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Fantomas Daemon


The F# advent calendar is a wonderful initiative that inspires the community to create new content in a short period of time. I’ve been fortunate enough to be part of this for some years now. This year, I had in mind to sit this one out actually. Sometimes, there are just no new tales to tell and it is better suited to give others an opportunity. When I noticed that Sergey still had some spots left to fill, I did happily volunteer, though didn’t think it through that much.

My original thought for this post was to create some content around the F# compiler. To the benefits of the Fantomas project, I have been contributing some PR’s to the compiler and I wanted to elaborate a bit how a change there can simplify Fantomas. This idea for a blogpost sounds reasonable, but life happens and I didn’t have this content ready at the time of writing. So, instead I’d like to write about some of the recent Fantomas changes.

Pre-Daemon mode

The next version of Fantomas 4.6 will contain an improvement in IDE integration. Versions 4.5 and prior were available in Ionide, Rider and the Visual Studio extensions. Installing those plugins or IDE was sufficient to format code using a shortcut or action. Doing this would call the format function that is present in the Fantomas NuGet package. Not only will this format the current file, it would also trigger the correct logic inside the editor that the file has changed. Imagine autocomplete and other refactoring options. And that really is a relevant integration aspect that you would not get when you format the code using the commandline.

The downside of this optimal integration is that you are not in control of the version that is being used in the editor. Fantomas bugfixes happen almost on a weekly basis. I encourage people to scratch their own itch and many people are able to contribute after checking out the Contribution Guidelines and watching some YouTube videos.

Solving your own bug is very rewarding and I try to publish these fixes to NuGet frequently. However, it’s important to note that having a fix on NuGet doesn’t make it available in your IDE.

The stars really have to align before newer versions of Fantomas can be integrated, mainly because the version of FCS has to be in sync with what the editor has, and that can be challenging.

Another downside that the IDE controlling the version of Fantomas is that you can have formatted a file yesterday and have a different result today. You notice this in your source control diff and that is overall just not what you want.

Daemon mode

To move beyond this limitation, we tried to rethink how the editor integration should look like. What if you could bring your own version of Fantomas? Well, you can do this today by installing it as a local .NET tool. This works, but lacks the editor integration.

In 4.6 we tried to solve this problem by introducing Fantomas.Client, a companion NuGet package for editors. The client exposes a FantomasService that can execute the traditional format requests as found in Fantomas, but it proxies those to fantomas-tool instead. When formatting inside an editor, Fantomas Client will try and find a compatible version for the current file and spin up a daemon process of fantomas-tool. This daemon process will act as an LSP server that send json messages back and forward to the FantomasService. As user of an editor, you don’t need to worry about Fantomas.Client, just know that the editor will use a locally or globally installed version of fantomas-tool. Meaning, you control the version and you upgrade at your own pace.

At the time of writing, there is support for VSCode (as part of the Ionide plugin). If you have Ionide installed, it will prompt you to install fantomas-tool if no compatible version can be found. After installing, you should be good to go as demonstrated in this video:

Post-Daemon mode

My hopes are that we can ship bug fixes to Fantomas at a high pace. If it gets on NuGet, it can be used in your IDE. Ever since 4.5, I have been releasing revisions on a frequent pace and people seem to have faith in this. Download numbers are significantly higher than say the first alpha of 4.4. Alpha and beta just don’t have a stable ring to it for most people. I completely understand this, and that is why I’d like to move forward to a model were bugfixes are considered stable and people should not fear to upgrade.

What about Rider and Visual Studio?

JetBrains has slightly different plans, although it is on their radar to support custom versions of Fantomas. They won’t go with Fantomas.Client for technical reason as can be read in this GitHub issue.

Integration of Fantomas.Client is ongoing for the Visual Studio extension. Progress can be tracked here. Please don’t hesitate to contribute any of the remaining work.


During the development of Fantomas.Client, I had a lot of mentoring by Chet Husk (the one and only @baronfel on GitHub). We worked together on the Ionide integration in FSAutocomplete (or F-Sack as the cool kids say it). And over a timespan of several months we were able to come up with a prototype.

For the Visual Studio integration I’d like to thank Christopher-Marcel Esser and Asti. Christopher-Marcel implemented the port to Fantomas.Client and Asti is the core maintainer of the project.

Closing thoughts

The whole Fantomas daemon thing is pretty mind-blowing. We can upgrade to different compiler versions in a breeze. This used to take months and always involved a lot of coordination. Now it happens overnight.

If you have any further questions about this, you can always reach out to our Gitter channel.

Enjoy the rest of the advent calendar, happy holidays and stay safe!



Photo by Donovan Reeves on Unsplash