Fantomas.Core 5.0.0-alpha-010

.NET Standard 2.0
This is a prerelease version of Fantomas.Core.
There is a newer prerelease version of this package available.
See the version list below for details.
Install-Package Fantomas.Core -Version 5.0.0-alpha-010
dotnet add package Fantomas.Core --version 5.0.0-alpha-010
<PackageReference Include="Fantomas.Core" Version="5.0.0-alpha-010" />
For projects that support PackageReference, copy this XML node into the project file to reference the package.
paket add Fantomas.Core --version 5.0.0-alpha-010
The NuGet Team does not provide support for this client. Please contact its maintainers for support.
#r "nuget: Fantomas.Core, 5.0.0-alpha-010"
#r directive can be used in F# Interactive, C# scripting and .NET Interactive. Copy this into the interactive tool or source code of the script to reference the package.
// Install Fantomas.Core as a Cake Addin
#addin nuget:?package=Fantomas.Core&version=5.0.0-alpha-010&prerelease

// Install Fantomas.Core as a Cake Tool
#tool nuget:?package=Fantomas.Core&version=5.0.0-alpha-010&prerelease
The NuGet Team does not provide support for this client. Please contact its maintainers for support.


Fantomas logo

F# source code formatter, inspired by scalariform for Scala, ocp-indent for OCaml and PythonTidy for Python.

Build Status Github Actions Discord

Quick install

How to use

Command line tool / API

Use this command to install Fantomas as a .NET 5 SDK global tool:

dotnet tool install -g fantomas

For detailed guidelines, please read Fantomas: How to use.

FAKE build system

Fantomas can be easily integrated with FAKE build system.<br /> Check out the sample.

JetBrains Rider

The resharper-fsharp uses fantomas under the hood to format the source code. No need for any additional plugins.

Using the latest version inside Rider

For technical reasons Rider cannot always use the latest version of Fantomas found on NuGet. As a workaround you could install fantomas locally with dotnet tool install fantomas-tool and configure it as an External tool.

Rider external tool window

Rider action window

This will have an impact on your editing experiencing in Rider, the external change to the file by the command line application might trigger more internal logic inside Rider than necessary. It could be noticeable in regards to the default formatting experience.

Visual Studio Code

The recommended way to use Fantomas is by using the Ionide plugin. Fantomas is integrated in FSAutoComplete which is the language server used by Ionide.

Alternatively, you can install the fantomas-fmt extension.

Visual Studio

The F# Formatting extension sets up Fantomas as the default formatter for F# files, configurable from Visual Studio's options.

Visual Studio for Mac

Install fantomas locally with dotnet tool install fantomas-tool and configure it as an [External tool]

VS Mac external tool window


Try the Fantomas online.

Early builds

Every once in a while an alpha or beta version is published to Our previous MyGet feed is now deprecated.


Some figures can be found at <br/> We use BenchmarkDotNet to collect data for each build on the master branch.


This project aims at formatting F# source files based on a given configuration. Fantomas will ensure correct indentation and consistent spacing between elements in the source files. We assume that the source files are parsable by F# compiler before feeding into the tool. Fantomas follows two F# style guides: the F# code formatting guidelines from Microsoft by default and the G-Research F# code formatting guidelines via various settings.

Use cases

The project is developed with the following use cases in mind:

  • Reformatting an unfamiliar code base. It gives readability when you are not the one originally writing the code. To illustrate, the following example

     type Type
         = TyLam of Type * Type
         | TyVar of string
         | TyCon of string * Type list
         with override this.ToString () =
                 match this with
                 | TyLam (t1, t2) -> sprintf "(%O -> %O)" t1 t2
                 | TyVar a -> a
                 | TyCon (s, ts) -> s

    will be rewritten to

     type Type =
         | TyLam of Type * Type
         | TyVar of string
         | TyCon of string * Type list
         override this.ToString() =
             match this with
             | TyLam(t1, t2) -> sprintf "(%O -> %O)" t1 t2
             | TyVar a -> a
             | TyCon(s, ts) -> s
  • Converting from verbose syntax to light syntax. Feeding a source file in verbose mode, Fantomas will format it appropriately in light mode. This might be helpful for code generation since generating verbose source files is much easier. For example, this code fragment

     let Multiple9x9 () =
         for i in 1 .. 9 do
             printf "\n";
             for j in 1 .. 9 do
                 let k = i * j in
                 printf "%d x %d = %2d " i j k;
     Multiple9x9 ();;

    is reformulated to

     let Multiple9x9() =
         for i in 1..9 do
             printf "\n"
             for j in 1..9 do
                 let k = i * j
                 printf "%d x %d = %2d " i j k
  • Formatting F# signatures, especially those generated by F# compiler and F# Interactive.

For more complex examples, please take a look at F# outputs of 20 language shootout programs and 10 CodeReview.SE source files.

Why the name "Fantomas"?

There are a few reasons to choose the name as such. First, it starts with an "F" just like many other F# projects. Second, Fantomas is my favourite character in the literature. Finally, Fantomas has the same Greek root as "phantom"; coincidentally F# ASTs and formatting rules are so mysterious to be handled correctly.

Contributing Guidelines

See the Contribution Guidelines.


We would like to gratefully thank the following persons for their contributions.


The library and tool are available under Apache 2.0 license. For more information see the License file.

Product Versions
.NET net5.0 net5.0-windows net6.0 net6.0-android net6.0-ios net6.0-maccatalyst net6.0-macos net6.0-tvos net6.0-windows
.NET Core netcoreapp2.0 netcoreapp2.1 netcoreapp2.2 netcoreapp3.0 netcoreapp3.1
.NET Standard netstandard2.0 netstandard2.1
.NET Framework net461 net462 net463 net47 net471 net472 net48
MonoAndroid monoandroid
MonoMac monomac
MonoTouch monotouch
Tizen tizen40 tizen60
Xamarin.iOS xamarinios
Xamarin.Mac xamarinmac
Xamarin.TVOS xamarintvos
Xamarin.WatchOS xamarinwatchos
Compatible target framework(s)
Additional computed target framework(s)
Learn more about Target Frameworks and .NET Standard.

NuGet packages (1)

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Package Downloads

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Version Downloads Last updated
5.0.0-beta-006 27 8/12/2022
5.0.0-beta-005 41 8/3/2022
5.0.0-beta-004 47 7/27/2022
5.0.0-beta-003 39 7/26/2022
5.0.0-beta-002 51 7/19/2022
5.0.0-beta-001 53 7/15/2022
5.0.0-alpha-011 38 7/8/2022
5.0.0-alpha-010 55 6/27/2022
5.0.0-alpha-009 47 6/21/2022
5.0.0-alpha-008 56 5/28/2022
5.0.0-alpha-007 58 5/16/2022
5.0.0-alpha-006 51 5/13/2022
5.0.0-alpha-005 53 5/7/2022
5.0.0-alpha-004 52 5/7/2022
5.0.0-alpha-003 54 5/6/2022

### Changed
* Restore the FormatSelection public API. [#2237](
* Refactor list of TriviaNodeAssigner to tree structure. [#2271](