Tighter Vite integration

[nojaf]

Hello,

As someone new to ReScript, I'm trying to find an optimal integration with Vite. The current approaches are either to run a separate terminal or to use jihchi/vite-plugin-rescript.

Running a separate terminal is less than ideal. The vite-plugin-rescript approach, which writes files to disk, poses its own problems. For instance, if there's an error in the ReScript code, the plugin might resort to using an outdated file, leading to confusion and loss of React state.

I'm curious about the feasibility of compiling ReScript code in-memory to address these issues. Such a method would ensure the Vite plugin always has access to the latest version of the code and might also enhance performance by reducing I/O operations.

I've initiated this discussion as I'm unsure if in-memory compilation is currently feasible with ReScript. I'm open to being corrected on this.

I'm also curious if you considered this @jihchi?

[jihchi]

Hi @nojaf , regarding

The vite-plugin-rescript approach, which writes files to disk, poses its own problems.

AFAIK, jihchi/vite-plugin-rescript does not write files to disk. Could you please elaborate on that?

What the plugin does is to simply spawn a process of the ReScript compiler in the background and redirect error logs from the .compiler.log (generated by ReScript compiler) to Vite.

[nojaf]

Could you please elaborate on that?

Sure, if I have a A.res file, the plugin will still compile it to A.res.mjs.
Which is what I would expect when looking at:

https://github.com/jihchi/vite-plugin-rescript/blob/54d751fc4b5e18f7f9168e2513213f8c5a93b042/src/index.ts#L21-L23

[jihchi]

...if I have a A.res file, the plugin will still compile it to A.res.mjs.

Sorry, I don't get your expectation in this scenario.

You'd get the same result without using the plugin. In other words, you'd get the same result by just executing rescript build --with-deps command.

[nojaf]

Yes, which is why I'm opening this discussion on the compiler side.
There is no real need for the compiler to write the result to disk.
It can stay in memory in the Vite plugin if this is possible.

In my naive mindset, any imports to a .res file in Vite could be compiled with rescript to the stdout.
The plugin could then return that result as the content of the requested files.

[jihchi]

Got it! Thank you for the explanation.

[cometkim]

There are two issues:

1. The ReScript compiler emitting JS is a design choice, not a capability. I remember an old official article about this, but I can't find it. It's related to the deprecation of bs-loader (BuckleScript loader for Webpack).
2. Making the compiler output to memory is not less work. Currently, the compiler is FS-dependent, so we need to add an FS shim to write to memory. In theory, this could be more efficient later, but it would require significant effort.

[nojaf]

Where would 2. need to be done? Is that in the Node wrapper or the compiler itself?

[nojaf]

So, I just saw the intro to the compiler by @cristianoc.
What I'm after might exist after all?

Running bunx bsc ./src/File.res does seem to output the JavaScript code to the std.
So, it is a matter of passing all the right compiler arguments to bunx bsc? I would assume that is what bunx rescript does.

If my hunch is correct, where could I find where rescript.exe is calling bsc? That might give me some inspiration to play around with.

[fhammerschmidt]

It's a bit more involved than that, the build system (either rescript.exe or rewatch) also decides when to rebuild/what to cache. In a naive approach you would probably just rebuild everything.

Regardless, here's an example where we call the compiler directly for the docs example tests: https://github.com/rescript-association/rescript-lang.org/blob/master/scripts/test-examples.mjs#L98-L115

[nojaf]

Yes, good point. I would basically want to spin up rewatch in some process and keep it running.
And ask it for transpiled .res file (in memory) based on what the vite plugin hook requires.
rewatch shouldn't watch actual .res files (as Vite does this) but should invalidate the correct parts of the project based on the input of Vite.