored. This is especially, true with Elmish.HMR in general. Perhaps with, pure React application it is a little bit different but it never bothered me a lot.