Does Fable provide a version of FSharp.Compiler.Service?

[njlr]

Does Fable provide a version of FSharp.Compiler.Service?

If so, what version is it?

And are any notable features not supported?

[alfonsogarciacaro]

The FCS embedded within dotnet fable tool is almost the same as the "official" with a few minor tweaks to help watch compilations, so they shouldn't be unsupported features (as long as we keep FCS in sync).

Unfortunately, it's tricky to provide a version because @ncave just syncs periodically with dotnet/fsharp/main, sometimes this means Fable provides feature before a new FCS package has been released. But it's true at least we should provide a commit hash 😅

[alfonsogarciacaro]

Sorry, closed by mistake. Please feel free to make further questions or suggestions.

[kant2002]

I have similar question of slightly different nature. Why there 2 verison of FSC in Fable repo.

• https://github.com/fable-compiler/Fable/tree/main/lib/fcs
• https://github.com/fable-compiler/Fable/tree/main/src/fcs-fable/src/Compiler
  Why for example fsc-fable cannot use DLL file from lib/fsc?

[ncave]

@kant2002

• lib/fcs is a light FCS façade, consumed by Fable .NET as binary assembly dependency.
• src/fcs-fable is a trimmed down FCS version that can actually be compiled by Fable to JavaScript.
  It's used in the Fable REPL and for the JS-only stand-alone version of the Fable compiler that can work even without .NET installed (but 3x slower than the managed .NET version of Fable, of course).

Both of these forks are frequently rebased to dotnet/fsharp latest, so should be up to date.
Those updates are not aligned to specific FCS versions, they generally track the latest F# main.

[kant2002]

Is there a way how, for example light FSC facade` can be upstreamed or being depends on vanilla FSC from some version nfrom F# main. Reason why I'm asking is that I have experiment on creation of F# dialect based on Ukrainian keywords. Frankly it's easy and I do that in my own fork. NP. I did try to customize Fable Repl to use my F# fork. That was successful, but now I have

• my own fork
• my own fork merged with ncave/fsharp
• apply my custom keywords to fsc-fable/src/Compiler
  which is quite fragile.

Reason why I speculate that maybe ncave/fsharp can be not used is initially I miss instructions in lib/fsc folder and try to replace content of lib/fsc using my own fork once I change FSharp.Compiler.SourceCodeServices to FSharp.Compiler.CodeAalysis and InteractiveChecker to FSharpChecker I was missing only some small bit which I did not figure out. So for my uninitiated view maybe that's possible?

Would this kind of attempt to unify at least one location desirable?

[ncave]

@kant2002 It's unlikely that either of those F# forks that Fable uses can be pushed upstream (although I did try, unsuccessfully, way back in the past), as they mainly provide functionality to Fable.

That said, those forks can be fairly easily rebased on top of (or below) your custom FCS version (if your FCS is recent enough, as there is a lot of churn in dotnet/fsharp).

You don't have to modify src/fcs-fable directly, you can just copy its contents from your (rebased) fork. See npm run build copy-fcs-repo, which copies source from Fable-compatible F# fork to Fable src/fcs-fable. Make sure to build your FCS fork first, see here.

P.S. Replacing the F# keywords sure looks fun, was this the only change needed in dotnet/fsharp?
It also needs support from the language service for syntax coloring etc, but that should be easy too.