Fantomas integration

[nojaf]

Continuation of the conversion in ionide/ionide-vscode-fsharp#274.
This is poor implementation meant to be improved over time (with the needed guidance of the maintainers). Please comment on everything little detail you see 😉.

First thing I noticed is that the latest beta of Fantomas is already on FCS 31. Currently somewhat waiting on #450.

Hopefully, this experiment will highlight if we need any changes to the Fantomas 3.0 API (See fsprojects/fantomas#454).

//cc: @Krzysztof-Cieslak @jindraivanek

[Krzysztof-Cieslak]

OK. Thanks a lot for driving this forward.

So the first step - to get access to the file source, AST and checker you need to implement functionality in Commands.fs file (it's in FsAutoComplete.Core project). The state object in Commands class will give you access to the file content (state.Files) ,to the AST of the current file (state.CurrentAST) (this probably should be enough as we always can format only current file) or to more-or-less up-to-date FSharpParseFileResults from any file tracked by FSAC (state.ParseResults).

In Commands class there exists checker field, but it's our custom wrapper, you'll probably need to expose "standard" FSharpChecker in CompilerServiceInterface.fs in FSharpCompilerServiceChecker class.

To get FSharpParsingOptions you can get FSharpProjectOptions from state.FileCheckOptions and then transform it with Utils.projectOptionsToParseOptions

[Krzysztof-Cieslak]

In meantime I'll poke @baronfel about finishing #450 ;-)

[nojaf]

Thanks for the pointers, Kris.
What should TextDocumentOnTypeFormatting do exactly?
And should TextDocumentRangeFormatting return the entire document or just the reformatted fragment?

[Krzysztof-Cieslak]

TextDocumentOnTypeFormatting - as far as I understand it's formatting that is run after every key stroke, I think we don't want to implement it? Not sure how fast is Fantomas ;-)

TextDocumentRangeFormatting - should return only the edited range.

The full LSP specification is available here: https://microsoft.github.io/language-server-protocol/specification#textDocument_formatting

[nojaf]

Ok thanks, so TextDocumentOnTypeFormatting someday maybe 😋.
Fantomas is getting faster but I would not implement that one right now.

TextDocumentRangeFormatting - should return only the edited range.

That is great as it is the same method Rider needs. I'm playing around with the idea to trim the public API of Fantomas down to the method we know that are in use by consumers. So maybe, just maybe we can drop https://github.com/nojaf/fantomas/blob/api-changes/src/Fantomas/CodeFormatter.fs#L37.

[nojaf]

@Krzysztof-Cieslak I think this is ready now, currently, it doesn't contain any options for settings but we can always add that later.